Phenotypic Characterization of Insulin-Resistant and Insulin-Sensitive Obesity.

CONTEXT: Whereas insulin resistance and obesity coexist, some obese individuals remain insulin sensitive. OBJECTIVE: We examined phenotypic and metabolic factors associated with insulin sensitivity in both muscle and liver in obese individuals. DESIGN AND PARTICIPANTS: Sixty-four nondiabetic obese adults (29 males) underwent hyperinsulinemic (15 and 80 mU/m(2) · min)-euglycemic clamps with deuterated glucose. Top tertile subjects for glucose infusion rate during the high-dose insulin clamp were assigned Musclesen and those in the lower two tertiles were assigned Muscleres. Secondarily, top tertile subjects for endogenous glucose production suppression during the low-dose insulin clamp were deemed Liversen and the remainder Liverres. MAIN OUTCOMES MEASURES: Clinical and laboratory parameters and visceral, subcutaneous, liver, and pancreatic fat were compared. RESULTS: Musclesen and Muscleres had similar body mass index and total fat (P > .16), but Musclesen had lower glycated hemoglobin (P < .001) and systolic (P = .01) and diastolic (P = .03) blood pressure (BP). Despite similar sc fat (P = 1), Musclesen had lower visceral (P < .001) and liver (P < .001) fat. Liversen had lower visceral (P < .01) and liver (P < .01) fat and C-reactive protein (P = .02) than Liverres. When subjects were grouped by both glucose infusion rate during the high-dose insulin clamp and endogenous glucose production suppression, insulin sensitivity at either muscle or liver conferred apparent protection from the adverse metabolic features that characterized subjects insulin resistant at both sites. High-density lipoprotein-cholesterol, 1-hour glucose, systolic BP, and triglycerides explained 54% of the variance in muscle insulin sensitivity. CONCLUSIONS: Obese subjects who were insulin sensitive at muscle and/or liver exhibited favorable metabolic features, including lower BP, liver and visceral adiposity. This study identifies factors associated with, and possibly contributing to, insulin sensitivity in obesity.

Eradicating hepatitis C virus ameliorates insulin resistance without change in adipose depots.

Chronic hepatitis C (CHC) is associated with lipid-related changes and insulin resistance; the latter predicts response to antiviral therapy, liver disease progression and the risk of diabetes. We sought to determine whether insulin sensitivity improves following CHC viral eradication after antiviral therapy and whether this is accompanied by changes in fat depots or adipokine levels. We compared 8 normoglycaemic men with CHC (genotype 1 or 3) before and at least 6 months post viral eradication and 15 hepatitis C antibody negative controls using an intravenous glucose tolerance test and two-step hyperinsulinaemic-euglycaemic clamp with [6,6-(2) H2 ] glucose to assess peripheral and hepatic insulin sensitivity. Magnetic resonance imaging and spectroscopy quantified abdominal fat compartments, liver and intramyocellular lipid. Peripheral insulin sensitivity improved (glucose infusion rate during high-dose insulin increased from 10.1 ± 1.6 to 12 ± 2.1 mg/kg/min/, P = 0.025), with no change in hepatic insulin response following successful viral eradication, without any accompanying change in muscle, liver or abdominal fat depots. There was corresponding improvement in incremental glycaemic response to intravenous glucose (pretreatment: 62.1 ± 8.3 vs post-treatment: 56.1 ± 8.5 mm, P = 0.008). Insulin sensitivity after viral clearance was comparable to matched controls without CHC. Post therapy, liver enzyme levels decreased but, interestingly, levels of glucagon, fatty acid-binding protein and lipocalin-2 remained elevated. Eradication of the hepatitis C virus improves insulin sensitivity without alteration in fat depots, adipokine or glucagon levels, consistent with a direct link of the virus with insulin resistance.

Effects of type 1 diabetes, sprint training and sex on skeletal muscle sarcoplasmic reticulum Ca2+ uptake and Ca2+-ATPase activity.

