Adipose expression of tumor necrosis factor-alpha: direct role in obesity-linked insulin resistance

Tumor necrosis factor-alpha (TNF-alpha) has been shown to have certain catabolic effects on fat cells and whole animals. An induction of TNF-alpha messenger RNA expression was observed in adipose tissue from four different rodent models of obesity and diabetes. TNF-alpha protein was also elevated locally and systemically. Neutralization of TNF-alpha in obese fa/fa rats caused a significant increase in the peripheral uptake of glucose in response to insulin. These results indicate a role for TNF-alpha in obesity and particularly in the insulin resistance and diabetes that often accompany obesity.

Serum immunoreactive-leptin concentrations in normal-weight and obese humans

BACKGROUND

Leptin, the product of the ob gene, is a hormone secreted by adipocytes. Animals with mutations in the ob gene are obese and lose weight when given leptin, but little is known about the physiologic actions of leptin in humans.

METHODS

Using a newly developed radioimmunoassay, wer measured serum concentrations of leptin in 136 normal-weight subjects and 139 obese subjects (body-mass index, > or = 27.3 for men and > or = 27.8 for women; the body-mass index was defined as the weight in kilograms divided by the square of the height in meters). The measurements were repeated in seven obese subjects after weight loss and during maintenance of the lower weight. The ob messenger RNA (mRNA) content of adipocytes was determined in 27 normal-weight and 27 obese subjects.

RESULTS

The mean (+/- SD) serum leptin concentrations were 31.3 +/- 24.1 ng per milliliter in the obese subjects and 7.5 +/- 9.3 ng per milliliter in the normal-weight subjects (P < 0.001). There was a strong positive correlation between serum leptin concentrations and the percentage of body fat (r = 0.85, P < 0.001). The ob mRNA content of adipocytes was about twice as high in the obese subjects as in the normal-weight subjects (P < 0.001) and was correlated with the percentage of body fat (r = 0.68, P < 0.001) in the 54 subjects in whom it was measured. In the seven obese subjects studied after weight loss, both serum leptin concentrations and ob mRNA content of adipocytes declined, but these measures increased again during the maintenance of the lower weight.

CONCLUSIONS

Serum leptin concentrations are correlated with the percentage of body fat, suggesting that most obese persons are insensitive to endogenous leptin production.

Leptin and the regulation of body weight in mammals

The assimilation, storage and use of energy from nutrients constitute a homeostatic system that is essential for life. In vertebrates, the ability to store sufficient quantities of energy-dense triglyceride in adipose tissue allows survival during the frequent periods of food deprivation encountered during evolution. However, the presence of excess adipose tissue can be maladaptive. A complex physiological system has evolved to regulate fuel stores and energy balance at an optimum level. Leptin, a hormone secreted by adipose tissue, and its receptor are integral components of this system. Leptin also signals nutritional status to several other physiological systems and modulates their function. Here we review the role of leptin in the control of body weight and its relevance to the pathogenesis of obesity.

Adipose tissue as an endocrine organ

Adipose tissue is a complex, essential, and highly active metabolic and endocrine organ. Besides adipocytes, adipose tissue contains connective tissue matrix, nerve tissue, stromovascular cells, and immune cells. Together these components function as an integrated unit. Adipose tissue not only responds to afferent signals from traditional hormone systems and the central nervous system but also expresses and secretes factors with important endocrine functions. These factors include leptin, other cytokines, adiponectin, complement components, plasminogen activator inhibitor-1, proteins of the renin-angiotensin system, and resistin. Adipose tissue is also a major site for metabolism of sex steroids and glucocorticoids. The important endocrine function of adipose tissue is emphasized by the adverse metabolic consequences of both adipose tissue excess and deficiency. A better understanding of the endocrine function of adipose tissue will likely lead to more rational therapy for these increasingly prevalent disorders. This review presents an overview of the endocrine functions of adipose tissue.

