BACKGROUND-Insulin resistance appears to be the best predictor of the development of diabetes in the children of patients with type 2 diabetes, but the mechanism responsible is unknown.

Insulin resistance is a major factor in the pathogenesis of type 2 diabetes in the elderly. To investigate how insulin resistance arises, we studied healthy, lean, elderly and young participants matched for lean body mass and fat mass. Elderly study participants were markedly insulinresistant as compared with young controls, and this resistance was attributable to reduced insulinstimulated muscle glucose metabolism. These changes were associated with increased fat accumulation in muscle and liver tissue assessed by 1 H nuclear magnetic resonance (NMR) spectroscopy, and with a ∼40% reduction in mitochondrial oxidative and phosphorylation activity, as assessed by in vivo 13 C/ 31 P NMR spectroscopy. These data support the hypothesis that an ageassociated decline in mitochondrial function contributes to insulin resistance in the elderly.Type 2 diabetes is the most common chronic metabolic disease in the elderly, affecting ∼30 million individuals 65 years of age or older in developed countries (1). The estimated economic burden of diabetes in the United States is ∼$100 billion per year, of which a substantial proportion can be attributed to persons with type 2 diabetes in the elderly age group (2). Epidemiological studies have shown that the transition from the normal state to overt type 2 diabetes in aging is typically characterized by a deterioration in glucose * To whom correspondence should be addressed. gerald.shulman@yale.edu. [11][12][13]. Increases in the intracellular concentration of fatty acid metabolites have been postulated to activate a serine kinase cascade leading to defects in insulin signaling in muscle (14-17) and the liver (18), which results in reduced insulinstimulated muscle glucose transport activity (14), reduced glycogen synthesis in muscle (19,20), and impaired suppression of glucose production by insulin in the liver (11-13).To examine whether insulin resistance in the elderly is associated with similar increases in intramyocellular and/or liver triglyceride content, we studied healthy elderly and young people that we matched for lean body mass (LBM) and fat mass. All study participants were non-smoking, sedentary, lean [body mass index (BMI) < 25 m 2 /kg], and taking no medications. Sixteen elderly volunteers (ages 61 to 84 years, 8 male and 8 female) were screened with a 3-hour oral glucose (75 g) tolerance test and underwent dual-energy x-ray absorptiometry to assess LBM and fat mass (21). One elderly man was excluded from the study because of an abnormal glucose profile. Thirteen young volunteers (ages 18 to 39 years, 6 male and 7 female), who had no family history of diabetes or hypertension, were matched to the older participants for BMI and habitual physical activity, which was assessed by means of an activity index questionnaire (22). All participants underwent a complete medical history and physical examination, as well as blood tests to confirm that they were in excellent health (23).Young and elderly participants had similar fat mass, percent fat mass, and LBM (Table 1...

Short term high fat feeding in rats results specifically in hepatic fat accumulation and provides a model of non-alcoholic fatty liver disease in which to study the mechanism of hepatic insulin resistance. Short term fat feeding (FF) caused a ϳ3-fold increase in liver triglyceride and total fatty acyl-CoA content without any significant increase in visceral or skeletal muscle fat content. Suppression of endogenous glucose production (EGP) by insulin was diminished in the FF group, despite normal basal EGP and insulin-stimulated peripheral glucose disposal. Hepatic insulin resistance could be attributed to impaired insulin-stimulated IRS-1 and IRS-2 tyrosine phosphorylation. These changes were associated with activation of PKC-⑀ and JNK1. Ultimately, hepatic fat accumulation decreased insulin activation of glycogen synthase and increased gluconeogenesis. Treatment of the FF group with low dose 2,4-dinitrophenol to increase energy expenditure abrogated the development of fatty liver, hepatic insulin resistance, activation of PKC-⑀ and JNK1, and defects in insulin signaling. In conclusion, these data support the hypothesis hepatic steatosis leads to hepatic insulin resistance by stimulating gluconeogenesis and activating PKC-⑀ and JNK1, which may interfere with tyrosine phosphorylation of IRS-1 and IRS-2 and impair the ability of insulin to activate glycogen synthase.

