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Subcutaneous and Visceral Adipose Tissue: Their Relation to the Metabolic Syndrome

Endocrine Service, Hospital das Clinicas of The University of São Paulo Medical School, São Paulo, SP, Brazil

	Abstract

Top Abstract I. Introduction II. Classification of Abdominal... III. Assessment of Abdominal... IV. Correlations of Abdominal... V. Adipose Tissue as... VI. Body Fat Distribution... VII. Pathology of the... VIII. Endocrinological... IX. Summary References

 
Methods for assessment, e.g., anthropometric indicators and imaging techniques, of several phenotypes of human obesity, with special reference to abdominal fat content, have been evaluated. The correlation of fat distribution with age, gender, total body fat, energy balance, adipose tissue lipoprotein lipase and lipolytic activity, adipose tissue receptors, and genetic characteristics are discussed. Several secreted or expressed factors in the adipocyte are evaluated in the context of fat tissue localization. The body fat distribution and the metabolic profile in nonobese and obese individuals is discussed relative to lipolysis, antilypolysis and lipogenesis, insulin sensitivity, and glucose, lipid, and protein metabolism. Finally, the endocrine regulation of abdominal visceral fat in comparison with the adipose tissue localized in other areas is presented.

I. Introduction

II. Classification of Abdominal Fat

III. Assessment of Abdominal Visceral Fat

A. Anthropometric indexes of abdominal visceral adipose tissue mass

B. Imaging techniques

IV. Correlations of Abdominal Visceral Fat

A. Age and gender

B. Total body fat

C. Energy balance

D. Adipose tissue LPL activity

E. Adipose tissue lipolytic activity

F. Adipose tissue receptors

G. Genetic characteristics

V. Adipose Tissue as an Endocrine Gland

A. Secreted proteins and triglyceride metabolism

B. Secreted proteins and cholesterol and retinoid metabolism

C. Protein related to blood coagulation: plasminogen activator inhibitor-1 (PAI-1)

D. Secreted factors with an endocrine function

E. Factors with an autocrine/paracrine activity regulating adipose tissue cellularity

VI. Body Fat Distribution and the Metabolic Profile

A. Comparison of lipolysis, antilipolysis and lipogenesis in visceral abdominal and subcutaneous fat in nonobese and obese individuals

B. Abdominal visceral fat and insulin sensitivity: role of FFAs

C. Glucose, insulin, lipid, and protein metabolism and its relationship to fat topography

VII. Pathology of the Abdominal Visceral Fat Without a Specific Endocrine Disorder

A. Increase in abdominal visceral fat

B. Decrease in abdominal visceral fat: congenital generalized lipodystrophy

VIII. Endocrinological Regulation of Abdominal Visceral Fat

A. Cortisol: Cushing’s syndrome and visceral obesity

B. Testosterone

C. GH

D. Estrogens

IX. Summary

	I. Introduction

Top Abstract I. Introduction II. Classification of Abdominal... III. Assessment of Abdominal... IV. Correlations of Abdominal... V. Adipose Tissue as... VI. Body Fat Distribution... VII. Pathology of the... VIII. Endocrinological... IX. Summary References

 
EPIDEMIOLOGICAL studies often report an association between severe obesity and mortality due to increased rates of cardiovascular and cerebrovascular diseases and diabetes (1, 2, 3, 4). In moderate obesity, regional distribution appears to be an important indicator for metabolic and cardiovascular alterations since an inconstant correlation between body mass index (BMI) and these disturbances has been found (2, 5). Over the last two decades, studies have reemphasized the notion put forward in 1947 by Vague (6) that obesity is not a homogeneous condition and that the regional distribution of adipose tissue is important to understanding the relation of obesity to disturbances in glucose and lipid metabolism (7). Many prospective studies have shown that excess fat in the upper part of the body (i.e., central or abdominal), considered by Vague (6) as "android or male-type obesity," more often correlates with increased mortality and risk for disorders such as diabetes, hyperlipidemia, hypertension, and atherosclerosis of coronary, cerebral, and peripheral vessels more often than the "gynoid" (lower body or gluteo-femoral or peripheral depot) female-type of fat distribution (8, 9, 10, 11, 12, 13). However, in these studies, the body fat distribution was assessed using anthropometric measurements such as skinfolds and waist-to-hip circumference ratios (WHR), particularly the latter. Although the WHR is simple and convenient for epidemiological studies and provides a useful estimation of the proportion of abdominal or upper-body fat (14, 15, 16), it does not distinguish between accumulations of deep abdominal (visceral) fat and subcutaneous abdominal fat. Imaging techniques, particularly computed tomography (CT), which clearly distinguishes fat from other tissues, allows the measurement of visceral and subcutaneous abdominal fat. Several studies have shown that the detrimental influence of abdominal obesity on metabolic processes is mediated by the intraabdominal fat depot. For example, the visceral fat area correlated with glucose intolerance in the presence of hyperinsulinemia during an oral glucose tolerance test, suggesting an insulin-resistant state (17, 18, 19). In addition, correlation analyses have shown that the effect of accumulation of deep abdominal fat on glucose tolerance was independent from total adiposity and subcutaneous abdominal adipose tissue and that no association was observed between total adiposity and glucose tolerance after control for visceral fat area (18, 19). In their study of a wide range of total body fat in both healthy young (20) and middle-aged (21) men, Park, Märin, and colleagues found that the intraabdominal fat area evaluated by CT was associated with a decrease in insulin sensitivity measured by an euglycemic hyperinsulinemic glucose clamp. In addition to being associated with disturbances in insulin-glucose homeostasis, abdominal obesity has been related to alterations in plasma lipoprotein-lipid levels (22, 23, 24), particularly increased plasma triglyceride and low high-density lipoprotein (HDL) cholesterol concentrations, as expected from the association of insulin resistance with disturbances in plasma lipid transport and lipoprotein levels (25, 26).

Although the cause-and-effect association has not been definitively established, the available evidence indicates that visceral fat is an important link between the many facets of the metabolic syndrome: glucose intolerance, hypertension, dyslipidemia, and insulin resistance (27). However, because of the considerable metabolic heterogeneity still remaining among obese patients with similar levels of visceral adipose tissue, it was proposed that genetic susceptibility plays a major role in modulating the risk associated with a given excess of visceral adipose tissue (28). In this regard, visceral obesity should be considered a factor that exacerbates an individual genetic susceptibility to the components of the metabolic syndrome (27). While there is a consensus that visceral fat has a strong association with cardiovascular risk factors, particularly dyslipidemia and hyperinsulinemia (29), the primary importance of visceral adipose tissue vis-à-vis subcutaneous abdominal obesity with regard to insulin sensitivity of glucose metabolism has been challenged by Abate et al. (30) and Goodpaster et al. (31). These researchers found that abdominal subcutaneous fat, as determined by magnetic resonance imaging and CT, was at least as strong a correlate of insulin sensitivity (evaluated by the euglycemic clamp) as visceral fat and retained independent significance after adjusting for visceral fat (31). Further, in a review of 23 published studies of intervention strategies to promote loss of visceral adipose tissue, measured by magnetic resonance imaging or CT, Smith and Zachwieja (32) concluded that individuals with greater visceral fat mass, either through an increase in body weight or the propensity to store fat in the visceral depot, lose more visceral fat when adjusted to the loss of body fat, regardless of the intervention applied (caloric restriction, pharmacological therapy, or exercise) because the visceral adipocyte has a higher lipolytic rate also in the steady state. In addition, it has been emphasized that the endocrine abnormalities described in obesity, which involve steroid hormones, GH, and insulin, may actually result in abdominal depot fat accumulation. This might cause the metabolic syndrome in the susceptible individual (33, 34).

