Trends in hyperinsulinemia among nondiabetic adults in the U.S

OBJECTIVE

Insulin resistance and compensatory hyperinsulinemia have been proposed as increasing risk for a variety of abnormalities and clinical syndromes, including type 2 diabetes and cardiovascular disease. Our aim was to assess the trends in the mean concentrations of fasting serum insulin and the prevalence of hyperinsulinemia among nondiabetic adults during the periods of 1988-1994 and 1999-2002 in the U.S.

RESEARCH DESIGN AND METHODS

We conducted analyses of data among men and nonpregnant women without diabetes aged >/=20 years from the Third National Health and Nutrition Examination Survey (NHANES III; 1988-1994; n = 7,926) and NHANES 1999-2002 (n = 2,993). Both surveys were designed to represent the noninstitutionalized civilian U.S. population. We calculated age-adjusted mean concentrations of fasting insulin and the prevalence of hyperinsulinemia defined using the 75th percentile of fasting insulin among nondiabetic individuals as the cutoff value.

RESULTS

The geometric mean concentrations of fasting insulin increased by approximately 5% from 1988-1994 to 1999-2002 among nondiabetic adults aged >/=20 years in the U.S. Mexican-American men, men and women aged 20-39 years, and non-Hispanic white women had a greater relative increase in the mean concentrations of fasting insulin than their counterparts. The prevalence of hyperinsulinemia increased by 35.1% overall (38.3% among men and 32.1% among women).

CONCLUSIONS

In parallel with the obesity epidemic, concentrations of fasting insulin and prevalence of hyperinsulinemia have increased remarkably among nondiabetic U.S. adults.

Comparison of body mass index versus waist circumference with the metabolic changes that increase the risk of cardiovascular disease in insulin-resistant individuals

This study compared the abilities of body mass index (BMI) and waist circumference (WC) to identify resistance to insulin-mediated glucose uptake and related metabolic abnormalities in 261 apparently healthy patients. Insulin resistance and associated metabolic abnormalities occur more commonly in the overweight/obese, and these changes increase the risk of cardiovascular disease (CVD). Determining either their BMI or WC can identify patients more likely to experience the adverse effects of excess adiposity on CVD risk, and the relative clinical utility of these measurements is not clear. Therefore, insulin-mediated glucose uptake was quantified in 261 apparently healthy adults by determining the steady-state plasma glucose concentrations during the insulin suppression test; the higher the concentration, the greater the defect in insulin action. The fasting plasma glucose, triglyceride, and total, low-density lipoprotein, and high-density lipoprotein cholesterol concentrations were also measured, and the associations between these variables and the measurements of BMI and WC were determined. The greater the degree of adiposity, the higher the steady-state plasma glucose, fasting plasma glucose, and triglyceride concentrations, irrespective of the index of adiposity used. However, increases in the total and low-density lipoprotein cholesterol and decreases in the high-density lipoprotein cholesterol concentrations were only seen in those with higher BMI values. In conclusion, because BMI performed at least as well as WC in identifying differences in insulin sensitivity and multiple CVD risk factors, either estimate can be used to identify patients at increased CVD risk.

Comparison of the 1997 and 2003 American Diabetes Association Classification of Impaired Fasting Glucose

OBJECTIVES

The goals of this study were to assess the effect of the 2003 American Diabetes Association definition of impaired fasting glucose (IFG) on prevalence of IFG, coronary heart disease (CHD) risk factors, and CHD compared with the 1997 IFG definition.

BACKGROUND

Although IFG is viewed as increasing CHD risk, this association is unclear and has not been well studied after changing the IFG criterion, especially in a clinical practice setting.

METHODS

This was a cross-sectional evaluation of 8,295 members (3,763 men and 4,532 women) of a community medical center who were between the ages of 30 and 69 years, without a history of diabetes mellitus, and who had available measurements of fasting plasma glucose and lipid concentrations within the past 2 years. The prevalence of IFG, CHD risk factors, and CHD with the 1997 and 2003 IFG definition was compared.