To examine the mechanism by which moderate weight reduction improves basal and insulin-stimulated rates of glucose metabolism in patients with type 2 diabetes, we used 1 H magnetic resonance spectroscopy to assess intrahepatic lipid (IHL) and intramyocellular lipid (IMCL) content in conjunction with hyperinsulinemiceuglycemic clamps using [6,6-2 H 2 ]glucose to assess rates of glucose production and insulin-stimulated peripheral glucose uptake. Eight obese patients with type 2 diabetes were studied before and after weight stabilization on a moderately hypocaloric very-low-fat diet (3%). The diabetic patients were markedly insulin resistant in both liver and muscle compared with the lean control subjects. These changes were associated with marked increases in IHL (12.2 ؎ 3.4 vs. 0.6 ؎ 0.1%; P ‫؍‬ 0.02) and IMCL (2.0 ؎ 0.3 vs. 1.2 ؎ 0.1%; P ‫؍‬ 0.02) compared with the control subjects. A weight loss of only ϳ8 kg resulted in normalization of fasting plasma glucose concentrations (8.8 ؎ 0.5 vs. 6.4 ؎ 0.3 mmol/l; P < 0.0005), rates of basal glucose production (193 ؎ 7 vs. 153 ؎ 10 mg/min; P < 0.0005), and the percentage suppression of hepatic glucose production during the clamp (29 ؎ 22 vs. 99 ؎ 3%; P ‫؍‬ 0.003). These improvements in basal and insulin-stimulated hepatic glucose metabolism were associated with an 81 ؎ 4% reduction in IHL (P ‫؍‬ 0.0009) but no significant change in insulin-stimulated peripheral glucose uptake or IMCL (2.0 ؎ 0.3 vs. 1.9 ؎ 0.3%; P ‫؍‬ 0.21). In conclusion, these data support the hypothesis that moderate weight loss normalizes fasting hyperglycemia in patients with poorly controlled type 2 diabetes by mobilizing a relatively small pool of IHL, which reverses hepatic insulin resistance and normalizes rates of basal glucose production, independent of any changes in insulin-stimulated peripheral glucose metabolism. Diabetes 54:603-608, 2005 P revious studies have demonstrated that relatively modest weight reduction in obese patients with poorly controlled type 2 diabetes can markedly reduce plasma glucose concentrations, but the mechanism responsible for this phenomenon is not known (1). Henry et al. (1) showed that a weight loss of 16.8 Ϯ 2.7 kg led to a reduction in fasting plasma glucose concentrations from 15.3 Ϯ 1.2 to 6.8 Ϯ 0.4 mmol/l and that the individual fasting glucose concentrations were highly correlated with rates of basal hepatic glucose production.We hypothesized that a relatively small pool of intrahepatic lipid (IHL) might be responsible for the hepatic insulin resistance and increased rates of glucose production in patients with poorly controlled type 2 diabetes and that hepatic steatosis and hepatic insulin resistance would reverse with modest weight reduction before any changes in peripheral insulin resistance and intramyocellular lipid (IMCL) content.To test these hypotheses, we used 1 H magnetic resonance spectroscopy (MRS) to noninvasively assess IHL and IMCL content in eight obese type 2 diabetic patients before and after weight stabilization on a hypocaloric diet,...

To further explore the nature of the mitochondrial dysfunction and insulin resistance that occur in the muscle of young, lean, normoglycemic, insulin-resistant offspring of parents with type 2 diabetes (IR offspring), we measured mitochondrial content by electron microscopy and insulin signaling in muscle biopsy samples obtained from these individuals before and during a hyperinsulinemic-euglycemic clamp. The rate of insulin-stimulated muscle glucose uptake was approximately 60% lower in the IR offspring than the control subjects and was associated with an approximately 60% increase in the intramyocellular lipid content as assessed by 1 H magnetic resonance spectroscopy. Muscle mitochondrial density was 38% lower in the IR offspring. These changes were associated with a 50% increase in IRS-1 Ser312 and IRS-1 Ser636 phosphorylation and an approximately 60% reduction in insulin-stimulated Akt activation in the IR offspring. These data provide new insights into the earliest defects that may be responsible for the development of type 2 diabetes and support the hypothesis that reductions in mitochondrial content result in decreased mitochondrial function, which predisposes IR offspring to intramyocellular lipid accumulation, which in turn activates a serine kinase cascade that leads to defects in insulin signaling and action in muscle. IntroductionRecent magnetic resonance spectroscopy (MRS) studies have revealed increased intramyocellular lipid content associated with reduced mitochondrial phosphorylation activity in the muscle of young, lean, normoglycemic, insulin-resistant offspring of parents with type 2 diabetes (IR offspring) (1). These data suggest a potential role of mitochondrial dysfunction in the pathogenesis of insulin resistance and type 2 diabetes; however, the underlying mechanism responsible for this reduced mitochondrial activity remains unknown.Increases in the intramyocellular concentration of fatty acid metabolites have been postulated to activate a serine kinase cascade, causing increased phosphorylation of IRS-1 on critical serine sites, which blocks insulin receptor phosphorylation of IRS-1 on tyrosine sites. This results in reduced insulin-stimulated IRS-1-associated PI3K activity (2-5), decreased insulin-stimulated glucose transport activity (3), and reduced muscle glycogen synthesis (6, 7). However, there is currently little evidence that serine phosphorylation of IRS-1 is a key molecular event for this process in humans or whether or not there are associated alterations in insu-