Muscles, exercise and obesity: skeletal muscle as a secretory organ

During the past decade, skeletal muscle has been identified as a secretory organ. Accordingly, we have suggested that cytokines and other peptides that are produced, expressed and released by muscle fibres and exert either autocrine, paracrine or endocrine effects should be classified as myokines. The finding that the muscle secretome consists of several hundred secreted peptides provides a conceptual basis and a whole new paradigm for understanding how muscles communicate with other organs, such as adipose tissue, liver, pancreas, bones and brain. However, some myokines exert their effects within the muscle itself. Thus, myostatin, LIF, IL-6 and IL-7 are involved in muscle hypertrophy and myogenesis, whereas BDNF and IL-6 are involved in AMPK-mediated fat oxidation. IL-6 also appears to have systemic effects on the liver, adipose tissue and the immune system, and mediates crosstalk between intestinal L cells and pancreatic islets. Other myokines include the osteogenic factors IGF-1 and FGF-2; FSTL-1, which improves the endothelial function of the vascular system; and the PGC-1α-dependent myokine irisin, which drives brown-fat-like development. Studies in the past few years suggest the existence of yet unidentified factors, secreted from muscle cells, which may influence cancer cell growth and pancreas function. Many proteins produced by skeletal muscle are dependent upon contraction; therefore, physical inactivity probably leads to an altered myokine response, which could provide a potential mechanism for the association between sedentary behaviour and many chronic diseases.

Macrophage-specific PPARgamma controls alternative activation and improves insulin resistance

Obesity and insulin resistance, the cardinal features of metabolic syndrome, are closely associated with a state of low-grade inflammation. In adipose tissue chronic overnutrition leads to macrophage infiltration, resulting in local inflammation that potentiates insulin resistance. For instance, transgenic expression of Mcp1 (also known as chemokine ligand 2, Ccl2) in adipose tissue increases macrophage infiltration, inflammation and insulin resistance. Conversely, disruption of Mcp1 or its receptor Ccr2 impairs migration of macrophages into adipose tissue, thereby lowering adipose tissue inflammation and improving insulin sensitivity. These findings together suggest a correlation between macrophage content in adipose tissue and insulin resistance. However, resident macrophages in tissues display tremendous heterogeneity in their activities and functions, primarily reflecting their local metabolic and immune microenvironment. While Mcp1 directs recruitment of pro-inflammatory classically activated macrophages to sites of tissue damage, resident macrophages, such as those present in the adipose tissue of lean mice, display the alternatively activated phenotype. Despite their higher capacity to repair tissue, the precise role of alternatively activated macrophages in obesity-induced insulin resistance remains unknown. Using mice with macrophage-specific deletion of the peroxisome proliferator activated receptor-gamma (PPARgamma), we show here that PPARgamma is required for maturation of alternatively activated macrophages. Disruption of PPARgamma in myeloid cells impairs alternative macrophage activation, and predisposes these animals to development of diet-induced obesity, insulin resistance, and glucose intolerance. Furthermore, gene expression profiling revealed that downregulation of oxidative phosphorylation gene expression in skeletal muscle and liver leads to decreased insulin sensitivity in these tissues. Together, our findings suggest that resident alternatively activated macrophages have a beneficial role in regulating nutrient homeostasis and suggest that macrophage polarization towards the alternative state might be a useful strategy for treating type 2 diabetes.

Adipocyte dysfunctions linking obesity to insulin resistance and type 2 diabetes

Acquired resistance to the action of insulin to stimulate glucose transport in skeletal muscle is associated with obesity and promotes the development of type 2 diabetes. In skeletal muscle, insulin resistance can result from high levels of circulating fatty acids that disrupt insulin signalling pathways. However, the severity of insulin resistance varies greatly among obese people. Here we postulate that this variability might reflect differences in levels of lipid-droplet proteins that promote the sequestration of fatty acids within adipocytes in the form of triglycerides, thereby lowering exposure of skeletal muscle to the inhibitory effects of fatty acids.

C-reactive protein in healthy subjects: associations with obesity, insulin resistance, and endothelial dysfunction: a potential role for cytokines originating from adipose tissue?

