To examine the mechanism by which lipids cause insulin resistance in humans, skeletal muscle glycogen and glucose-6-phosphate concentrations were measured every 15 min by simultaneous 13 C and 31 P nuclear magnetic resonance spectroscopy in nine healthy subjects in the presence of low (0.18 Ϯ 0.02 mM [mean Ϯ SEM]; control) or high (1.93 Ϯ 0.04 mM; lipid infusion) plasma free fatty acid levels under euglycemic ( ‫ف‬ 5.2 mM) hyperinsulinemic ( ‫ف‬ 400 pM) clamp conditions for 6 h. During the initial 3.5 h of the clamp the rate of whole-body glucose uptake was not affected by lipid infusion, but it then decreased continuously to be ‫ف‬ 46% of control values after 6 h ( P Ͻ 0.00001). Augmented lipid oxidation was accompanied by a ‫ف‬ 40% reduction of oxidative glucose metabolism starting during the third hour of lipid infusion ( P Ͻ 0.05). Rates of muscle glycogen synthesis were similar during the first 3 h of lipid and control infusion, but thereafter decreased to ‫ف‬ 50% of control values (4.0 Ϯ 1.0 vs. 9.3 Ϯ 1.6 mol/[kg и min], P Ͻ 0.05). Reduction of muscle glycogen synthesis by elevated plasma free fatty acids was preceded by a fall of muscle glucose-6-phosphate concentrations starting at ‫ف‬ 1.5 h (195 Ϯ 25 vs. control: 237 Ϯ 26 M; P Ͻ 0.01). Therefore in contrast to the originally postulated mechanism in which free fatty acids were thought to inhibit insulin-stimulated glucose uptake in muscle through initial inhibition of pyruvate dehydrogenase these results demonstrate that free fatty acids induce insulin resistance in humans by initial inhibition of glucose transport/phosphorylation which is then followed by an ‫ف‬ 50% reduction in both the rate of muscle glycogen synthesis and glucose oxidation. ( J. Clin. Invest. 1996. 97:2859-2865.) Key words: free fatty acids • muscle glycogen • glucose transport • nuclear magnetic resonance spectroscopy • glucose-6-phosphate

Insulin resistance is the best prediction factor for the clinical onset of type 2 diabetes. It was suggested that intramuscular triglyceride store may be a primary pathogenic factor for its development. To test this hypothesis, 14 young lean offspring of type 2 diabetic parents, a model of in vivo insulin resistance with increased risk to develop diabetes, and 14 healthy subjects matched for anthropomorphic parameters and life habits were studied with 1) euglycemic-hyperinsulinemic clamp to assess whole body insulin sensitivity, 2) localized 1H nuclear magnetic resonance (NMR) spectroscopy of the soleus (higher content of fiber type I, insulin sensitive) and tibialis anterior (higher content of fiber type IIb, less insulin sensitive) muscles to assess intramyocellular triglyceride content, 3) 13C NMR of the calf subcutaneous adipose tissue to assess composition in saturated/unsaturated carbons of triglyceride fatty acid chains, and 4) dual X-ray energy absorption to assess body composition. Offspring of diabetic parents, notwithstanding normal fat content and distribution, were characterized by insulin resistance and increased intramyocellular triglyceride content in the soleus (P < 0.01) but not in the tibialis anterior (P = 0.19), but showed a normal content of saturated/unsaturated carbons in the fatty acid chain of subcutaneous adipocytes. Stepwise regression analysis selected intramyocellular triglyceride soleus content and plasma free fatty acid levels as the main predictors of whole body insulin sensitivity. In conclusion, 1H and 13C NMR spectroscopy revealed intramyocellular abnormalities of lipid metabolism associated with whole body insulin resistance in subjects at high risk of developing diabetes, and might be useful tools for noninvasively monitoring these alterations in diabetes and prediabetic states.

Exercise increases insulin sensitivity in both normal subjects and the insulin-resistant offspring of diabetic parents because of a twofold increase in insulin-stimulated glycogen synthesis in muscle, due to an increase in insulin-stimulated glucose transport-phosphorylation.

OBJECTIVESome obese individuals have normal insulin sensitivity. It is controversial whether this phenotype is associated with increased all-cause mortality risk.RESEARCH DESIGN AND METHODSFifteen-year all-cause mortality data were obtained through the Regional Health Registry for 2,011 of 2,074 Caucasian middle-aged individuals of the Cremona Study, a population study on the prevalence of diabetes in Italy. Individuals were divided in four categories according to BMI (nonobese: <30 kg/m2; obese: ≥30 kg/m2) and estimated insulin resistance (insulin sensitive: homeostasis model assessment of insulin resistance <2.5; insulin resistant ≥2.5).RESULTSObese insulin-sensitive subjects represented 11% (95% CI 8.1–14.5) of the obese population. This phenotype had similar BMI but lower waist circumference, blood pressure, fasting glucose, triglycerides, and fibrinogen and higher HDL cholesterol than obese insulin-resistant subjects. In the 15-year follow-up, 495 deaths (cardiovascular disease [CVD]: n = 221; cancer: n = 180) occurred. All-cause mortality adjusted for age and sex was higher in the obese insulin-resistant subjects (hazard ratio 1.40 [95% CI 1.08–1.81], P = 0.01) but not in the obese insulin-sensitive subjects (0.99 [0.46–2.11], P = 0.97) when compared with nonobese insulin-sensitive subjects. Also, mortality for CVD and cancer was higher in the obese insulin-resistant subjects but not in the obese insulin-sensitive subjects when compared with nonobese insulin-sensitive subjects.CONCLUSIONSIn contrast to obese insulin-resistant subjects, metabolically healthy obese individuals are less common than previously thought and do not show increased all-cause, cancer, and CVD mortality risks in a 15-year follow-up study.