RESULTS

The prevalence of IFG increased from 8% to 35% with the 2003 criterion. Individuals with glucose of 100 to 109 mg/dl had lower prevalence of most CHD risk factors (hypertension, triglyceride > or =150 mg/dl, high-density lipoprotein cholesterol <40 mg/dl, meeting 2 components of the metabolic syndrome criteria, CHD risk > or =10% by Framingham score) compared with individuals with glucose 110 to 125 mg/dl. Individuals identified with the 2003 IFG definition did not have an increase in known CHD when adjusted for covariates (odds ratio 1.4 [95% confidence interval (CI) 0.7 to 2.3] vs. 3.2 [95% CI 1.8 to 5.9]).

CONCLUSIONS

One-third of the population has IFG with the 2003 definition, yet many of these individuals do not have increased prevalence of CHD risk factors or CHD.

The metabolic syndrome: is this diagnosis necessary?

Values of insulin-mediated glucose disposal vary continuously throughout a population of apparently healthy persons, and a difference of > or = 600% exists between the most insulin-sensitive and the most insulin-resistant persons. Approximately 50% of this variability can be attributed to differences in adiposity (25%) and fitness (25%), with the remaining 50% likely of genetic origin. The more insulin-resistant a person, the more likely that he or she will develop some degree of glucose intolerance, high triacylglycerol and low HDL concentrations, essential hypertension, and procoagulant and proinflammatory states, all of which increase the risk of cardiovascular disease (CVD). To identify persons at greater CVD risk because of these abnormalities, the World Health Organization, the Adult Treatment Panel III, and the International Diabetes Federation created a new diagnostic category, the metabolic syndrome. Although the components of the 3 versions of the metabolic syndrome are similar, the specific values for those components that define an abnormality are somewhat different, and the manner in which the abnormalities are used to make a positive diagnosis varies dramatically from version to version. This review will summarize the similarities in and differences between the 3 versions of the metabolic syndrome, point out that the clustering of components that make up all 3 definitions of the metabolic syndrome is not accidental and occurs only in insulin-resistant persons, develop the argument that diagnosing the metabolic syndrome in a person has neither pedagogical nor clinical utility, and suggest that the clinical emphasis should be on treating effectively any CVD risk factor that is present.

Plasma asymmetric dimethylarginine concentrations are elevated in obese insulin-resistant women and fall with weight loss

CONTEXT

Plasma asymmetric dimethylarginine (ADMA) concentrations are higher in apparently healthy, insulin-resistant (IR) individuals and decrease in response to thiazolidenedione treatment.

OBJECTIVE

The objective of the study was to determine whether ADMA concentrations would also fall when insulin sensitivity is enhanced with weight loss in obese individuals. DESIGN/SETTING/PATIENTS/INTERVENTION: Twenty obese women classified as IR or insulin sensitive (IS) on the basis of their steady-state plasma glucose (SSPG) concentration during the insulin suppression test underwent 12 wk of dietary weight loss.

OUTCOME MEASURES

Plasma glucose, insulin, and ADMA were measured at baseline and after weight loss; change in insulin resistance was quantified by repeating the SSPG after the dietary intervention.

RESULTS

Although weight loss was similar in the two groups, significant improvements in SSPG, glucose, and insulin concentrations were confined to the IR group. Baseline plasma ADMA concentrations (mean +/- sd) were higher in IR subjects (1.69 +/- 0.44 vs. 1.18 +/- 0.45 micromol/liter, P = 0.02) and decreased to 1.20 +/- 0.22 micromol/liter (P < 0.001) with weight loss. In contrast, ADMA levels did not change with a similar extent of weight loss in the IS group.

CONCLUSION

Plasma ADMA levels are higher in obese, IR women than in equally obese, IS women and decrease in response to weight loss when associated with enhancement of insulin sensitivity.

Relation of plasma insulin levels to forearm flow-mediated dilatation in healthy volunteers