C-reactive protein, a hepatic acute phase protein largely regulated by circulating levels of interleukin-6, predicts coronary heart disease incidence in healthy subjects. We have shown that subcutaneous adipose tissue secretes interleukin-6 in vivo. In this study we have sought associations of levels of C-reactive protein and interleukin-6 with measures of obesity and of chronic infection as their putative determinants. We have also related levels of C-reactive protein and interleukin-6 to markers of the insulin resistance syndrome and of endothelial dysfunction. We performed a cross-sectional study in 107 nondiabetic subjects: (1) Levels of C-reactive protein, and concentrations of the proinflammatory cytokines interleukin-6 and tumor necrosis factor-alpha, were related to all measures of obesity, but titers of antibodies to Helicobacter pylori were only weakly and those of Chlamydia pneumoniae and cytomegalovirus were not significantly correlated with levels of these molecules. Levels of C-reactive protein were significantly related to those of interleukin-6 (r=0.37, P<0.0005) and tumor necrosis factor-alpha (r=0.46, P<0.0001). (2) Concentrations of C-reactive protein were related to insulin resistance as calculated from the homoeostasis model assessment model, blood pressure, HDL, and triglyceride, and to markers of endothelial dysfunction (plasma levels of von Willebrand factor, tissue plasminogen activator, and cellular fibronectin). A mean standard deviation score of levels of acute phase markers correlated closely with a similar score of insulin resistance syndrome variables (r=0.59, P<0.00005), this relationship being weakened only marginally by removing measures of obesity from the insulin resistance score (r=0.53, P<0.00005). These data suggest that adipose tissue is an important determinant of a low level, chronic inflammatory state as reflected by levels of interleukin-6, tumor necrosis factor-alpha, and C-reactive protein, and that infection with H pylori, C pneumoniae, and cytomegalovirus is not. Moreover, our data support the concept that such a low-level, chronic inflammatory state may induce insulin resistance and endothelial dysfunction and thus link the latter phenomena with obesity and cardiovascular disease.

The dielectric properties of biological tissues: II. Measurements in the frequency range 10 Hz to 20 GHz

Three experimental techniques based on automatic swept-frequency network and impedance analysers were used to measure the dielectric properties of tissue in the frequency range 10 Hz to 20 GHz. The technique used in conjunction with the impedance analyser is described. Results are given for a number of human and animal tissues, at body temperature, across the frequency range, demonstrating that good agreement was achieved between measurements using the three pieces of equipment. Moreover, the measured values fall well within the body of corresponding literature data.

The dielectric properties of biological tissues: III. Parametric models for the dielectric spectrum of tissues

A parametric model was developed to describe the variation of dielectric properties of tissues as a function of frequency. The experimental spectrum from 10 Hz to 100 GHz was modelled with four dispersion regions. The development of the model was based on recently acquired data, complemented by data surveyed from the literature. The purpose is to enable the prediction of dielectric data that are in line with those contained in the vast body of literature on the subject. The analysis was carried out on a Microsoft Excel spreadsheet. Parameters are given for 17 tissue types.

PPAR-gamma: adipogenic regulator and thiazolidinedione receptor

The past several years have seen an explosive increase in our understanding of the transcriptional basis of adipose cell differentiation. In particular, a key role has been illustrated for PPAR-gamma, a member of the nuclear hormone receptor superfamily. PPAR-gamma has also been recently identified as the major functional receptor for the thiazolidinedione class of insulin-sensitizing drugs. This review examines the evidence that has implicated this transcription factor in the processes of adipogenesis and systemic insulin action. In addition, several models are discussed that may explain how a single protein can be involved in these related but distinct physiological actions. I also point out several important areas where our knowledge is incomplete and more research is needed. Finally, I discuss how advances in our understanding of nuclear receptor function, particularly the docking of cofactors in a ligand-dependent fashion, should lead to improved drugs that utilize the PPAR-gamma system for the treatment of insulin resistance.

The dielectric properties of biological tissues: I. Literature survey

The dielectric properties of tissues have been extracted from the literature of the past five decades and presented in a graphical format. The purpose is to assess the current state of knowledge, expose the gaps there are and provide a basis for the evaluation and analysis of corresponding data from an on-going measurement programme.

Leptin

The discovery of the adipose-derived hormone leptin has generated enormous interest in the interaction between peripheral signals and brain targets involved in the regulation of feeding and energy balance. Plasma leptin levels correlate with fat stores and respond to changes in energy balance. It was initially proposed that leptin serves a primary role as an anti-obesity hormone, but this role is commonly thwarted by leptin resistance. Leptin also serves as a mediator of the adaptation to fasting, and this role may be the primary function for which the molecule evolved. There is increasing evidence that leptin has systemic effects apart from those related to energy homeostasis, including regulation of neuroendocrine and immune function and a role in development.