A fatty liver, which is a common feature in insulin-resistant states, can lead to chronic liver disease. It has been hypothesized that a fatty liver can also increase the rates of non-hepaticrelated morbidity and mortality. Therefore, we wanted to determine whether the fatty liver index (FLI), a surrogate marker and a validated algorithm derived from the serum triglyceride level, body mass index, waist circumference, and c-glutamyltransferase level, was associated with the prognosis in a population study. The 15-year all-cause, hepatic-related, cardiovascular disease (CVD), and cancer mortality rates were obtained through the Regional Health Registry in 2011 for 2074 Caucasian middle-aged individuals in the Cremona study, a population study examining the prevalence of diabetes mellitus in Italy. During the 15-year observation period, 495 deaths were registered: 34 were hepatic-related, 221 were CVD-related, 180 were cancer-related, and 60 were attributed to other causes. FLI was independently associated with the hepatic-related deaths (hazard ratio 5 1.04, 95% confidence interval 5 1.02-1.05, P < 0.0001). Age, sex, FLI, cigarette smoking, and diabetes were independently associated with all-cause mortality. Age, sex, FLI, systolic blood pressure, and fibrinogen were independently associated with CVD mortality; meanwhile, age, sex, FLI, and smoking were independently associated with cancer mortality. FLI correlated with the homeostasis model assessment of insulin resistance (HOMA-IR), a surrogate marker of insulin resistance (Spearman's q 5 0.57, P < 0.0001), and when HOMA-IR was included in the multivariate analyses, FLI retained its association with hepatic-related mortality but not with all-cause, CVD, and cancer-related mortality. Conclusion: FLI is independently associated with hepatic-related mortality. It is also associated with all-cause, CVD, and cancer mortality rates, but these associations appear to be tightly interconnected with the risk conferred by the correlated insulin-resistant state. (HEPATOLOGY 2011;54:145-152) See Editorial on Page 6 N onalcoholic fatty liver disease (NAFLD) is common in insulin-resistant subjects 1 and affects 20% to 30% of the adult population and more than 50% of overweight and obese individuals. 2

OBJECTIVE -Fatty liver may be involved in the pathogenesis of type 2 diabetes. Physical exercise is a tool to improve insulin sensitivity, but little is known about its effect on intrahepatic fat (IHF) content. The purpose of this study was to examine the association of habitual physical activity, insulin resistance, and adiponectin with IHF content.RESEARCH DESIGN AND METHODS -Participants were 191 (77 female and 114 male) apparently healthy, nonalcoholic individuals (aged 19 -62 years; BMI 17.0 -35.5 kg/m 2 ). IHF content was assessed in a quantitative fashion and noninvasively as a continuous variable by means of 1 H magnetic resonance spectroscopy (MRS), and habitual physical activity was assessed by means of a questionnaire. Fatty liver was defined as IHF content of Ͼ5% wet weight, and insulin sensitivity was estimated using the computer homeostasis model assessment (HOMA)-2 indexes.RESULTS -A reduced prevalence of fatty liver in the quartile of the most physically active individuals (25,11,25, and 2% in quartile 1, 2, 3, and 4, respectively; 2 ϭ 15.63; P ϭ 0.001) was found along with an inverse correlation between the physical activity index and the IHF content when plotted as continuous variables (Pearson's r ϭ Ϫ0.27; P Ͻ 0.000). This association was not attenuated when adjusted for age, sex, BMI, HOMA-2, and adiponectin (partial correlation r ϭ Ϫ0.25; P Ͻ 0.001).CONCLUSIONS -This study demonstrated that a higher level of habitual physical activity is associated with a lower IHF content and suggested that this relationship may be due to the effect of exercise per se. Diabetes Care 30:683-688, 2007A lanine aminotransferase and ␥-glutamyltransferase are associated with type 2 diabetes risk (1-3), and it is thought that the link is represented by the intrahepatic fat (IHF) content. Ectopic fat accumulation within the liver, in fact, has been reported in association with impairment of insulin-stimulated glucose metabolism, of suppression of endogenous glucose production, and of whole-body lipolysis in nondiabetic individuals with nonalcoholic fatty liver disease (NAFLD) (4,5). Additional results also demonstrated that decreased levels of circulating adiponectin in NAFLD are related to hepatic insulin sensitivity and to the IHF content, suggesting that hypoadiponectinemia may be involved in excessive hepatic fat accumulation (6).Physical exercise was found to be associated with a reduced risk of development of type 2 diabetes (7,8) and is a wellrecognized tool to improve insulin sensitivity at the level of the skeletal muscle (9). However, whether physical exercise may affect insulin sensitivity and diabetes risk via an effect on the IHF content and adiponectin remains unknown.The IHF content may be assessed as a continuous variable by means of 1 H magnetic resonance spectroscopy (MRS) (10), and recently this technique was found to be a sensitive, quantitative, and noninvasive method also when applied to a large population (11) without the need for the use of a more invasive approach such as liver biopsy. The ...