In this review I will evaluate the methods for assessment of abdominal fat content, from anthropometric indicators to imaging techniques, and their usefulness for predicting changes in visceral fat. The correlations of abdominal visceral fat with age, gender, total body fat, energy balance, adipose tissue lipoprotein lipase (LPL) and lipolytic activity, and genetic characteristics will be presented. The pathology of intraabdominal visceral fat without a specific endocrine disorder, considering the increase in intraabdominal fat content in nonobese and obese subjects and, on the other hand, the lipodystrophic syndromes with a reduction of visceral fat, will be analyzed. Finally, I will discuss the endocrine regulation of abdominal visceral fat, taking into consideration the factors expressed and released by adipose tissue and the hormones known to have a role in human obesity: cortisol, testosterone, estrogens, and GH.

	II. Classification of Abdominal Fat

Top Abstract I. Introduction II. Classification of Abdominal... III. Assessment of Abdominal... IV. Correlations of Abdominal... V. Adipose Tissue as... VI. Body Fat Distribution... VII. Pathology of the... VIII. Endocrinological... IX. Summary References

 
As described by Märin et al. (21), abdominal fat is composed of abdominal subcutaneous fat and intraabdominal fat, as clearly shown by CT and magnetic resonance imaging (MRI); intraabdominal adipose tissue is composed of visceral, or intraperitoneal, fat, mainly composed of omental and mesenteric fat and retroperitoneal fat masses by a delineation along the dorsal borderline of the intestines and the ventral surface of the kidney.

According to Abate et al. (30) the two intraabdominal compartments are separated on MRI using anatomical points, such as ascending and descending colon, and aorta and inferior vena cava; such a procedure has been validated in human cadavers (35). However, the lack of exact borderlines between these two depots on CT or MRI makes this subdivision only an approximation. Even a large error in the delineation between these two tissues would lead to the conclusion that, at least in men, the retroperitoneal fat mass is a minor part of intraabdominal adipose mass, comprising only approximately one fourth of visceral fat (21). On the other hand, the intraperitoneal and retroperitoneal adipose tissue masses measured after dissection in three cadavers were 61–71% and 29–33%, respectively, of the intraabdominal adipose tissue mass (35). Abate et al. (30) studied healthy middle-aged men with a wide range of adiposity and found that the retroperitoneal fat mass decreased from 42 to 31% of the intraabdominal adipose tissue mass, respectively, in lean and obese subjects. Märin et al. (21) have shown a stronger correlation for visceral than for retroperitoneal adipose mass with systemic metabolic variables, including plasma insulin, blood glucose levels, glucose disposal rate (euglycemic clamp), and systolic blood pressure. Similarly, Abate et al. (30) showed that while there was a significant and negative correlation with glucose disposal during an euglycemic hyperinsulinemic clamp, no such relationship was observed with the retroperitoneal fat mass and, as mentioned previously, the subcutaneous adipose tissue in the truncal region, including thorax and abdomen, contributed more to insulin resistance than the adipose tissue elsewhere in the body. Thus, their investigation, as well as that of Goodpaster et al. (31), suggested that subcutaneous abdominal fat, as a component of central obesity, has as strong an association with insulin resistance as visceral fat and retained independent significance after adjusting for visceral fat.

	III. Assessment of Abdominal Visceral Fat

Top Abstract I. Introduction II. Classification of Abdominal... III. Assessment of Abdominal... IV. Correlations of Abdominal... V. Adipose Tissue as... VI. Body Fat Distribution... VII. Pathology of the... VIII. Endocrinological... IX. Summary References

 
A. Anthropometric indexes of abdominal visceral adipose tissue mass
1. WHR. The WHR is the most widely used index of regional adipose tissue distribution and is measured in a standing position. Waist circumference is defined as the minimal circumference measured at the navel, and the hip circumference is defined as the widest circumference measured at the hips and buttocks (36).

There is a well documented sex dimorphism in regional adipose tissue distribution (37). Indeed, despite the fact that women are usually more obese as a group than men, male subjects more frequently have significantly higher mean waist circumference and higher mean WHR in agreement with the greater propensity of men to accumulate excess fat within the abdominal cavity. Thus, the threshold values suggested by Pouliot et al. (38) of 0.85 for women and 0.95 for men are in agreement with those proposed in previous studies (39) and in our small series of normal men and women where the mean + 2 SD was 0.97 and 0.86, respectively (unpublished data). Since the WHR has been shown to be associated, albeit moderately, with the amount of abdominal visceral adipose tissue measured by CT or MRI [the "gold standards" for such determination (40, 41, 42)], this index has been widely used to investigate the relations between regional adipose tissue distribution and metabolic profile. Thus it was effective in predicting aberrations in glucose and insulin levels and also showed a strong correlation between plasma lipids and blood pressure (14). WHR predicted subsequent diabetes in men (13) and coronary heart disease in both men and women (8, 9) and was more predictive of these endpoints than either the BMI or a more complex procedure using the sum of multiple skinfold thicknesses. Its effects are independent of the overall level of obesity. However, Pouliot et al. (38), in a study using a large sample of men and women, showed that the use of WHR as a single anthropometric index of cardiovascular risk, as well as the use of critical threshold values indicated by their current data, is limited by the fact that, for a given WHR value, there may be large variations in the level of total body fat and in the level of abdominal visceral adipose tissue that are most likely to be associated with important variations in the metabolic profile. Thus, according to their data, the WHR determines the regional distribution of adipose tissue, which is relatively independent of the degree of obesity and appears less closely related to the amount of abdominal visceral adipose tissue. In this study (38), other simple anthropometric indexes were evaluated that appeared to be superior to the WHR in providing assessment of visceral obesity, waist circumference, and abdominal sagittal diameter (to be discussed below). Similar conclusions were reached by Sjöström et al. (43).

2. Waist circumference. Of the body circumferences, the measurement at the abdomen or "waist" is the most variable in term of its location or position, especially among obese and elderly persons. For example, the waist circumference is correctly measured at the level of the umbilicus, but in many obese individuals, the umbilicus may be directed downward because of the excessive curvatures of the abdominal wall.

Waist circumference measured at the midpoint between the lower border of the rib cage and the iliac crest has been reported to be more closely correlated with the level of abdominal visceral adipose tissue and associated metabolic variables than the WHR in both sexes (38, 42, 44, 45). According to Pouliot et al. (38), a waist circumference greater than 100 cm is most likely to be associated with disturbances in lipoprotein metabolism and in plasma glucose-insulin homeostasis (at least in French Canadians). The threshold value is similar in men and women in that for a given waist circumference, men and women had comparable levels of abdominal visceral adipose tissue. Thus, waist circumference, a convenient and simple measurement unrelated to height (46) and correlated with BMI and WHR (47), determines the extension of abdominal obesity, which appears closely linked to abdominal visceral adipose tissue deposition. Furthermore, while changes in waist girth reflect changes in risk factors for cardiovascular disease (48) and other forms of chronic disease, the risks vary in different populations; therefore, globally applicable cut-off points cannot be developed. For example, abdominal fatness has been shown to be less strongly associated with risk factors for cardiovascular disease and type 2 diabetes in black women than in white women (49). Risk factors such as total and HDL cholesterol were correlated with subcutaneous and abdominal fat areas by CT as well as their sum in healthy nonobese Asian Indians. On the other hand, while there was an association of visceral adiposity with insulin secretion during an oral glucose test in men, such was not found in women (50). In addition, it has been reported that visceral obesity is strongly related to coronary heart disease risk factors in nonobese Japanese-American men (51). Also, people of South Asian (Indian, Pakistani, and Bangladeshi) descent living in urban societies have a higher incidence of obesity complications than other ethnic groups (52). These complications are seen to be associated with abdominal fat distribution, which is markedly higher for a given level of BMI than in Europeans. Finally, although women have an almost equivalent absolute risk of coronary heart disease (CHD) to men at the same WHR (53, 54), they show increases in relative risk of CHD at lower waist circumferences than men. For example, in a random sample of 2,183 men and 2,689 women from the Netherlands, aged 20–59 yr (54), risk of obesity-associated metabolic complications was either increased or substantially increased in men with a waist girth of 94 and 102 cm, respectively. In women, the correspondent values were 80 and 88 cm, respectively. Thus, there is a need to develop sex-specific waist circumference cut-off points appropriate for different populations.