Although several observations suggest that insulin resistance/compensatory hyperinsulinemia (IR/CH) has a direct effect on endothelial function, independently of the metabolic abnormalities associated with the defect in insulin action, this relation has not been evaluated in apparently healthy individuals. To address this issue, we measured endothelial-dependent vasodilation in response to forearm ischemia (flow-mediated dilation [FMD]) in 47 nonsmoking, healthy volunteers without known risk factors for atherosclerosis. Measurements were also made of multiple anthropometric, metabolic, and hemodynamic variables related to IR/CH. Decreases in FMD were significantly correlated (analysis of variance for linear trend) with (1) male gender (p = 0.003), (2) waist circumference (p = 0.038), (3) higher fasting plasma insulin (p = 0.015) and triglyceride concentrations (p = 0.023), and (4) lower concentrations of high-density lipoprotein cholesterol (p = 0.001). Multivariate linear regression analysis indicated that only plasma insulin (beta -0.424) was independently associated (p <0.001) with changes in FMD, and individual differences in insulin concentrations, along with gender and brachial artery diameter at baseline, accounted for approximately 39% of the variability in FMD. In conclusion, IR/CH is an independent predictor of decreases in endothelial-dependent vasodilation in apparently healthy individuals, in the absence of traditional risk factors for atherosclerosis.

Improvements in insulin resistance with weight loss, in contrast to rosiglitazone, are not associated with changes in plasma adiponectin or adiponectin multimeric complexes

It has been suggested that changes in adiponectin levels may contribute to improved insulin sensitivity in insulin-resistant individuals both after weight loss and after treatment with thiazolidinedione compounds. If this is correct, then changes in total circulating adiponectin and/or distribution of its multimeric complexes should coincide with improvements in insulin sensitivity after both interventions. To address this issue, fasting adiponectin concentrations and distribution of adiponectin complexes were measured in plasma samples in 24 insulin-resistant, nondiabetic subjects before and after 3-4 mo of treatment with either rosiglitazone or caloric restriction. The degree of insulin resistance in each group of 12 subjects was equal at baseline and improved to a similar extent ( approximately 30%) after each therapy. Whereas total adiponectin levels increased by nearly threefold and the relative amount of several higher molecular weight adiponectin complexes increased significantly in the rosiglitazone treatment group, there were no discernible changes in adiponectin levels or in the distribution between high or low molecular weight complexes in the weight loss group. These data indicate that, although changes in total adiponectin and several specific adiponectin complexes paralleled improvements in insulin resistance in thiazolidinedione-treated subjects, neither circulating adiponectin concentrations nor multimeric complexes changed in association with enhanced insulin sensitivity after moderate weight loss in 12 insulin-resistant, obese individuals.

Body mass index and waist circumference both contribute to differences in insulin-mediated glucose disposal in nondiabetic adults

BACKGROUND

Overweight and obese individuals are more likely to be insulin resistant and at increased risk of adverse clinical outcomes. Questions remain as to whether waist circumference (WC) or body mass index (BMI) most effectively identifies insulin-resistant individuals.

OBJECTIVE

This study quantified insulin-mediated glucose uptake (IMGU) in 330 apparently healthy volunteers and compared the relation between this value and measurements of WC and BMI.

DESIGN

IMGU was quantified via determination of the steady-state plasma glucose (SSPG) concentration during the insulin-suppression test. Differences in SSPG concentrations due to variations in WC within a given BMI category, as well as those due to differences in BMI within a given WC classification, were then compared.

RESULTS

BMI and WC correlated with each other (r = 0.78, P < 0.001) and equally with SSPG concentrations (r = 0.58 and 0.57, respectively; P < 0.001). When stratified by BMI, abdominally obese subjects within the overweight BMI category had higher SSPG concentrations than did those with a normal WC (P < 0.05). When classified by WC, subjects in the overweight BMI category had greater SSPG concentrations than did subjects in the normal BMI category within the normal WC category (P < 0.01), as did subjects in the obese BMI category in comparison with subjects in the overweight BMI category within the obese WC category (P < 0.01).

CONCLUSIONS

The more overweight or obese a person, the greater the degree of insulin resistance; differences in adiposity accounted for approximately one-third of the variation in IMGU, irrespective of the index used. Furthermore, there was no difference in the relation between the degree of insulin resistance and either index of adiposity.

Insulin resistance/compensatory hyperinsulinemia predict carotid intimal medial thickness in patients with essential hypertension

BACKGROUND AND AIM

Approximately 50% of subjects with essential hypertension (EH) are insulin resistant, and this defect in insulin action could contribute to increased cardiovascular disease (CVD) risk in these patients. To test this hypothesis, we attempted to see if there was a link between insulin resistance (IR) and carotid intimal medial thickness (IMT), an early index of CVD, in patients with essential hypertension.