Calcium cycling is integral to muscle performance during the rapid muscle contraction and relaxation of high-intensity exercise. Ca(2+) handling is altered by diabetes mellitus, but has not previously been investigated in human skeletal muscle. We investigated effects of high-intensity exercise and sprint training on skeletal muscle Ca(2+) regulation among men and women with type 1 diabetes (T1D, n = 8, 3F, 5M) and matched non-diabetic controls (CON, n = 8, 3F, 5M). Secondarily, we examined sex differences in Ca(2+) regulation. Subjects undertook 7 weeks of three times-weekly cycle sprint training. Before and after training, performance was measured, and blood and muscle were sampled at rest and after high-intensity exercise. In T1D, higher Ca(2+)-ATPase activity (+28%) and Ca(2+) uptake (+21%) than in CON were evident across both times and days (P < 0.05), but performance was similar. In T1D, resting Ca(2+)-ATPase activity correlated with work performed until exhaustion (r = 0.7, P < 0.01). Ca(2+)-ATPase activity, but not Ca(2+) uptake, was lower (-24%, P < 0.05) among the women across both times and days. Intense exercise did not alter Ca(2+)-ATPase activity in T1D or CON. However, sex differences were evident: Ca(2+)-ATPase was reduced with exercise among men but increased among women across both days (time × sex interaction, P < 0.05). Sprint training reduced Ca(2+)-ATPase (-8%, P < 0.05), but not Ca(2+) uptake, in T1D and CON. In summary, skeletal muscle Ca(2+) resequestration capacity was increased in T1D, but performance was not greater than CON. Sprint training reduced Ca(2+)-ATPase in T1D and CON. Sex differences in Ca(2+)-ATPase activity were evident and may be linked with fibre type proportion differences.

Impaired Akt phosphorylation in insulin-resistant human muscle is accompanied by selective and heterogeneous downstream defects.

AIMS/HYPOTHESIS: Muscle insulin resistance, one of the earliest defects associated with type 2 diabetes, involves changes in the phosphoinositide 3-kinase/Akt network. The relative contribution of obesity vs insulin resistance to perturbations in this pathway is poorly understood. METHODS: We used phosphospecific antibodies against targets in the Akt signalling network to study insulin action in muscle from lean, overweight/obese and type 2 diabetic individuals before and during a hyperinsulinaemic-euglycaemic clamp. RESULTS: Insulin-stimulated Akt phosphorylation at Thr309 and Ser474 was highly correlated with whole-body insulin sensitivity. In contrast, impaired phosphorylation of Akt substrate of 160 kDa (AS160; also known as TBC1D4) was associated with adiposity, but not insulin sensitivity. Neither insulin sensitivity nor obesity was associated with defective insulin-dependent phosphorylation of forkhead box O (FOXO) transcription factor. In view of the resultant basal hyperinsulinaemia, we predicted that this selective response within the Akt pathway might lead to hyperactivation of those processes that were spared. Indeed, the expression of genes targeted by FOXO was downregulated in insulin-resistant individuals. CONCLUSIONS/INTERPRETATION: These results highlight non-linearity in Akt signalling and suggest that: (1) the pathway from Akt to glucose transport is complex; and (2) pathways, particularly FOXO, that are not insulin-resistant, are likely to be hyperactivated in response to hyperinsulinaemia. This facet of Akt signalling may contribute to multiple features of the metabolic syndrome.

Insulin-sensitive obesity in humans - a 'favorable fat' phenotype?

In most humans, obesity and insulin resistance coexist. However, a unique group of obese individuals, who exhibit better insulin sensitivity than expected for their adiposity, has been the focus of recent research interest. We critically examine cross-sectional and lifestyle intervention studies in obese humans classified as 'insulin-sensitive' versus 'insulin-resistant' and review the few longitudinal studies comparing rates of cardiovascular disease, type 2 diabetes and all-cause mortality in these groups of individuals. We suggest that reduced deposition of fat, particularly of bioactive lipid intermediates, in muscle and liver is potentially protective. We propose that dynamic interventional studies in insulin-sensitive obese humans may increase understanding of the metabolic factors that play a role in obesity-associated insulin resistance in humans.

The effects of high-intensity intermittent exercise training on fat loss and fasting insulin levels of young women.