The studies by Ferland et al. (42) and Pouliot et al. (38) revealed that the shared variance between waist circumference and visceral adipose tissue reached 75%, which suggests that waist girth by itself may be a useful variable for the crude assessment of visceral fat accumulation. Indeed, their results indicate that more than 90% of the variation in waist girth could be explained by differences in total body fatness and in visceral adipose tissue accumulation in both men and women.

Therefore, the waist circumference, and the abdominal sagittal diameter (as will be discussed below), are the anthropometric indexes preferred over the WHR to estimate the amount of abdominal visceral fat and related cardiovascular risk profile.

Using the equations for prediction, multiscan CT was used to determined visceral adipose tissue volume from the waist circumference in a sample of 17 males and 10 females with different degrees of obesity (43). The waist circumference explained 60% and 64% of the visceral adipose tissue volume variance in males and females, respectively, and the SE was 26% and 29%, respectively. The corresponding figures for the WHR were 26% and 69%, and 36% and 27%, respectively. Again, it was concluded that the WHR is a suboptimal predictor of visceral adipose tissue volume.

3. Abdominal sagittal diameter. Abdominal sagittal diameter is derived either from a CT abdominal scan (38) or by using a carpenter’s spirit level placed over the abdomen perpendicular to the length axis of the trunk at the iliac crest level when the subject is placed on a firm examination table. The sagittal diameter is measured with a ruler as the vertical distance from the horizontal spirit level to the examination table after a normal expiration (43).

Kvist et al. (55) were the first to demonstrate that the sagittal diameter (measured on a CT scan) was closely related to the volume of visceral fat. The correlation of the sagittal diameter with visceral fat volume was 0.94 in 19 women and 0.92 in 24 men, the subjects presenting a wide range of BMI. The correlations between the waist circumference and visceral fat were, respectively, 0.85 and 0.88. These correlations are considerably higher than those observed between anthropometric variables and the visceral fat area measured at the level of the umbilicus in obese men and women (56). Ferland et al. (42) also observed markedly lower correlations in obese women. Desprès et al. (45), in a study of men covering a wide range of fatness, also observed higher correlations but there was not much difference between the visceral fat area and the correlations with the waist circumference (r = 0.82) and the sagittal diameter (r = 0.85). Busetto et al. (57) demonstrated that the waist girth was more closely related to the visceral fat area in nonobese compared with obese subjects. It is very likely, therefore, that the range of fatness in subjects studied greatly influences the magnitude of the correlations and perhaps also the comparison between the sagittal diameter and the waist circumference with regard to their utility in predicting intraabdominal fat. In addition, the distinction between studies that used only visceral fat area and those that calculated visceral fat volume from multiple scans may be important to make (58). Ross et al. (59) showed that correlations of the waist with the visceral fat area (r = 0.65) were weaker than those with the visceral fat volume (r = 0.78) in obese women.

A study from the Canadian group (38) conducted in a large group of males and females evaluated systematically the three anthropometric indexes and their association with abdominal visceral adipose and subcutaneous areas measured by CT (between the fourth and fifth lumbar vertebrae) and metabolic profile. As seen in Table 1, there was a strong association between waist girth and body fat mass, the slope of the regression line being steeper in women (data not shown). With relation to the abdominal visceral fat area, for a given waist circumference, men and women had similar levels and the slopes of the regression lines were not different between genders. Essentially similar results were observed with the abdominal sagittal diameter. However, in contrast with waist circumference, the slopes of regression of abdominal sagittal diameter to abdominal visceral fat area were significantly different between genders and were steeper in men (data not shown). Finally, it can be seen that the WHR was less strongly correlated with total body fat mass and abdominal visceral and subcutaneous areas than the other indexes. This study demonstrated that most of the variance in waist girth and abdominal sagittal diameter can be explained by variations in body fat mass and in abdominal visceral and subcutaneous adipose tissue areas (0.85 R² 0.95), whereas a lower proportion of the variance in the WHR could be explained by these adipose variables (R² = 0.46 and 0.60 in men and women, respectively). With relation to the metabolic variables related to cardiovascular risk (plasma triglycerides and high-density lipoprotein cholesterol levels, fasting and postglucose glucose and insulin levels), in women, the waist circumference and the abdominal sagittal diameter were more closely related to the metabolic variables than the WHR, whereas such differences were not apparent in men. They concluded that waist circumference values above approximately 100 cm, abdominal sagittal diameter values greater than 25 cm, and WHR values greater than 0.8 in women and 1.00 in men were likely to be associated with disturbances in lipoprotein metabolism and plasma insulin-glucose homeostasis, suggesting that the waist girth or the abdominal sagittal diameter, rather than the WHR, should be used as indexes of abdominal visceral adipose tissue deposition for the assessment of cardiovascular risk.

B. Imaging techniques
1. Computed tomography (CT). CT can be considered the gold standard not only for adipose tissue evaluation but also for multicompartment body measurement (61, 43). The reported error for the determination of total adipose tissue volume after performing 28 scans is 0.4%, which supports the high reproducibility of CT. The subcompartments of adipose tissue volume, visceral and subcutaneous adipose tissue, can be accurately measured with errors of 1.2 and 0.5%, respectively. Since the visceral fat volume has been determined from the visceral adipose tissue area of several scans, it is independent of individual visceral adipose tissue distributions; the precision error of this volume determination was reported to be in the order of 1% by Chowdhury et al. (61). In eight nonobese Swedish males evaluated by the multiscan CT technique, the volume of visceral abdominal adipose tissue in the intraperitoneal and retroperitoneal compartments was found to be 1.96 ± 1.13 and 0.78 ± 0.51 L (mean ± SD), respectively (62). It was found that the sagittal diameter (L4-L5) was a more specific predictor of visceral adipose tissue volume than waist circumference and WHR; the estimates based on sagittal diameter had errors in the order of 20%, while those based on visceral fat had only slightly lower errors (10–14%) (43). Using a multislice magnetic resonance protocol, Abate et al. (30) and Ross et al. (59, 63) found values for visceral adipose tissue and its subcompartments, particularly retroperitoneal fat, within the range found by Sjöstrom (62). In effect, in 13 lean males, Abate et al. (30) found 1.1 ± 0.5 and 0.8 ± 0.3 kg for intraperitoneal and retroperitoneal fat, respectively.

If only one scan is used to measure the visceral adipose tissue area, a strictly defined longitudinal level is very important since the average visceral adipose tissue area shifts if there is a change in position, even of a few centimeters. This, according to Sjöström et al. (43), implies that examinations at the so called umbilical level, as performed by many investigators, are not sufficiently exact since the umbilicus of obese subjects may be located at a lower position. Instead, the longitudinal level must be defined in a strict relation to the skeleton, usually between the L4 and L5 vertebrae. Kvist et al. (55) have found that visceral fat areas from a single scan in the L4-L5 region are highly correlated to the total visceral fat volume in both sexes.