METHODS AND RESULTS

Ultrasound quantification of carotid IMT was performed in 79 hypertensive patients, and 63 patients (31 m and 32 f), defined as being free of plaque (IMT < 1.3 mm), were further subdivided into normal (<1.0 mm) and thickened (1-1.3 mm) IMT groups. Subjects in the thickened IMT group were older and had significantly (p < 0.05) higher plasma concentrations of fasting insulin, nitric oxide (NO(x)) and intercellular adhesion molecule 1 (ICAM-1). However, the two groups were not significantly different in terms of blood pressure, overall or regional obesity, fasting lipid levels, uric acid, concentrations of other cellular adhesion molecules or levels of C-reactive protein. There were significant (p < 0.05) correlations in the whole population between IMT and age, fasting insulin and NO(x), and multiple regression analysis identified fasting insulin as an independent predictor of IMT.

CONCLUSIONS

The presence of increased IMT is significantly related to several metabolic and endothelial abnormalities associated with IR/hyperinsulinemia, and fasting insulin independently predicts the thickness of the intima-media layer. These results support the view that CVD risk is greatest in those patients with essential hypertension who are also IR/hyperinsulinemic.

Hyperinsulinemia predicts hepatic fat content in healthy individuals with normal transaminase concentrations

Although the prevalence of insulin resistance (IR) and compensatory hyperinsulinemia (CH) is increased in patients with nonalcoholic fatty liver disease, the role of IR/CH in regulation of hepatic fat content in healthy volunteers with normal concentrations of alanine transaminase (ALT) has not been defined. To address this issue, hepatic fat content was quantified by ultrasound in 69 (30 men, 39 women) healthy individuals, without known risk factors for liver disease and with plasma ALT concentrations of less than 30 U/L. Experimental variables quantified included body mass index, waist circumference, systolic and diastolic blood pressures, and fasting plasma glucose, fasting plasma insulin (FPI), and lipid concentrations. Subjects were classified as having no (55%), mild (27%), or moderate to severe (18%) hepatic steatosis on the basis of the ultrasound results. Statistically significant (P < .05-.001) correlations (Spearman rho values) existed between liver fat content and ALT (0.26), body mass index (0.52), waist circumference (0.50), systolic blood pressure (0.28), diastolic blood pressure (0.27), fasting plasma glucose (0.47), FPI (0.56), triglycerides (0.30), and high-density lipoprotein cholesterol (-0.35). Multivariate general discriminant analysis and multiple linear regression analysis indicated that FPI was the only independent predictor (P < .001) of both liver fat content and ALT concentrations. Fasting plasma insulin (a surrogate estimate of IR/CH) predicts hepatic fat content and ALT in healthy volunteers with normal transaminase concentrations, independently of the other anthropometric and metabolic variables measured.