Anatomy and regulation of the central melanocortin system

The central melanocortin system is perhaps the best-characterized neuronal pathway involved in the regulation of energy homeostasis. This collection of circuits is unique in having the capability of sensing signals from a staggering array of hormones, nutrients and afferent neural inputs. It is likely to be involved in integrating long-term adipostatic signals from leptin and insulin, primarily received by the hypothalamus, with acute signals regulating hunger and satiety, primarily received by the brainstem. The system is also unique from a regulatory point of view in that it is composed of fibers expressing both agonists and antagonists of melanocortin receptors. Given that the central melanocortin system is an active target for development of drugs for the treatment of obesity, diabetes and cachexia, it is important to understand the system in its full complexity, including the likelihood that the system also regulates the cardiovascular and reproductive systems.

Adipose tissue as an endocrine organ

Obesity is characterized by increased storage of fatty acids in an expanded adipose tissue mass and is closely associated with the development of insulin resistance in peripheral tissues such as skeletal muscle and the liver. In addition to being the largest source of fuel in the body, adipose tissue and resident macrophages are also the source of a number of secreted proteins. Cloning of the obese gene and the identification of its product, leptin, was one of the first discoveries of an adipocyte-derived signaling molecule and established an important role for adipose tissue as an endocrine organ. Since then, leptin has been found to have a profound role in the regulation of whole-body metabolism by stimulating energy expenditure, inhibiting food intake and restoring euglycemia, however, in most cases of obesity leptin resistance limits its biological efficacy. In contrast to leptin, adiponectin secretion is often diminished in obesity. Adiponectin acts to increase insulin sensitivity, fatty acid oxidation, as well as energy expenditure and reduces the production of glucose by the liver. Resistin and retinol binding protein-4 are less well described. Their expression levels are positively correlated with adiposity and they are both implicated in the development of insulin resistance. More recently it has been acknowledged that macrophages are an important part of the secretory function of adipose tissue and the main source of inflammatory cyokines, such as TNFalpha and IL-6. An increase in circulating levels of these macrophage-derived factors in obesity leads to a chronic low-grade inflammatory state that has been linked to the development of insulin resistance and diabetes. These proteins commonly known as adipokines are central to the dynamic control of energy metabolism, communicating the nutrient status of the organism with the tissues responsible for controlling both energy intake and expenditure as well as insulin sensitivity.

Relationship of adiponectin to body fat distribution, insulin sensitivity and plasma lipoproteins: evidence for independent roles of age and sex

AIMS/HYPOTHESIS

Increased intra-abdominal fat is associated with insulin resistance and an atherogenic lipoprotein profile. Circulating concentrations of adiponectin, an adipocyte-derived protein, are decreased with insulin resistance. We investigated the relationships between adiponectin and leptin, body fat distribution, insulin sensitivity and lipoproteins.

METHODS

We measured plasma adiponectin, leptin and lipid concentrations, intra-abdominal and subcutaneous fat areas by CT scan, and insulin sensitivity index (S(I)) in 182 subjects (76 M/106F).

RESULTS

Adiponectin concentrations were higher in women than in men (7.4+/-2.9 vs 5.4+/-2.3 micro g/ml, p<0.0001) as were leptin concentrations (19.1+/-13.7 vs 6.9+/-5.1 ng/ml, p<0.0001). Women were more insulin sensitive (S(I): 6.8+/-3.9 vs 5.9+/-4.4 x 10(-5) min(-1)/(pmol/l), p<0.01) and had more subcutaneous (240+/-133 vs 187+/-90 cm(2), p<0.01), but less intra-abdominal fat (82+/-57 vs 124+/-68 cm(2), p<0.0001). By simple regression, adiponectin was positively correlated with age ( r=0.227, p<0.01) and S(I) ( r=0.375, p<0.0001), and negatively correlated with BMI ( r=-0.333, p<0.0001), subcutaneous ( r=-0.168, p<0.05) and intra-abdominal fat ( r=-0.35, p<0.0001). Adiponectin was negatively correlated with triglycerides ( r=-0.281, p<0.001) and positively correlated with HDL cholesterol ( r=0.605, p<0.0001) and Rf, a measure of LDL particle buoyancy ( r=0.474, p<0.0001). By multiple regression analysis, adiponectin was related to age ( p<0.0001), sex ( p<0.005) and intra-abdominal fat ( p<0.01). S(I) was related to intra-abdominal fat ( p<0.0001) and adiponectin ( p<0.0005). Both intra-abdominal fat and adiponectin contributed independently to triglycerides, HDL cholesterol and Rf.