Hepatic glycogen concentration was measured in six subjects with insulin-dependent diabetes mellitus (IDDM) and nine weight-matched control subjects using '3C nuclear magnetic resonance spectroscopy during a day in which three isocaloric mixed meals were ingested. The relative fluxes of the direct and indirect (3 carbon units -+ -+ glycogen) pathways of hepatic glycogen synthesis were also assessed using [1-3C]glucose in combination with acetaminophen to noninvasively sample the hepatic UDP-glucose pool. Mean fasting hepatic glycogen content was similar in the two groups. After each meal, hepatic glycogen content increased, peaking 4-5 h after the meal in both groups. By 11:00 p.m. the IDDM subjects had synthesized only 30% of the glycogen that was synthesized by the control group [IDDM subjects, net increment = 44±20 (mean±SE) mM; control subjects, net increment = 144±14 mM; P < 0.05]. After breakfast the flux through the gluconeogenic pathway relative to the direct pathway of hepatic glycogen synthesis was 1.7-fold greater in the IDDM subjects (59±4%) than in the control subjects (35±4%, P < 0.0003). In conclusion, under mixed meal conditions, subjects with poorly controlled IDDM have a major defect in net hepatic glycogen synthesis and augmented hepatic gluconeogenesis. The former abnormality may result in an impaired glycemic response to counterregulatory hormones, whereas both abnormalities may contribute to postprandial hyperglycemia. (J. Clin. Invest. 1995. 95:783-787.)

Insulin resistance is a key pathogenic factor of type 2 diabetes (T2DM); in contrast, in type 1 diabetes (T1DM) it is considered a secondary alteration. Increased intramyocellular lipid (IMCL) content accumulation and reduced plasma adiponectin were suggested to be pathogenic events of insulin resistance in T2DM. This study was designed to assess whether IMCL content and plasma adiponectin were also associated with the severity of insulin resistance in T1DM. We studied 18 patients with T1DM, 7 older and overweight/obese patients with T2DM, and 15 nondiabetic, insulin-resistant offspring of T2DM parents (OFF) and 15 healthy individuals (NOR) as appropriate control groups matched for anthropometric features with T1DM patients by means of the euglycemic hyperinsulinemic clamp combined with the infusion of [6,6-2 H2]glucose and 1 H magnetic resonance spectroscopy of the calf muscles. T1DM and T2DM patients showed reduced insulin-stimulated glucose metabolic clearance rate (MCR: 5.1 Ϯ 0.6 and 3.2 Ϯ 0.8 ml ⅐ kg Ϫ1 ⅐ min Ϫ1 ) similar to OFF (5.3 Ϯ 0.4 ml ⅐ kg Ϫ1 ⅐ min Ϫ1 ) compared with NOR (8.5 Ϯ 0.5 ml ⅐ kg Ϫ1 ⅐ min Ϫ1 , P Ͻ 0.001). Soleus IMCL content was increased in T1DM (112 Ϯ 15 AU), T2DM (108 Ϯ 10 AU) and OFF (82 Ϯ 13 AU) compared with NOR (52 Ϯ 7 AU, P Ͻ 0.05) and the result was inversely proportional to the MCR (R 2 ϭ 0.27, P Ͻ 0.001); an association between IMCL content and Hb A1c was found only in T1DM (R 2 ϭ 0.57, P Ͻ 0.001). Fasting plasma adiponectin was reduced in T2DM (7 Ϯ 1 g/ml, P ϭ 0.01) and OFF (11 Ϯ 1 g/ml, P ϭ 0.03) but not in T1DM (25 Ϯ 6 g/ml), whose plasma level was increased with respect to both OFF (P ϭ 0.03) and NOR (16 Ϯ 2 g/ml, P ϭ 0.05). In conclusion, in T1DM, T2DM, and OFF, IMCL content was associated with insulin resistance, demonstrating that IMCL accretion is a marker of insulin resistance common to both primary genetically determined and secondary metabolic (chronic hyperglycemia) alterations. The increased adiponectin levels in insulin-resistant patients with T1DM, in contrast to the reduced levels found in patients with T2DM and in OFF, demonstrated that the relationship of adiponectin to insulin resistance in humans is still unclear. intramyocellular triglyceride content; leptin; magnetic resonance spectroscopy