To determine the visceral intraabdominal and subcutaneous abdominal areas, a simple CT (or MR) scan is taken either at the level of L4-L5 or the umbilicus, with an attenuation range of -30 to -190 Hounsfield units (64, 65). The subjects are examined in a supine position with their arms stretched above their heads. The choice to perform the scan at the level of the umbilicus was initially proposed by Borkan et al. (66), who found that at the level of the umbilicus there is the highest percentage of body fat, and it best allows differentiation of subcutaneous from intraabdominal fat. Subsequently, Tokunaga et al. (67) also suggested taking the measurement at the umbilicus. In addition to the recommendations of the Japanese investigators, studies from Korea (20) and from our clinic use the scan at the umbilicus.

Visceral fat is defined as intraabdominal fat bound by parietal peritoneum or transversalis fascia, excluding the vertebral column and the paraspinal muscles; subcutaneous fat is fat superficial to the abdominal and back muscles. Subcutaneous fat area is calculated by subtracting the intraabdominal fat area from the total fat area. The ratio of intraabdominal visceral fat (V) to the sc fat area (S)— V/S—as a relative index of intraabdominal fat accumulation, was shown to be strongly related with disorders of glucose and lipid metabolism in obese subjects, these metabolic parameters being significantly higher in the so-called visceral group (with a V/S ratio of 0.4) than in the subcutaneous group (with a V/S ratio of < 0.4) (17). The same authors (17) have found that glucose and lipid metabolism in the visceral group was disordered independent of sex, age, and BMI, with males having a higher V/S ratio than females and the individuals with high V/S ratios tending to be older than those with lower V/S ratios. In addition, visceral fat increases with age (68). Figure 1 shows cross-sectional abdominal areas obtained by CT at the level of the umbilicus in two women matched for the same BMI, who differed markedly in the accumulation of fat in the abdominal cavity but less so in the subcutaneous abdominal fat.

View larger version (66K): [in this window] [in a new window]   Figure 1. Computed tomography showing cross-sectional abdominal areas at umbilicus level in two patients demonstrating variation in fat distribution. A, Visceral type (49-yr-old female, 23.1 of BMI, visceral fat area: 146 cm2; subcutaneous fat area, 115 cm2; V/S ratio, 1.27). B, Subcutaneous type (40-yr-old female, 24.0 of BMI, visceral fat area: 60 cm2; subcutaneous fat area, 190 cm2; V/S ratio, 0.31).

Regarding the reproducibility of CT measurement of visceral adipose tissue area, Thaete et al. (72) evaluated duplicate cross-sectional CT scans of the abdomen at the L4 level in 16 healthy premenopausal women, who ranged from lean to obese. The duplication occurred after the initial scan; the subjects were repositioned before repeat scanning. Excellent reproducibility was shown by a high correlation between duplicate measurements (r = 0.99) and by small precision errors: 1.2% of the mean value for total adipose tissue cross-sectional area, 1.9% for subcutaneous adipose tissue area, and 3.9% for visceral adipose area.

As indicated in the Introduction, individuals with a high accumulation of visceral abdominal fat, as shown by CT scans, had an increased risk for development of type 2 diabetes, dyslipidemia, and coronary heart disease. Table 2 shows the thresholds above which metabolic complications would be more likely to be observed in visceral adipose tissue areas. Desprès and Lamarche (73), Hunter et al. (74), and Williams et al. (75) studied men and/or women with a wide range of body weight. They found that a value above 110 cm² was associated with an increased risk of coronary heart disease in pre and postmenopausal women (75); the same group (74) found that males with abdominal visceral fat cross-section areas measuring more than 131 cm² were clearly at an increased risk for coronary disease. On the other hand, Desprès and Lamarche (73) found that in both men and women a value of 100 cm² was associated with significant alterations in cardiovascular disease risk profile and that a further deterioration of the metabolic profile was observed when values greater than 130 cm² of visceral adipose tissue were reached. From the same center, Lemieux et al. (76) determined in a sample of 213 men and 190 women the threshold values of the anthropometric parameters corresponding to an accumulation of visceral adipose tissue of 130 cm²: a waist girth of approximately 95 cm in both sexes, sagittal diameters of 22.8 cm in men and 25.2 cm in women, and WHR values of 0.94 in men and 0.88 in women. In both sexes, threshold values of those anthropometric indexes were generally lower in subjects who were 40 yr old than in younger individuals. It was concluded that waist circumference was a more convenient anthropometric correlate to visceral adipose tissue because its threshold values did not appear to be influenced by sex or by the degree of obesity. Anderson et al. (77) examined the relationship between visceral abdominal fat area by MRI and cardiovascular risk factors in Chinese type 2 diabetes and found a threshold value of 132 cm².

The most extensive studies using a single CT scan at umbilical level was done by Matsuzawa and colleagues (17, 78). As indicated above, the data were expressed by the ratio of visceral fat area (V)/subcutaneous fat area (S), the cut-off point for the risk factors for cardiovascular disease, particularly those related to glucose and lipid metabolism and hypertension, being > 0.4. However, they did not present the raw data on visceral and subcutaneous areas but only their ratios, thus precluding their inclusion in Table 2. In a study of fat distribution in 29 nonobese coronary heart disease patients in comparison with 21 nonobese controls in which the data on abdominal visceral and subcutaneous abdominal fat areas were available, 34% of the patients had visceral areas above the maximal level found in the controls (132 cm²), there being a significant difference (P < 0.01) between the two groups while no differences were found in the subcutaneous abdominal areas (78). In another study, performed in Japan by Saito et al. (79) in nonobese and obese males and females, fat areas at the umbilicus level as determined by CT had threshold values 100 for men and 90 for women and the V/S was also > 0.4. Lottenberg et al. (80) indicated a threshold value of 107 cm² for abdominal visceral fat area in a group of obese individuals.

2. Magnetic resonance imaging (MRI). MRI provided results similar to CT without exposure to ionizing radiation, the main problem with CT multislice measurements. It demonstrated good reproducibility for total and visceral adipose tissue volumes (63), which were slightly lower than previously reported using CT (55), although the percent contribution of visceral to total adipose tissue volume was similar (18 vs. 20%). Subcutaneous adipose tissue and visceral fat areas at the L4-L5 level determined in 27 healthy men by MRI were 252.8 ± 132.9 and 117.9 ± 62.1 cm² (mean ± SD), respectively, and the differences between two measurements for a single scan ranged from 1.4 to 4.2% (63). These areas were highly predictive of the corresponding volume measurements computed from the 41-scan MRI, confirming the CT studies of Kvist et al. (55), who made similar observations in both male and female subjects.

Two studies have compared estimates of subcutaneous and visceral adipose tissue by CT and MRI. Comparison between MRI and CT in seven subjects showed a high degree of agreement in measurement of total subcutaneous adipose tissue area but not visceral adipose tissue area (81). Moreover, it has been shown (82) that MRI when compared with CT overestimates subcutaneous adipose tissue (+8%) and visceral adipose tissue (+22%). As already mentioned, MRI has been validated in three cadavers, confirming its accuracy (35).

3. Ultrasound (US). US subcutaneous and intraabdominal thicknesses, the latter corresponding to the distance between abdominal muscle and aorta, were measured 5 cm from the umbilicus on the xipho-umbilical line with a 7.5-MHz probe for subcutaneous adipose tissue and a 3.5-MHz probe for intraabdominal fat (71). The intraindividual reproducibility of US measurements was very high both for intraabdominal and subcutaneous thickness as well as for interoperators (83, 84).