OBJECTIVE: To determine the effects of a 15-week high-intensity intermittent exercise (HIIE) program on subcutaneous and trunk fat and insulin resistance of young women. DESIGN AND PROCEDURES: Subjects were randomly assigned to one of the three groups: HIIE (n=15), steady-state exercise (SSE; n=15) or control (CONT; n=15). HIIE and SSE groups underwent a 15-week exercise intervention. SUBJECTS: Forty-five women with a mean BMI of 23.2+/-2.0 kg m(-2) and age of 20.2+/-2.0 years. RESULTS: Both exercise groups demonstrated a significant improvement (P<0.05) in cardiovascular fitness. However, only the HIIE group had a significant reduction in total body mass (TBM), fat mass (FM), trunk fat and fasting plasma insulin levels. There was significant fat loss (P<0.05) in legs compared to arms in the HIIE group only. Lean compared to overweight women lost less fat after HIIE. Decreases in leptin concentrations were negatively correlated with increases in VO(2peak) (r=-0.57, P<0.05) and positively correlated with decreases in TBM (r=0.47; P<0.0001). There was no significant change in adiponectin levels after training. CONCLUSIONS: HIIE three times per week for 15 weeks compared to the same frequency of SSE exercise was associated with significant reductions in total body fat, subcutaneous leg and trunk fat, and insulin resistance in young women.

Circulating fatty acids, non-high density lipoprotein cholesterol, and insulin-infused fat oxidation acutely influence whole body insulin sensitivity in nondiabetic men.

Circulating lipids and tissue lipid depots predict insulin sensitivity. Associations between fat oxidation and insulin sensitivity are variable. We examined whether circulating lipids and fat oxidation independently influence insulin sensitivity. We also examined interrelationships among circulating lipids, fat oxidation, and tissue lipid depots. Fifty-nine nondiabetic males (age, 45.4 +/- 2 yr; body mass index, 29.1 +/- 0.5 kg/m(2)) had fasting circulating nonesterified fatty acids (NEFAs) and lipids measured, euglycemic-hyperinsulinemic clamp for whole body insulin sensitivity [glucose infusion rate (GIR)], substrate oxidation, body composition (determined by dual energy x-ray absorptiometry), and skeletal muscle triglyceride (SMT) measurements. GIR inversely correlated with fasting NEFAs (r = -0.47; P = 0.0002), insulin-infused NEFAs (n = 38; r = -0.62; P < 0.0001), low-density lipoprotein cholesterol (r = -0.50; P < 0.0001), non-high-density lipoprotein cholesterol (r = -0.52; P < 0.0001), basal fat oxidation (r = -0.32; P = 0.03), insulin-infused fat oxidation (r = -0.40; P = 0.02), SMT (r = -0.28; P < 0.05), and central fat (percentage; r = -0.59; P < 0.0001). NEFA levels correlated with central fat, but not with total body fat or SMT. Multiple regression analysis showed non-high-density lipoprotein cholesterol, fasting NEFAs, insulin-infused fat oxidation, and central fat to independently predict GIR, accounting for approximately 60% of the variance. Circulating fatty acids, although closely correlated with central fat, independently predict insulin sensitivity. Insulin-infused fat oxidation independently predicts insulin sensitivity across a wide range of adiposity. Therefore, lipolytic regulation as well as amount of central fat are important in modulating insulin sensitivity.

Fat oxidation, body composition and insulin sensitivity in diabetic and normoglycaemic obese adults 5 years after weight loss.

OBJECTIVE: To investigate whether normal glucose-tolerant and type II diabetic overweight adults differ in response to weight regain with regard to substrate oxidation and metabolic parameters. METHODS: A total of 15 overweight-obese subjects: seven normal glucose tolerant (NGT) and eight with type II diabetes (DM) were restudied 5 y after significant weight loss. Prediet, after 28 days calorie restriction and at 5 y, subjects were characterised for weight, height, waist-to-hip ratio (WHR) and body composition by dual-energy X-ray absorptiometry. Fasting glucose, insulin, leptin and lipid levels were measured and subjects underwent euglycaemic-hyperinsulinaemic clamp (insulin 0.25 U/kg/h for 150 min). Indirect calorimetry was performed resting and in the final 30 min of the clamp. Dietary assessment was by 4-day diet-diary. RESULTS: Both NGT and DM groups regained weight at 5 y and were not different to prediet. Total body fat (%) and WHR were higher at 5 y compared to prediet in both groups. Fasting glucose was increased in NGT subjects at 5 y, and fasting insulin was higher in both groups at 5 y compared to prediet. Insulin sensitivity (GIR) was similar at 5 y compared to prediet, but at 5 y DM subjects were more insulin resistant than NGT subjects. At 5 y, both DM and NGT groups had significantly reduced basal fat oxidation and no significant suppression of fat oxidation with insulin. Clamp respiratory quotient levels at 5 y were significantly higher in NGT compared to DM subjects. CONCLUSION: Reduced basal fat oxidation, and reduced variation in substrate oxidation in response to insulin develop with fat regain and fasting hyperinsulinaemia in both NGT and DM obese adults.