CONCLUSION/INTERPRETATION

These data suggest that adiponectin concentrations are determined by intra-abdominal fat mass, with additional independent effects of age and sex. Adiponectin could link intra-abdominal fat with insulin resistance and an atherogenic lipoprotein profile.

Adipose-derived adult stem cells: isolation, characterization, and differentiation potential

Adipose tissue is an abundant, accessible, and replenishable source of adult stem cells that can be isolated from liposuction waste tissue by collagenase digestion and differential centrifugation. These adipose-derived adult stem (ADAS) cells are multipotent, differentiating along the adipocyte, chondrocyte, myocyte, neuronal, and osteoblast lineages, and can serve in other capacities, such as providing hematopoietic support and gene transfer. ADAS cells have potential applications for the repair and regeneration of acute and chronically damaged tissues. Additional pre-clinical safety and efficacy studies will be needed before the promise of these cells can be fully realized.

Increased adipose tissue in male and female estrogen receptor-alpha knockout mice

Estrogen regulates the amount of white adipose tissue (WAT) in females, but its role in males and whether WAT effects involve estrogen receptor-alpha (ERalpha) or ERbeta were unclear. We analyzed the role of ERalpha in WAT and brown adipose tissue by comparing these tissues in wild-type (WT) and ERalpha-knockout (alphaERKO) male and female mice. Brown adipose tissue weight was similar in alphaERKO and WT males at all ages. Progressive increases in WAT were seen in alphaERKO males with advancing age. Epididymal, perirenal, and inguinal WAT weighed 139-185% more in alphaERKO than in WT males by 270-360 days of age. Epididymal and perirenal adipocyte size was increased 20% in alphaERKO males. Adipocyte number was 82-168% greater in fat pads of alphaERKO vs. WT males. Compared with WT, 90-day-old alphaERKO females had increases in fat pad weights (54-103%), adipocyte size, and number. Both alphaERKO males and females had insulin resistance and impaired glucose tolerance, similar to humans lacking ERalpha or aromatase. Energy intake was equal in WT and alphaERKO males, indicating that obesity was not induced by hyperphagia. In contrast, energy expenditure was reduced by 11% in alphaERKO compared with WT males, indicating that altered energy expenditure may be important for the observed obesity. In summary, ERalpha absence causes adipocyte hyperplasia and hypertrophy, insulin resistance, and glucose intolerance in both sexes. These results are evidence that estrogen/ERalpha signaling is critical in female and male WAT; obesity in alphaERKO males involves a mechanism of reduced energy expenditure rather than increased energy intake.

Increased energy expenditure, decreased adiposity, and tissue-specific insulin sensitivity in protein-tyrosine phosphatase 1B-deficient mice

Protein-tyrosine phosphatase 1B (PTP-1B) is a major protein-tyrosine phosphatase that has been implicated in the regulation of insulin action, as well as in other signal transduction pathways. To investigate the role of PTP-1B in vivo, we generated homozygotic PTP-1B-null mice by targeted gene disruption. PTP-1B-deficient mice have remarkably low adiposity and are protected from diet-induced obesity. Decreased adiposity is due to a marked reduction in fat cell mass without a decrease in adipocyte number. Leanness in PTP-1B-deficient mice is accompanied by increased basal metabolic rate and total energy expenditure, without marked alteration of uncoupling protein mRNA expression. In addition, insulin-stimulated whole-body glucose disposal is enhanced significantly in PTP-1B-deficient animals, as shown by hyperinsulinemic-euglycemic clamp studies. Remarkably, increased insulin sensitivity in PTP-1B-deficient mice is tissue specific, as insulin-stimulated glucose uptake is elevated in skeletal muscle, whereas adipose tissue is unaffected. Our results identify PTP-1B as a major regulator of energy balance, insulin sensitivity, and body fat stores in vivo.

Molecular regulation of adipogenesis

Adipogenesis, or the development of fat cells from preadipocytes, has been one of the most intensely studied models of cellular differentiation. In part this has been because of the availability of in vitro models that faithfully recapitulate most of the critical aspects of fat cell formation in vivo. More recently, studies of adipogenesis have proceeded with the hope that manipulation of this process in humans might one day lead to a reduction in the burden of obesity and diabetes. This review explores some of the highlights of a large and burgeoning literature devoted to understanding adipogenesis at the molecular level. The hormonal and transcriptional control of adipogenesis is reviewed, as well as studies on a less well known type of fat cell, the brown adipocyte. Emphasis is placed, where possible, on in vivo studies with the hope that the results discussed may one day shed light on basic questions of cellular growth and differentiation in addition to possible benefits in human health.