Several studies demonstrated a highly significant correlation between the intraabdominal adipose tissue determined by CT and by US. A decade ago, Armellini et al. (85) found a reasonable correlation (r = 0.67) of intraabdominal US measurements with CT at the L4-L5 level. In a more recent study, Tornaghi et al. (84) found a highly significant correlation between intraabdominal thickness and CT visceral adipose tissue area (r = 0.89–0.91) and Radominski (86), at The Hospital das Clinicas of São Paulo, Brazil, also observed in 24 subjects an excellent correlation between ultrasonography and CT (r = 0.79 for sc abdominal thickness and r = 0.84 for visceral adipose tissue), again indicating that US intraabdominal thickness is an excellent predictor of visceral abdominal adipose tissue (83, 87, 88). Furthermore, the correlation between abdominal sagittal diameter and CT in Radominski’s study was lower than that observed between US and CT. In a cross-validation study, intraabdominal adipose tissue measured by CT was significantly correlated with intraabdominal adipose tissue predicted from an equation using primarily US intraabdominal thickness (r = 0.84) (88).

In a study of 191 men (C. C. Leite, D. Matsuda, B. L. Wajchenberg, G. G. Cerri, and A. Halpern, unpublished data), in which 53.5% presented some of the risk factors for cardiovascular disease, it was shown that intraabdominal thickness by US measurements was a better predictor of cardiovascular risk [Odds ratio (OR) = 2.27 (95% CI = 1.05–4.8)] than the anthropometric measurements (waist circumference [OR=1.53 (95% CI=0.47–5.0)] and sagittal diameter [OR = 0.8 (95% CI = 0.3–1.9)]). The cut-off points for moderate risk (2 or more of the following: total serum cholesterol > 200 and < 240 mg/dl; triglycerides > 200 mg/dl + HDL cholesterol > 35 and < 45 mg/dl; systolic > 140 and diastolic blood pressure > 90 mm Hg) and high risk (2 or more of the factors: cholesterol > 240 mg/dl; HDL cholesterol < 35 mg/dl; triglycerides > 200 + HDL < 35 mg/dl; plasma glucose > 126 mg/dl and systolic > 140 and diastolic blood pressure > 90 mm Hg) were 7 cm (sensitivity of 72% and specificity of 53%) and 9 cm (53% sensitivity and 83% specificity), respectively.

In obese women, after a 6-kg weight loss, a significant decrease was found in intraabdominal fat but not in subcutaneous adipose tissue, as determined by both CT and US (87). There was also a significant correlation between changes in intraabdominal adipose tissue using both techniques, indicating that US can be used in the evaluation of body fat distribution modifications during weight loss. Further, by subdividing a group of 119 obese women with a wide range of BMIs into tertiles of intraabdominal adipose tissue as evaluated by CT (<114, 114–170, >170 cm²), Armellini et al. (83) observed that intraabdominal US measurements were significantly different in the intraabdominal CT tertiles (16 ± 10, 32 ± 13, 50 ± 22 mm, respectively; P < 0.001) while neither sagittal diameter nor WHR was able to distinguish between the two >114 cm² groups. This is another confirmation of the reliability of the US intraabdominal determinations.

	IV. Correlations of Abdominal Visceral Fat

Top Abstract I. Introduction II. Classification of Abdominal... III. Assessment of Abdominal... IV. Correlations of Abdominal... V. Adipose Tissue as... VI. Body Fat Distribution... VII. Pathology of the... VIII. Endocrinological... IX. Summary References

 
A. Age and gender
The amount of visceral fat increases with age in both genders, and this increase is present in normal weight (BMI, 18.5 to 24.9) as well as in overweight (BMI, 25 to 29.9) and obese subjects (BMI > 30 kg/m²) but more so in men than in women (7, 68, 90). In a study of 130 subjects (62 males and 68 females with a wide range of age and weight), Enzi et al. (68) found that in young females, either lean or obese, the subcutaneous abdominal fat area was predominant over abdominal visceral fat, both measured by CT at the upper renal pole. This fat topography was retained in young and middle-aged females up to about 60 yr of age, at which point there was a change to an android type of fat distribution. This age-related redistribution of fat is due to an absolute as well as relative increment in visceral fat depots, particularly in obese women, which could be related to an increase in androgenic activity in postmenopausal subjects. On the other hand, they showed that males at any age tend to accumulate fat at the visceral depot, increasing with age and BMI increase. In the male, a close linear correlation between age and visceral fat volume was shown, suggesting that visceral fat increased continuously with age (78). Although this correlation was also present in women, the slope was very gentle in the premenopausal condition. It became steeper in postmenopausal subjects, almost the same as in males (78). Further, Enzi et al. (68) found that 7.3% of the females in their study had an android type of body fat topography and 6.5% of the males had a gynoid type of fat distribution.

From the published data (68, 90), it can be concluded that both subcutaneous and visceral abdominal fat increase with increasing weight in both sexes but while abdominal subcutaneous adipose tissue decreases after the age of 50 yr in obese men, it increases in women up to the age of 60–70 yr, at which point it starts to decline (71). Fowler et al. (91) evaluated total and subcutaneous adipose tissue with a multislice MRI in obese and lean women and found that the main difference between them was in the percentage of subcutaneous fat in the abdomen, less than in the thighs in lean, while no significant differences between both sites of subcutaneous adipose tissue were observed in obese individuals. Finally, as previously indicated, visceral fat is more sensitive to weight reduction than subcutaneous adipose tissue because omental and mesenteric adipocytes, the major components of visceral abdominal fat, have been shown to be more metabolically active and sensitive to lipolysis (92).

Lemieux et al. (93) have indicated that the gender difference in visceral adipose tissue accumulation was an important factor in explaining the gender differences in cardiovascular risk profile. In addition, the adjustment for differences in visceral fat between men and women eliminated most of the sex differences in cardiovascular risk factors. There is evidence supporting the notion that abdominal visceral fat accumulation is an important correlate of the features of the insulin-resistant syndrome (23, 24, 29) but this should not be interpreted as supporting the notion of a cause and effect relationship between these variables (27). This subject will be discussed later on.

B. Total body fat
The correlations of abdominal visceral fat mass evaluated by CT or MRI scans with total body fat range from 0.4 to 0.8, with higher values obtained when a large range of fatness, from lean to obese, is present in the population (40, 42, 44, 45, 64). They tend to be lower in the lean and normal weight subjects than in the obese (44). As indicated by Bouchard et al. (7) it is important to recognize that individual differences in abdominal visceral fat remains considerable even when subjects with relatively similar BMI and percent body fat are investigated. When they examined the relationship of total body fat mass to visceral adipose tissue accumulation in men and in premenopausal women, Lemieux et al. (94) reported that for a given amount of total body fat men had about twice the amount of visceral adipose tissue than what is found in premenopausal women. Furthermore, the relationship of visceral adipose tissue to metabolic complications was found to be independent of concomitant variation in total body fat, and it was concluded that the assessment of cardiovascular risk in obese patients solely from the measurement of body weight or of total body fatness may be completely misleading (19, 22, 36, 95). Indeed, it appears that only the subgroup of obese individuals characterized by a high accumulation of visceral adipose fat show the complications predictive of type 2 diabetes and cardiovascular disease (27). On the other hand, after adjustment for total body fat, Abate et al. (30) have shown that intraperitoneal adipose tissue lost its significant correlation to the parameters of insulin resistance during an hyperinsulinemic euglycemic clamp, failing to demonstrate that intraperitoneal (visceral) fat uniquely enhances insulin resistance and thus the associated risks.