Insulin resistance, the insulin resistance syndrome, and cardiovascular disease

Insulin-mediated glucose disposal varies widely in apparently healthy human beings, and the more insulin resistant an individual, the more insulin they must secrete in order to prevent the development of type 2 diabetes. However, the combination of insulin resistance and compensatory hyperinsulinemia increases the likelihood that an individual will be hypertensive, and have a dyslipidemia characterized by a high plasma triglyceride (TG) and low high-density lipoprotein cholesterol (HDL-C) concentration. These changes increase risk of cardiovascular disease (CVD), and in 1988, this cluster of related abnormalities was designated as comprising a syndrome (X). Several other clinical syndromes are now known to be associated with insulin resistance and compensatory hyperinsulinemia. For example, polycystic ovary syndrome appears to be secondary to insulin resistance and compensatory hyperinsulinemia. More recently, studies have shown that the prevalence of insulin resistance/hyperinsulinemia is increased in patients with nonalcoholic fatty liver disease, and there are reports that certain forms of cancer are more likely to occur in insulin resistant/hyperinsulinemic persons. Finally, there is substantial evidence of an association between insulin resistance/hyperinsulinemia, and sleep disordered breathing. Given the rapid increase in the number of clinical syndromes and abnormalities associated with insulin resistance/hyperinsulinemia, it seems reasonable to suggest that the cluster of these changes related to the defect in insulin action be subsumed under the term of the insulin resistance syndrome. In addition to the identification of additional clinical syndromes related to insulin resistance/hyperinsulinemia, a number of new risk factors have been recognized that would increase CVD risk in these individuals. Thus, in addition to a high TG and a low HDL-C, the atherogenic lipoprotein profile in insulin resistant/hyperinsulinemic individuals also includes the appearance of smaller and denser low density lipoprotein particles, and the enhanced postprandial accumulation of remnant lipoproteins; changes identified as increasing risk of CVD. Elevated plasma concentrations of plasminogen activator inhibitor-1 (PAI-1) have been shown to be associated with increased CVD, and there is evidence of a significant relationship between PAI-1 and fibrinogen levels and both insulin resistance and hyperinsulinemia. Evidence is also accumulating that sympathetic nervous system (SNS) activity is increased in insulin resistant, hyperinsulinemic individuals, and, along with the salt sensitivity associated with insulin resistance/hyperinsulinemia, increases the likelihood that these individuals will develop essential hypertension. The first step in the process of atherogenesis is the binding of mononuclear cells to the endothelium, and mononuclear cells isolated from insulin resistant/hyperinsulinemic individuals adhere with greater avidity. This process is modulated by adhesion molecules produced by endothelial cells, and there is a significant relationship between degree of insulin resistance and the plasma concentration of the several of these adhesion molecules. Further evidence of the relationship between insulin resistance and endothelial dysfunction is the finding that asymmetric dimethylarginine, an endogenous inhibitor of the enzyme nitric oxide synthase, is increased in insulin resistant/hyperinsulinemic individuals. Finally, plasma concentrations of several inflammatory markers are elevated in insulin resistant subjects. It is obvious that the cluster of abnormalities associated with insulin resistance and compensatory hyperinsulinemia contains many well-recognized CVD risk factors, choosing which one, or ones, that are primarily responsible for the accelerated atherogenesis that characterizes this syndrome is not a simple task. Indeed, efforts to try to do so by the use of multiple regression analysis of epidemiological data may be more misleading than helpful.

Effect of a high-carbohydrate versus a high--cis-monounsaturated fat diet on blood pressure in patients with type 2 diabetes

OBJECTIVE

To investigate whether blood pressure is different in type 2 diabetic patients on a diet rich in carbohydrates versus a diet rich in cis-monounsaturated fatty acids. Data on the dietary effects on these diets' glucose and lipid metabolism have been previously published.

RESEARCH DESIGN AND METHODS

The study compared the effect of feeding 42 type 2 diabetic patients a carefully controlled isoenergic high-carbohydrate (high-carb; 55% energy as carbohydrate, 30% as fat, and 10% as monounsaturated fat) and high-monounsaturated fat (high-mono; 45% energy as fat, 25% as monounsaturated fat, and 40% as carbohydrate) diet for 6 weeks each in a four-center, randomized, cross-over study on blood pressure. Twenty-one patients continued the diet they received during the second phase for an additional 8 weeks.

RESULTS

According to repeated-measures ANOVA, blood pressure during the last 3 days of each phase was similar after 6 weeks of the high-carb and high-mono diets (systolic blood pressure: 128 +/- 16 vs. 127 +/- 15 mmHg, P = 0.9; diastolic blood pressure: 75 +/- 7 vs. 75 +/- 8 mmHg, P = 0.7). However, after 14 weeks of the high-carb diet (n = 13), there was a significant increase in blood pressure compared with 6 weeks of the high-mono diet (systolic blood pressure: 132 +/- 13 vs. 126 +/- 11 mmHg, P = 0.04; diastolic blood pressure: 83 +/- 6 vs. 76 +/- 7 mmHg, P = 0.002). After 14 weeks of the high-mono diet (n = 8), the reduction in blood pressure was not significant compared with 6 weeks of the high-carb diet (systolic blood pressure: 118 +/- 14 vs. 121 +/- 16 mmHg, P = 0.4; diastolic blood pressure: 71 +/- 8 vs. 75 +/- 10 mmHg, P = 0.3).

CONCLUSION

Although the exchange of carbohydrates with monounsaturated fats may not affect blood pressure in the short term, long-term consumption of a high-carbohydrate diet may modestly raise blood pressure in type 2 diabetic patients.