Central fat predicts deterioration of insulin secretion index and fasting glycaemia: 6-year follow-up of subjects at varying risk of Type 2 diabetes mellitus.

AIMS: To examine the relationships between body composition and changes in fasting glycaemia, and in indices of insulin secretion and insulin action over 6 years in females with a family history of Type 2 diabetes with or without prior gestational diabetes ('at risk' group, AR) and control females (control group, C). METHODS: At baseline and at follow-up, an oral glucose tolerance test and dual energy X-ray absorptiometry assessment of body composition were performed. Indices of insulin resistance (HOMA R') and insulin secretion (HOMA beta') were obtained from fasting insulin and glucose concentrations. RESULTS: At baseline, the groups were similar for age, body mass index, fasting levels of plasma glucose and insulin, HOMA R' and HOMA beta'. Despite similar total body fatness, AR had significantly greater waist circumference and central fat (both P < 0.02) compared with C. At follow-up there was a significant increase in central adiposity only in AR, and the fasting plasma glucose (FPG) level was higher in AR compared with C (5.0 +/- 0.2 vs. 4.3 +/- 0.2 mmol/l, P = 0.02). This rise in plasma glucose in AR was related to a decline in HOMA beta' (r = 0.45, P = 0.0065). Both the baseline and the increments in total and central abdominal fat mass were associated with the time-related decline in HOMA beta'. CONCLUSIONS: Six years after initial assessment, AR showed deterioration in FPG levels due predominantly to a decline in insulin secretion index without major change in insulin resistance index. Importantly, baseline body fatness (especially central adiposity), as well as increases in fatness with time, were the major predictors of the subsequent decline of insulin secretion index and the consequent rise in FPG.

Independent influences of central fat and skeletal muscle lipids on insulin sensitivity.

OBJECTIVE: Insulin resistance is closely associated with two disparate aspects of lipid storage: the intracellular lipid content of skeletal muscle and the magnitude of central adipose beds. Our aim was to determine their relative contribution to impaired insulin action. RESEARCH METHODS AND PROCEDURES: Eighteen older (56 to 75 years of age) men were studied before elective knee surgery. Insulin sensitivity (M/Delta I) was determined by hyperinsulinemic-euglycemic clamp. Central abdominal fat (CF) was assessed by DXA. Skeletal muscle was excised at surgery and assayed for content of metabolically active long-chain acyl-CoA esters (LCAC). RESULTS: Significant inverse relationships were observed between LCAC and M/Delta I (R(2) = 0.34, p = 0.01) and between CF and M/Delta I (R(2) = 0.38, p = 0.006), but not between CF and LCAC (R(2) = 0.0005, p = 0.93). In a multiple regression model (R(2) = 0.71, p < 0.0001), both CF (p = 0.0006) and LCAC (p = 0.0009) were independent statistical predictors of M/Delta I. Leptin levels correlated inversely with M/Delta I (R(2) = 0.60, p = 0.0002) and positively with central (R(2) = 0.41, p = 0.006) and total body fat (R(2) = 0.63, p = 0.0001). DISCUSSION: The mechanisms by which altered lipid metabolism in skeletal muscle influences insulin action may not be related directly to those linking central fat and insulin sensitivity. In particular, it is unlikely that muscle accumulation of lipids directly derived from labile central fat depots is a principal contributor to peripheral insulin resistance. Instead, our results imply that circulating factors, other than nonesterified fatty acids or triglyceride, mediate between central fat depots and skeletal muscle tissue. Leptin was not exclusively associated with central fat, but other factors, secreted specifically from central fat cells, could modulate muscle insulin sensitivity.

Expression of genes involved in lipid metabolism correlate with peroxisome proliferator-activated receptor gamma expression in human skeletal muscle.