C. Energy balance
Intraabdominal visceral fat is associated with an increase in energy intake but this is not an absolute requirement. Positive energy balance is a strong determinant of truncal-abdominal fat as shown by Bouchard and colleagues (96) in overfeeding experiments in identical twins. The correlations between gains in body weight or total fat mass with those in subcutaneous fat on the trunk reached about 0.7 in their 100-day overfeeding study in 12 pairs of male identical twins. In contrast, these correlations attained only 0.3 with the gains in abdominal visceral fat, corresponding to a common variance of less than 10% (7, 96). Thus, positive energy balance does not appear to be a strong determinant of abdominal visceral fat as is the case with other body fat phenotypes (7). In effect, as discussed in the CT section of imaging techniques for evaluation of intraabdominal visceral fat, some investigators (70, 71) have shown that either when the subjects lose or increase their weight, particularly females, visceral fat is lost or gained, respectively, less than subcutaneous fat at the abdominal level. However, at variance from these data, Zamboni et al. (97) have shown in premenopausal women that after weight loss, on a very low energy intake from 2 weeks to 3 months, visceral fat decreased more than subcutaneous fat, confirming (according to these authors) that visceral fat is more sensitive to weight reduction because omental and mesenteric adipocytes have been shown to be more metabolically active and sensitive to lipolysis (92). Similarly, as already mentioned, Smith and Zachwieja (32) noted that all forms of weight loss affect visceral fat more than subcutaneous fat (percentage wise), and there was a gender difference, with men appearing to lose more visceral fat than women for any given weight loss. This subject will be discussed later on.

D. Adipose tissue LPL activity
LPL activity, being related to the liberation of the lipolytic products [from chylomicra and very-low-density lipoproteins (VLDL)] to the adipocytes for deposit as triglycerides, is a key regulator of fat accumulation in various adipose areas, since human adipose tissue derives most of its lipid for storage from circulating triglycerides. However, adipocytes can synthesize lipid de novo if the need arises, as in patients with LPL deficiency (98).

It was demonstrated in men with a wide variation of body fat that the uptake of labeled triglycerides was higher in omental than in subcutaneous abdominal adipose tissue, amounting to as much as approximately 50% more in omental than in abdominal subcutaneous depots; however, this did not correlate with LPL activity in the tissue (21). This suggests that other factors may be as important as LPL for the regulation of triglyceride uptake in vivo in adipose tissue (21), such as the "acylation stimulating protein" (ASP), a strong stimulator of FFA reesterification and triglyceride synthesis in human adipose tissue that has an insulin-like effect and thus possibly plays a role in initiating and maintaining the obese state. According to Sniderman et al. (99), as fatty acids are being liberated from triglycerides as a result of LPL, ASP is also generated to ensure that the rate of triglyceride synthesis within adipocytes is sufficiently rapid that fatty acids in the microcirculation will not increase unduly, allowing the rapid hydrolysis of triglyceride-rich lipoproteins to continue and, consequently, rapid triglyceride clearance to occur. The increase of visceral fat masses with increasing total body fat was explained by an increase of fat cell size only up to a certain adipocyte weight. However, with further enlargement of intraabdominal fat masses with severe obesity, the number of adipocytes seems to be elevated (100, 101). In women, but not in men, omental adipose tissue has smaller adipocytes and lower LPL activity than subcutaneous fat depots since variations in LPL activity parallel differences in fat cell size (7). When adipocytes enlarge in relation to a gain in body weight, the activity of LPL increases in parallel, possibly as a consequence of obesity-related hyperinsulinism. The higher basal activity of adipose tissue LPL in obesity is accompanied by a lower increment after acute hyperinsulinemia (102). Lipid accumulation is favored in the femoral region of premenopausal women in comparison with men (103). In the latter, LPL activity as well as the LPL mRNA levels were greater in the abdominal than in gluteal fat cells, while the opposite was observed in women, suggesting that regional variation of gene expression and posttranslational modification of LPL could potentially account for the differences between genders in fat distribution (103). With progressive obesity, adipose tissue LPL is increased in the depots of fat in parallel with serum insulin. However, when obese subjects lost weight and became less hyperinsulinemic, adipose LPL increased further and the patients who were most obese showed the largest increase in LPL, suggesting that very obese patients are most likely to have abnormal LPL regulation, independent of the influence of insulin. This probably indicates that adipose tissue LPL activity may represent an adipocyte "set point" that is intended to limit adipocyte shrinkage induced by a hypocaloric diet (98). In response to feeding, the increase in LPL is, as indicated, due to posttranslational changes in the LPL enzyme. However, the increased LPL after weight loss involved an increase in LPL mRNA levels, followed by parallel increases in LPL protein and activity (104). Because the response to weight loss occurred via a different cellular mechanism, it is probably controlled by factors different from the day-to-day regulatory forces. In addition, because the very obese patients demonstrated a larger increase in LPL with weight loss than the less obese patients, these data suggest a genetic regulation of LPL that is most operative in the very obese (98). The role of sex steroids, glucocorticoids, and catecholamines in the regulation of adipose tissue LPL activity in various fat depots will be discussed in the section on hormonal regulation of abdominal visceral fat.

E. Adipose tissue lipolytic activity
Lipid mobilization and the release of FFA and glycerol are modulated by the sympathetic nervous system. Catecholamines are the most potent regulators of lipolysis in human adipocytes through stimulatory ß_l- and ß₂-adrenoreceptors or inhibitory 2-adrenoreceptors (105). A gene that codes for a third stimulatory ß-adrenoreceptor, ß₃-adrenoreceptor, is functionally active principally in omental adipocytes (106) but also present in mammary fat and subcutaneous fat in vivo (107). The main systems involved in the inhibitory control of lipolysis are insulin/insulin receptor and adenosine/adenosine receptor (102).

Regional differences in catecholamine-induced lipolysis and sensitivity to insulin’s antilipolytic effects have been extensively described in in vitro studies. In both genders and independently of the degree of obesity, femoral and gluteal fat cells exhibit a lower lipolytic response to catecholamines than subcutaneous abdominal adipocytes, the latter showing both increased ß_l- and ß₂-adrenoreceptor density and sensitivity and reduced 2-adrenoreceptor affinity and number (Refs. 7, 102 ; Table 3). Abdominal visceral adipocytes, compared with subcutaneous abdominal or femoral adipose cells, are more sensitive to catecholamine-induced lipolysis, equally (or slightly less) sensitive to both 2- and adenosine receptor-dependent inhibition of lipolysis, and less sensitive to insulin’s antilipolytic effects. The increased sensitivity to catecholamine-induced lipolysis in omental fat in nonobese individuals is paralleled by an increase in the amount of ß_l- and ß₂-receptors, with normal receptor affinity and normal lipolytic action of agonists acting at postadrenoreceptor steps in the lipolytic cascade (108, 109); this is associated with enhanced ß₃-adrenoreceptor sensitivity, which usually reflect changes in receptor number in comparison with subcutaneous adipocytes (110, 111).

An increased ß₃-adrenoreceptor sensitivity to catecholamine stimulation may lead to an increased delivery of FFA into the portal venous system, with several possible effects on liver metabolism. These include glucose production, VLDL secretion, and interference with hepatic clearance of insulin (115), resulting in dyslipoproteinemia, glucose intolerance, and hyperisulinemia.

Lönnqvist et al. (116) investigated sex differences in visceral fat mobilization in obese males and females matched for BMI and age who were undergoing elective surgery. They observed that males had a higher fat cell volume with no sex differences in the lipolytic sensitivity to ß_l- and ß₂-adrenoreceptor-specific agonists or in the antilipolytic effect of insulin. However, the lipolytic ß₃-adrenoreceptor sensitivity was 12 times higher in men, and the antilipolytic 2-adrenoreceptor sensitivity was 17 times lower in men. It was concluded that in obesity, the catecholamine-induced rate of FFA mobilization from visceral fat to the portal venous system is higher in men than women. This phenomenon is partly due to a larger fat cell volume, a decrease in the function of 2-adrenoceptors, and an increase in the function of ß₃-adrenoreceptors. These factors may contribute to gender-specific differences observed in the metabolic disturbances accompanied by obesity, i.e., males have higher abdominal sagittal diameter, blood pressure, plasma insulin, glucose, and triglyceride, and lower HDL cholesterol than females.