Body mass index and waist circumference correlate to the same degree with insulin-mediated glucose uptake

To compare the relationship between insulin-mediated glucose uptake (IMGU) and excess adiposity as determined by measurement of either body mass index (BMI) or waist circumference (WC), IMGU was quantified by determining the steady-state plasma glucose (SSPG) concentration with the insulin suppression test and the relationship between the SSPG concentration and BMI or WC evaluated in a study of 208 healthy individuals (128 women/80 men). The results indicated that BMI and WC were correlated (P < .001) to a similar degree in both men (r = 0.90) and women (r = 0.86). Steady-state plasma glucose and both indices of excess adiposity were also significantly correlated (P < .001) to an essentially identical extent in men (r values of 0.71 vs 0.70) and women (r values of 0.54 vs 0.53). When the population was divided into tertiles on the basis of SSPG concentrations, 96% of those in the most insulin-resistant tertile were identified as being overweight/obese by BMI criteria and 84% as abdominally obese by WC criteria. However, a substantial number of those in the most insulin-sensitive tertile also demonstrated excess adiposity as defined by either BMI (45%) or WC (33%). To summarize, (1) BMI and WC correlate closely within an individual and equally well with IMGU, and (2) BMI is as effective as WC in identifying insulin-resistant individuals.

The insulin resistance syndrome: definition and dietary approaches to treatment

The ability of insulin to stimulate glucose disposal varies at least sixfold in apparently healthy individuals, and approximately one-third of the population that is most resistant to this action of insulin is at greatly increased risk to develop a number of adverse clinical outcomes. Type 2 diabetes occurs when insulin resistant individuals are unable to secrete enough insulin to compensate for the defect in insulin action, and this was the first clinical syndrome identified as being related to insulin resistance. Although the majority of insulin-resistant individuals are able to maintain the level of compensatory hyperinsulinemia needed to prevent the development of a significant degree of hyperglycemia, the combination of insulin resistance and hyperinsulinemia greatly increases the likelihood of developing a cluster of closely related abnormalities and the resultant clinical diagnoses that can be considered to make up the insulin resistance syndrome (IRS). Since being overweight/obese and sedentary decreases insulin sensitivity, it is not surprising that the prevalence of the manifestations of the IRS is increasing at a rapid rate. From a dietary standpoint, there are two approaches to attenuating the manifestations of the IRS: (a) weight loss to enhance insulin sensitivity in those overweight/obese individuals who are insulin resistant/hyperinsulinemic; and (b) changes in macronutrient content of diets to avoid the adverse effects of the compensatory hyperinsulinemia. This chapter will focus on defining the abnormalities and clinical syndromes that compose the IRS and evaluating the dietary changes that can ameliorate its adverse consequences.

Metabolic and ovarian effects of rosiglitazone treatment for 12 weeks in insulin-resistant women with polycystic ovary syndrome

BACKGROUND

Insulin sensitizers have favourable metabolic and ovarian effects in polycystic ovary syndrome (PCOS). This study examined rosiglitazone, a thiazolidinedione, in PCOS.

METHODS

In a prospective, open-label study, the effects of rosiglitazone on metabolism and ovarian function were examined in 42 non-diabetic women with PCOS classified according to the National Institute of Child Health and Human Development criteria and insulin resistance (IR) by steady-state plasma glucose (SSPG) > or =10 mmol/l on octreotide-modified insulin suppression testing. Participants were randomized to rosiglitazone 2, 4 or 8 mg daily for 12 weeks. Endpoints included ovulation and menstrual pattern; serum testosterone, sex hormone-binding globulin (SHBG), and LH; and changes in IR and glucose-insulin responses on 8 h mixed-meal profile.

RESULTS

After rosiglitazone 8 mg daily for 12 weeks, SSPG declined and insulinaemia fell by 46%; lower doses gave lesser effects. Serum LH, total and free testosterone were unchanged; SHBG increased. With rosiglitazone, ovulation occurred in 23/42 women (55%), without significant dose dependence. Both before and during treatment, ovulators on rosiglitazone had lower circulating insulin and free testosterone and higher SHBG than non-ovulators. Testosterone declined only in a subgroup of ovulators with early vaginal bleeding after starting rosiglitazone.

CONCLUSIONS

Rosiglitazone in insulin-resistant PCOS promoted ovulation and dose-dependently decreased IR and insulinaemia; ovulators had lower circulating insulin and testosterone.