Adipose tissue as an endocrine organ

The discovery of leptin in the mid-1990s has focused attention on the role of proteins secreted by adipose tissue. Leptin has profound effects on appetite and energy balance, and is also involved in the regulation of neuroendocrine and immune function. Sex steroid and glucocorticoid metabolism in adipose tissue has been implicated as a determinant of body fat distribution and cardiovascular risk. Other adipose products, for example, proinflammatory cytokines, complement factors and components of the coagulation/fibrinolytic cascade, may mediate the metabolic and cardiovascular complications associated with obesity.

Attenuation of skeletal muscle and strength in the elderly: The Health ABC Study

Although loss of muscle mass is considered a cause of diminished muscle strength with aging, little is known regarding whether composition of aging muscle affects strength. The skeletal muscle attenuation coefficient, as determined by computed tomography, is a noninvasive measure of muscle density, and lower values reflect increased muscle lipid content. This investigation examined the hypothesis that lower values for muscle attenuation are associated with lower voluntary isokinetic knee extensor strength at 60 degrees/s in 2,627 men and women aged 70-79 yr participating in baseline studies of the Health ABC Study, a longitudinal study of health, aging, and body composition. Strength was higher in men than in women (132.3 +/- 34.5 vs. 81.4 +/- 22.0 N x m, P < 0.01). Men had greater muscle attenuation values (37.3 +/- 6.5 vs. 34.7 +/- 7.0 Hounsfield units) and muscle cross-sectional area (CSA) at the midthigh than women (132.7 +/- 22.4 vs. 93.3 +/- 17.5 cm(2), P < 0.01 for both). The strength per muscle CSA (specific force) was also higher in men (1.00 +/- 0.21 vs. 0.88 +/- 0.21 N x m x cm(-2)). The attenuation coefficient was significantly lower for hamstrings than for quadriceps (28.7 +/- 8.7 vs. 41.1 +/- 6.9 Hounsfield units, P < 0.01). Midthigh muscle attenuation values were lowest (P < 0.01) in the eldest men and women and were negatively associated with total body fat (r = -0.53, P < 0.01). Higher muscle attenuation values were also associated with greater specific force production (r = 0.26, P < 0.01). Multivariate regression analysis revealed that the attenuation coefficient of muscle was independently associated with muscle strength after adjustment for muscle CSA and midthigh adipose tissue in men and women. These results demonstrate that the attenuation values of muscle on computed tomography in older persons can account for differences in muscle strength not attributed to muscle quantity.

Quantification of the relationship between insulin sensitivity and beta-cell function in human subjects. Evidence for a hyperbolic function

To determine the relationship between insulin sensitivity and beta-cell function, we quantified the insulin sensitivity index using the minimal model in 93 relatively young, apparently healthy human subjects of varying degrees of obesity (55 male, 38 female; 18-44 yr of age; body mass index 19.5-52.2 kg/m2) and with fasting glucose levels < 6.4 mM. SI was compared with measures of body adiposity and beta-cell function. Although lean individuals showed a wide range of SI, body mass index and SI were related in a curvilinear manner (P < 0.0001) so that on average, an increase in body mass index was associated generally with a lower value for SI. The relationship between the SI and the beta-cell measures was more clearly curvilinear and reciprocal for fasting insulin (P < 0.0001), first-phase insulin response (AIRglucose; P < 0.0001), glucose potentiation slope (n = 56; P < 0.005), and beta-cell secretory capacity (AIRmax; n = 43; P < 0.0001). The curvilinear relationship between SI and the beta-cell measures could not be distinguished from a hyperbola, i.e., SI x beta-cell function = constant. This hyperbolic relationship described the data significantly better than a linear function (P < 0.05). The nature of this relationship is consistent with a regulated feedback loop control system such that for any difference in SI, a proportionate reciprocal difference occurs in insulin levels and responses in subjects with similar carbohydrate tolerance. We conclude that in human subjects with normal glucose tolerance and varying degrees of obesity, beta-cell function varies quantitatively with differences in insulin sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)