F. Adipose tissue receptors
1. Glucocorticoid receptors. Glucocorticoid receptors, one of the most important receptors for human adipose tissue function, are involved in metabolic regulation and distribution of body fat under normal as well as pathophysiological conditions. Glucocorticoid receptors in adipose tissue show a regional variation in density with elevated concentrations in visceral adipose tissue (117). With exposure to high concentrations of cortisol, such as in Cushing’s syndrome, the density of glucocorticoid receptors is down-regulated; however, the differences in glucocorticoid receptor density between adipose tissues remain proportionally similar, but on a lower level, with visceral glucocorticoid receptor density remaining higher than subcutaneous adipose tissue (34). In spite of the lower receptor density, the elevated cortisol secretion results in clearly increased net effects of cortisol.

2. Androgen and estrogen receptors. Adipocytes have specific receptors for androgens, with a higher density in visceral fat cells than in adipocytes isolated from subcutaneous fat. Unlike most hormones, testosterone induces an increase in the number of androgen receptors after exposure to fat cells (118), thereby affecting lipid mobilization. This is more apparent in visceral fat (omental, mesenteric, and retroperitoneal) because of higher density of adipocytes and androgen receptors, in addition to other factors (34). However, at variance with the effects of testosterone, dihydrotestosterone treatment does not influence lipid mobilization (118). In females, there is an association between visceral fat accumulation and hyperandrogenicity, despite the documented effects of testosterone on lipid mobilization and the expected decrease in visceral fat depots. The observation that visceral fat accumulation occurs only in female-to-male transsexuals after oophorectomy (119) suggests that the remaining estrogen production before oophorectomy was protective (120). The androgen receptor in female adipose tissue seems to have the same characteristics as that found in male adipose tissue. However, estrogen treatment down-regulates the density of this receptor, which might be a mechanism whereby estrogen protects adipose tissue from androgen effects. Estrogen by itself seems to protect postmenopausal women receiving replacement therapy from visceral fat accumulation (121). Estrogen receptors are expressed in human adipose tissue (122) and show a regional variation of density, but whether the quantity of these receptors is of physiological importance has not been clearly established (34).

With regard to progesterone, adipose cells seem to lack binding sites and mRNA for progesterone receptors, indicating that progesterone acts through glucocorticoid receptors (123).

3. GH receptors. While it is well established that GH has specific and receptor-mediated effects in adipose tissue of experimental animals, the importance of GH receptors in human adipose tissue is not fully elucidated at present although the available data indicate a functional role. However, GH is clearly involved in the regulation of visceral fat mass in humans. Acromegaly, a state of GH excess, is associated with decreased visceral fat while in GH deficiency there is an increase in visceral fat and in adults with GH deficiency, recombinant human GH replacement therapy results in adipose tissue redistribution from visceral to subcutaneous locations; however, the regulation of adipose tissue metabolism requires synergism with steroid hormones (34). A direct demonstration of a regulation of the GH receptor in human fat cells has not yet been performed (124).

4. Thyroid hormone receptors. Thyroid hormones have multiple catabolic effects on fat cells as a result of interactions with the adrenergic receptor signal transduction system, and most of these interactions are also present in human fat cells (125). There are data regarding the characterization of the nuclear T₃ receptor in human fat cells (126). Although receptor regulation has not yet been demonstrated, there is little doubt that the thyroid hormone receptors are important for the function of human adipose tissue (125). Further, no data are available on the correlation between visceral fat mass and thyroid hormone levels.

5. Adenosine receptors. Adenosine behaves as a potent antilipolytic and vasodilator agent and can be considered as an autocrine regulator of both lipolysis and insulin sensitivity in human adipose tissue. Site differences in ambient adenosine concentration, perhaps controlled by blood flow, may also modulate adipose tissue metabolism (7). Adenosine content is higher in omental than in abdominal subcutaneous adipose tissue, but the receptor-dependent inhibition of lipolysis is, as indicated before (102), less pronounced in the former than in the latter depot (127). However, despite strong antilipolytic effect of adenosine analogs, human adipocytes contain few adenosine type A_l receptors, regardless of the fat depot considered (128).

According to Arner (124), the 2-, ß_l -, ß₂-, and ß₃-adrenoreceptors and receptors for insulin, adenosine, and glucocorticoids, as well as for PGE₂, a potent antilipolytic agent with high affinity receptors identified in adipocytes (129), have a major functional role, as shown by relevant biological receptor-mediated effects, the presence of a receptor molecule, and receptor regulation. The receptors for GH, thyroid hormones, estrogen, and testosterone, as well as for acetylcholine and TSH, probably have an important functional role but complete evidence, indicated in the previous group of receptors, is not present so far; however, there is little doubt of a regulatory role.

G. Genetic characteristics
1. Genetic epidemiology: heritability and segregation analysis. Studies performed in individuals from families of French descent living in Quebec City [Quebec Family Study (QFS)] allowed the estimation of the fraction of the phenotypic variance that could be attributed to the genetic and environmental factors among the obesity phenotypes or in the distribution of the adipose tissue, taking into account the BMI and amount of subcutaneous fat (by the sum of the measurement of skinfolds in six different sites), lean body mass, fat mass, percentage of fat derived from underwater weighing, and visceral fat by CT (130, 131). While the genetic effect was no more than 5% for the BMI and subcutaneous fat, it was 25% for the fat mass and percentage of fat and 56% for the visceral fat area after adjustment for age, sex, and total fat mass (130). A similar level of heritability (48%) was observed by the same group of investigators in another family study (HERITAGE Family Study) (132). The residual variance corresponded to environmental factors, but some factors (cultural, nongenetic) could be transmitted from parents to descendents and sometimes were confounded by genetic effects (131).

Segregation analysis studies have recently concluded that visceral fat is similarly influenced by a gene with a major effect in the QFS and HERITAGE families (133, 134). In the QFS families, the gene with the major effect, with an autosomic recessive transmission, corresponded to 51% of the variance for the visceral fat and the polygenic effects contributing to 21% of the variance. However, after adjustment of the visceral adipose tissue for the fat mass, the effect of the gene with the major effect was not more compatible with a mendelian transmission. These results suggested the presence of a pleiotropism: the gene with the major effect, identified by the fat mass (135), could similarly influence the amount of visceral fat (133). Similar results were obtained with the same type of analysis in the HERITAGE cohort (134).

To test the hypothesis of a genetic pleiotropism, Rice et al. (136) tried to determine whether, in addition to the genetic effects specific for the fat mass and visceral fat, there would be common genetic effects for the two phenotypes. The results of this study (Fig. 2) indicate that fat mass and visceral fat are influenced by genetic (G₁ and G₂) and environmental effects (E₁ and E₂), which are unique to them and the heritability of the two phenotypes is approximately 25 and 55%, respectively. Figure 2 also indicates that the common variance for the two phenotypes within the QFS cohort is about 43%. This phenotypic covariation is characterized by familial resemblances and the existence of common genetic factors for the two phenotypes (G₃) explaining their 30% covariation. These results have confirmed the presence of a genetic pleiomorphism and suggested the presence of genes affecting simultaneously the amounts of fat mass and visceral abdominal fat.