Rosiglitazone reduces glucose-stimulated insulin secretion rate and increases insulin clearance in nondiabetic, insulin-resistant individuals

Compensatory hyperinsulinemia permitting insulin-resistant individuals to maintain normal glucose tolerance is associated with a left shift in the glucose-stimulated insulin secretion rate (GS-ISR) dose-response curve and decrease in the insulin metabolic clearance rate (I-MCR). To see whether these changes would reverse with improvement in insulin sensitivity, 14 nondiabetic insulin-resistant subjects received rosiglitazone for 12 weeks (4 mg daily for 4 weeks and then 8 mg daily for 8 weeks). Insulin-mediated glucose uptake was quantified by measuring the steady-state plasma glucose concentration during the insulin suppression test. GS-ISR and I-MCR were determined during a 240-min graded intravenous glucose infusion. I-MCR was also calculated during the insulin suppression test. After rosiglitazone treatment, insulin sensitivity improved with significant fall in steady-state plasma glucose (means +/- SE from 13.5 +/- 0.62 to 9.8 +/- 1.02 mmol/l, P < 0.001). In response, the integrated GS-ISR decreased by 21% (P < 0.001), with a right shift in the dose-response curve. Calculated I-MCR increased by 34% (P = 0.008) during the insulin suppression test and by 21% (P = 0.03) during the graded glucose infusion. In conclusion, enhanced insulin sensitivity in rosiglitazone-treated nondiabetic insulin-resistant individuals was associated with a shift to the right in the GS-ISR dose-response curve and an increase in I-MCR.

Increased aortic stiffness in the insulin-resistant Zuckerfa/farat

Accumulating clinical evidence indicates increased aortic stiffness, an independent risk factor for cardiovascular and all-cause mortality, in type 2 diabetic and glucose-intolerant individuals. The present study sought to determine whether increased mechanical stiffness, an altered extracellular matrix, and a profibrotic gene expression profile could be observed in the aorta of the insulin-resistant Zucker fa/fa rat. Mechanical testing of Zucker fa/fa aortas showed increased vascular stiffness in longitudinal and circumferential directions compared with Zucker lean controls. Unequal elevations in developed strain favoring the longitudinal direction resulted in a loss of anisotropy. Real-time quantitative PCR and immunohistochemistry revealed increased expression of fibronectin and collagen IVα3in the Zucker fa/fa aorta. In addition, expression of transforming growth factor-β and several Smad proteins was increased in vessels from insulin-resistant animals. In rat vascular smooth muscle cells, 12–18 h of exposure to insulin (100 nmol/l) enhanced transforming growth factor-β1 mRNA expression, implicating a role for hyperinsulinemia in vascular stiffness. Thus there is mechanical, structural, and molecular evidence of arteriosclerosis in the Zucker fa/fa rat at the glucose-intolerant, hyperinsulinemic stage.

Prevalence of insulin resistance and the metabolic syndrome with alternative definitions of impaired fasting glucose

We sought to evaluate the effect of the new definition of impaired fasting glucose (IFG = fasting glucose concentration 100-125 mg/dL among people without diabetes) on the ability to identify insulin resistance, as well as the prevalence of the metabolic syndrome in apparently healthy individuals. From the Stanford General Clinical Research Center data-base, we used data from 230 men and 260 women aged 19-79 years who had had an insulin suppression test to measure insulin resistance. From the Third National Health and Nutrition Examination Survey (1988-1994), we used data from 8814 participants aged > or =20 years to estimate the impact of adopting the new IFG criteria on the prevalence of the metabolic syndrome. Among the 490 participants, the prevalence of IFG increased from 5.5% under the old definition of IFG to 20.4% under the new definition. Using the old definition of IFG, the sensitivity, specificity, and positive predictive value (PPV) of IFG of identifying an individual as being insulin resistant were 10%, 97%, and 63%, respectively. Using the new definition, these parameters were 33%, 88%, and 61%, respectively. If the new IFG criteria were adopted, the prevalence of the metabolic syndrome would increase from 21.8% to 26.3%. The new definition of IFG expands the population with insulin resistance by almost four-fold and could expand the population with the metabolic syndrome by about 20%. The clinical and public health implications of the new IFG definition remain to be elucidated.