Peroxisome proliferator-activated receptor gamma (PPAR-gamma) activation in adipose tissue is known to regulate genes involved in adipocyte differentiation and lipid metabolism. However, the role of PPAR-gamma in muscle remains unclear. To examine the potential regulation of genes by PPAR-gamma in human skeletal muscle, we used semiquantitative RT-PCR to determine the expression of PPAR-gamma, lipoprotein lipase (LPL), muscle carnitine palmitoyl transferase-1 (mCPT1), fatty acid-binding protein (FABP), carnitine acylcarnitine transferase (CACT), and glucose transporter-4 (GLUT4) in freeze-dried muscle samples from 14 male subjects. These samples were dissected free of adipose and other tissue contamination, as confirmed by minimal or absent adipsin expression. Between individuals, the messenger ribonucleic acid concentration of PPAR-gamma varied up to 3-fold, whereas LPL varied up to 6.5-fold, mCPT1 13-fold, FABP 4-fold, CACT 4-fold, and GLUT4 up to 3-fold. The expression of LPL (r2 = 0.54; P = 0.003), mCPT1 (r2 = 0.42; P = 0.012), and FABP (r2 = 0.324; P = 0.034) all correlated significantly with PPAR-gamma expression in the same samples. No significant correlation was observed between the expression of CACT and PPAR-gamma or between GLUT4 and PPAR-gamma. These findings demonstrate a relationship between PPAR-gamma expression and the expression of other genes of lipid metabolism in muscle and support the hypothesis that PPAR-gamma activators such as the antidiabetic thiazolidinediones may regulate fatty acid metabolism in skeletal muscle as well as in adipose tissue.

Long-chain acyl-CoA esters as indicators of lipid metabolism and insulin sensitivity in rat and human muscle.

Long-chain acyl-CoAs (LCACoA) are an activated lipid species that are key metabolites in lipid metabolism; they also have a role in the regulation of other cellular processes. However, few studies have linked LCACoA content in rat and human muscle to changes in nutritional status and insulin action. Fasting rats for 18 h significantly elevated the three major LCACoA species in muscle (P < 0.001), whereas high-fat feeding of rats with a safflower oil (18:2) diet produced insulin resistance and increased total LCACoA content (P < 0.0001) by specifically increasing 18:2-CoA. The LCACoA content of red muscle from rats (4-8 nmol/g) was 4- to 10-fold higher than adipose tissue (0.4-0.9 nmol/g, P < 0.001), suggesting that any contamination of muscle samples with adipocytes would contribute little to the LCACoA content of muscle. In humans, the LCACoA content of muscle correlated significantly with a measure of whole body insulin action in 17 male subjects (r(2) = 0.34, P = 0.01), supporting a link between muscle lipid metabolism and insulin action. These results demonstrate that the LCACoA pool reflects lipid metabolism and nutritional state in muscle. We conclude that the LCACoA content of muscle provides a direct index of intracellular lipid metabolism and its links to insulin action, which, unlike triglyceride content, is not subject to contamination by closely associated adipose tissue.

Predicting the occurrence of diabetes mellitus in recipients of heart transplants.

AIMS: To establish the incidence of post-transplant diabetes mellitus (PTDM) and factors predictive of its development. METHODS: This was a retrospective review (using hospital records and transplant database) of 97 consecutive adult patients who underwent cardiac transplantation at St Vincent's Hospital, Sydney, Australia. RESULTS: Mean follow-up was 27 months. Excluding five patients who had pre-existing diabetes, the cumulative incidence of PTDM was 15.7%. Pre-transplant random blood glucose (5.6 +/- 0.8 vs. 5.2 +/- 0.6 mmol/l, P<0.05), family history (46% vs. 15%, P<0.05) and a continuing requirement for insulin on the second post-transplant day (54% vs. 15%, P< 0.01) differed in those who developed PTDM as opposed to those who remained free of diabetes. Patients who developed PTDM had received slightly higher mean doses of prednisolone at three months (0.21 +/- 0.03 vs. 0.19 +/- 0.03 mg. kg(-1)/day(-1), P<0.01). Of the factors identifiable prior to initial hospital discharge, only family history of diabetes mellitus and second post-transplant day insulin requirement independently predicted the occurrence of PTDM. CONCLUSIONS: A family history of diabetes and the need for insulin beyond the first 24 h after transplantation are factors identifiable prior to hospital discharge, which predict patients at risk of developing PTDM. In such patients, consideration to minimizing the dose of glucocorticoids should be given where possible.