View larger version (16K): [in this window] [in a new window]   Figure 2. Schematic representation of the genetic effects on total fat mass and visceral fat (adjusted for the fat mass) and on the co-variation between the two phenotypes (Quebec Family Study, 1996). G1 and G2 represent the genetic effects specific for the total fat mass and visceral fat, respectively. E1 and E2 represent the specific effects of the environment on total fat mass and visceral fat, respectively. G3 and E3 indicate the genetic and environment effects common to both phenotypes. [Reprinted with permission from L. Pérusse et al.: Médecine/Sciences 14:914–924, 1998 (131 ).]

2. Molecular genetics: association and linkage studies. Several candidate genes as well as random genetic markers were found to be associated with obesity as well as body fat and fat distribution in humans. The current human obesity gene map, based on results from animal and human studies, indicates that all chromosomes, with the exception of the Y chromosome, include genes or loci potentially involved in the etiology of obesity (138). Initial findings from the QFS showed that significant but marginal associations with body fat were found with LPL (139) and the 2-subunit of the sodium-potassium ATPase genes (140). The Trp64Arg mutation of the ß₃-adrenergic receptor gene (ß₃ AR), prevalent in some ethnic groups, is associated with visceral obesity and insulin resistance in Finns (141) as well as increased capacity to gain weight (142). This mutation was also shown to be associated with abdominal visceral obesity in Japanese subjects, with lower triglycerides in the Trp64Arg homozygotes but not heterozygotes (143). It has been suggested that those with the mutation may describe a subset of subjects characterized by decreased lipolysis in visceral adipose tissue. On the other hand, Vohl et al. (144) found that triglyceride levels were positively correlated with visceral obesity and hyperinsulinemia only in the subjects homozygous for the presence of the LPL HindIII polymorphism. Previously, it was reported by the same group that apo-B-100 gene EcoR-1 polymorphism appeared to modulate the magnitude of the dyslipidemia generally found in the insulin-resistant state linked with visceral obesity (145). These studies are a demonstration of a significant interaction between visceral obesity and a polymorphism for a gene playing an important role in lipoprotein metabolism.

When the genes related to the hormonal regulation of body fat distribution studied in the QFS families (sex hormone-binding globulin, 3ß-hydroxysteroid dehydrogenase, and glucocorticoid receptor genes) were considered along with the knowledge that body fat distribution is influenced by nonpathological variations in the responsiveness to cortisol, it was shown that the less frequent 4.5-kb allele detected with the BclI restriction enzyme at the glucocorticoid receptor gene locus was associated with higher abdominal visceral fat area independently of total body fat mass. However, the association with abdominal visceral fat area was seen only in subjects of the lower tertile of the percent body fat level. In these subjects, the polymorphism was found to account for 41% and 35%, in men and women, respectively, of the total variance in abdominal visceral fat area. The consistent association between the glucocorticoid receptor polymorphism detected with BclI and abdominal visceral fat area suggested that this gene or a locus in linkage disequilibrium with the BclI restriction site may contribute to the accumulation of abdominal visceral adipose tissue (146).

With respect to the linkage studies, only a few studies of body fat or fat distribution with random genetic markers or candidate genes have been reported using the sibling-pair linkage method. One of the few reported studies relative to the visceral fat mass was the evaluation of a 122 sib-pair linkage analysis from the QFS between five microsatellite markers encompassing about 20 cM in the Mob-1 region of the human chromosome 16p12-p11.2 (the corresponding region in mice is significantly linked to body fat and blood lipids) (147). This study showed evidence of linkage between the marker D16S287, serum cholesterol and its fractions, and visceral fat (P = 0.01), while another marker (D16S401) located about 19 cM further centromeric also exhibited good evidence of linkage to abdominal visceral fat (P = 0.007). These results suggested to the authors that this region of the human genome contains a locus affecting the amount of visceral fat and lipid metabolism as also shown by the association studies indicated above. The other population and intrafamily association study used a polymorphic marker (LIPE) in the hormone-sensitive lipase gene, located on chromosome 19q13.1–13.2. This study suggests that the LIPE marker is in linkage disequilibrium with an allele and/or gene that increases susceptibility to abdominal obesity and thereby possibly to type 2 diabetes (148).

In conclusion, despite the fact that the genetic architecture of obesity has just begun, the results obtained so far suggest that a great number of genes, loci, or chromosomal regions distributed on different chromosomes could play a role in determining body fat and fat distribution in humans. This reflects the complex and heterogeneous nature of obesity. The accumulation of adipose tissue in the abdominal region is at least partially influenced by genes, which becomes more evident as the number of involved genes are identified.

	V. Adipose Tissue as an Endocrine Gland

Top Abstract I. Introduction II. Classification of Abdominal... III. Assessment of Abdominal... IV. Correlations of Abdominal... V. Adipose Tissue as... VI. Body Fat Distribution... VII. Pathology of the... VIII. Endocrinological... IX. Summary References

 
The concept that adipocytes are secretory cells has emerged over the past few years. Adipocytes synthesize and release a variety of peptide and nonpeptide compounds; they also express other factors, in addition to their ability to store and mobilize triglycerides, retinoids, and cholesterol. These properties allow a cross-talk of adipose tissue with other organs as well as within the adipose tissue. The important finding that adipocytes secrete leptin as the product of the ob gene has established adipose tissue as an endocrine organ that communicates with the central nervous system.

A. Secreted proteins and triglyceride metabolism
1. LPL. As already mentioned, LPL is the key regulator of fat cell triglyceride deposition from circulating triglycerides. LPL is found, after transcytosis, associated with the glycosaminoglycans present in the luminal surface of the endothelial cells. The regulation of LPL secretion, stimulated by the most important hormonal regulator, insulin, is related to posttranslational changes in the LPL enzyme, at the level of the Golgi cisternae and exocytotic vesicles, insulin possibly having a positive role in this secretory process (149). Genes encoding LPL were not differentially expressed in omental when compared with subcutaneous adipocytes (150). However, in very obese individuals omental adipocytes express lower levels of LPL protein and mRNA than do subcutaneous fat cells (151). The regulation of LPL in obesity has been presented in the Section on correlations of abdominal visceral fat.

With respect to the hormonal regulation of LPL, insulin and glucocorticoids are the physiological stimulators of the LPL activity, and their association plays an important role in the regulation of body fat topography. In effect, omental adipose tissue is known to be less sensitive to insulin, both in the suppression of lipolysis (152) and in the stimulation of LPL (151). However, when exposed to the combination of insulin plus dexamethasone in culture for 7 days, large increases in adipose LPL were observed because of increases in LPL mRNA (151). Significant differences were observed between men and women. The omental/subcutaneous LPL mRNA ratio was higher in men than in women, and omental LPL was more responsive to insulin plus dexamethasone in men. The increase in LPL in response to dexamethasone suggests that the well known steroid-induced adipose redistribution (especially in the abdomen) may be caused by increases in LPL, which would lead to a preferential distribution of plasma triglyceride fatty acids to the abdominal depot. Therefore, these data suggest that LPL is central to the development of abdominal visceral obesity (98). On the other hand, catecholamines, GH, and testosterone (in males) reduce adipose tissue LPL (149).

2. Acylation-stimulating protein (ASP). ASP is considered the most potent stimulant of triglyceride synthesis in human adipocytes yet described. Its generation is as follows (99). Human adipocytes secrete three proteins of the alternate complement pathway: C3 (the third component of the complement), factor B, and factor D (adipsin), which interact extracellularly to produce a 77-amino-terminal fragment of C3 known as C3a. Excess carboxypeptidases in plasma rapidly cleave the terminal arginine from C3a to produce the 76-amino acid peptide known as C3a desarg or ASP, which then acts back upon the adipocyte, causing triglyceride synthesis to increase. As fatty acids are being liberated fro