The metabolic syndrome: requiescat in pace

Values for insulin-mediated glucose disposal vary continuously throughout a population of apparently healthy individuals, with at least a sixfold variation between the most insulin sensitive and most insulin resistant of these individuals. The more insulin resistant a person, the more insulin must be secreted to prevent decompensation of glucose tolerance. Insulin resistance is not a disease, but a description of a physiologic state, and approximately one third of an apparently healthy population is sufficiently insulin resistant to be at increased risk to develop a cluster of abnormalities and related clinical syndromes. The primary value of the concept of insulin resistance is that it provides a conceptual framework with which to place a substantial number of apparently unrelated biological events into a pathophysiological construct. In contrast, the metabolic syndrome was introduced as a diagnostic category to identify individuals that satisfy three of five relatively arbitrarily chosen criteria to initiate lifestyle changes with the goal of decreasing risk of cardiovascular disease. Consequently, the value of the notion of the metabolic syndrome must be considered not in pathophysiologic terms, but as a pragmatic approach to obtain a better clinical outcome. In this review, an effort is made to critically evaluate the concept of the metabolic syndrome, the criteria chosen to identify individuals with the syndrome, and the clinical utility of making, or not making, a diagnosis of the metabolic syndrome.

Compensatory hyperinsulinemia and the development of an atherogenic lipoprotein profile: the price paid to maintain glucose homeostasis in insulin-resistant individuals

The ability of insulin to stimulate glucose disposal varies sixfold to eightfold among apparently healthy individuals. The only way that insulin-resistant persons can prevent the development of type 2 diabetes is by secreting the increased amount of insulin that is necessary to compensate for the resistance to insulin action. The greater the magnitude of muscle and adipose tissue insulin resistance, the more insulin must be secreted to maintain normal or near-normal glucose tolerance. Although compensatory hyperinsulinemia may prevent the development of fasting hyperglycemia in insulin-resistant individuals, the price paid is the untoward physiologic effects of increased circulating insulin concentrations on tissues that retain normal insulin sensitivity. This article focused on the interplay between insulin resistance at the level of the muscle and adipose tissue and normal hepatic insulin sensitivity; this leads to the atherogenic lipoprotein profile that is characteristic of insulin-resistant individuals. It would be inappropriate to minimize the importance of differential insulin sensitivity in the genesis of the changes in lipoprotein metabolism that increase CVD risk in insulin-resistant persons. It would be equally remiss not to emphasize that differential tissue insulin resistance also is necessary to explain why insulin-resistant/hyperinsulinemic individuals are more likely to develop the clinical syndromes (with the exception of type 2 diabetes mellitus) that are listed in Box 1.

Effect of rosiglitazone treatment on circulating vascular and inflammatory markers in insulin-resistant subjects

Thiazolidinedione (TZD) compounds enhance insulin sensitivity and attenuate inflammation. The effect of the TZD compound, rosiglitazone (RSG) on both actions was evaluated in two groups of insulin-resistant subjects with minimal elevations of fasting plasma glucose (PG) concentration: group A (n=15, PG < 7.0 mmol/L) and group B (n=14, PG 7.0-8.3 mmol/L). Insulin action, quantified by the insulin suppression test, improved after three months of treatment in both groups, and concentrations of C-reactive protein, plasminogen activator inhibitor-1 and Eselectin all fell. Significant decreases in L-selectin and P-selectin were confined to group B, and concentrations of interleukin-6, intercellular adhesion molecule-1 and vascular cellular adhesion molecule-1 did not fall in either group. Significant relationships were not discerned between enhanced insulin sensitivity and related variables and decreases in inflammatory/vascular markers, suggesting that RSG-induced changes in the latter variables in insulin-resistant individuals might be at least partly independent of the effects of the drug on insulin action.

Why Syndrome X? From Harold Himsworth to the insulin resistance syndrome

Although the concept of Syndrome X was introduced in the Banting Medal address of 1988 (Reaven, 1988), the notion that led to its genesis had started approximately 50 years earlier. In this short history, an attempt will be made to trace the two paths of scientific discovery that were formally merged in New Orleans in 1988 to form the scientific foundation of Syndrome X. In addition, the developments in the last 16 years that have led from the notion of Syndrome X to the broader concept of an Insulin Resistance Syndrome (IRS) will be briefly summarized.