Increased plasma free fatty acid (FFA) concentrations are typically associated with many insulin-resistant states including obesity and type 2 diabetes mellitus (1-3). Furthermore, raising plasma FFA levels in healthy humans, by triglyceride/heparin infusions, can also acutely induce insulin resistance (4-11). Over thirty years ago, Randle et al. (12,13) demonstrated that FFAs compete with glucose for oxidation in isolated rat heart and diaphragmatic muscle preparations, and they speculated that increased fat oxidation may cause the insulin resistance associated with diabetes and obesity. They proposed that increased FFA oxidation leads to an increase in the intramitochondrial acetyl-coenzyme A (acetyl-CoA) and reduced/oxidized nicotinamide adenine dinucleotide (NADH/NAD + ) ratios, resulting in inactivation of pyruvate dehydrogenase activity. The consequent increase in intracellular citrate concentration causes inhibition of phosphofructokinase resulting in an increase in glucose-6-phosphate levels. The elevated glucose-6-phosphate levels would inhibit hexokinase II activity and then lead to decreased glucose uptake. However, recent studies by our group (14) and others (15,16) have called this mechanism into question. Boden and coworkers have shown that a reduction in carbohydrate oxidation was responsible for only one-third of the fatty acid-dependent decrease in glucose uptake, while impaired non-oxidative glucose metabolism accounted for the remainder (16). These workers suggested that two different defects might contribute to the impairment in nonoxidative glucose metabolism. At FFA concentrations of ∼0.75 mM, they found an increase in glucose-6-phosphate concentrations in muscle biopsies, suggesting an inhibitory effect of FFA on glycogen synthase activity, whereas at lower FFA concentrations (∼0.50 mM) they observed no difference in intramuscular glucose-6-phosphate concentration. In contrast, using carbon-13/phosphorous-31 nuclear magnetic resonance (NMR) spectroscopy under increased plasma FFA concentrations (∼1.8 mM), we observed a decrease in intramuscular glucose-6-phosphate concentration associated with a 50% reduction in insulin-stimulated muscle glycogen synthesis (14). These data suggest that acute elevations in plasma FFA levels in humans cause insulin resistance by initial inhibition of glucose transport and/or phosphorylation activity that is concurrently followed by a reduction in the rate of both muscle glycogen synthesis and glucose oxidation. Because glucose-6-phosphate (and not intracellular glucose) concentration was measured, it was not possible to distinguish between To examine the mechanism by which free fatty acids (FFA) induce insulin resistance in human skeletal muscle, glycogen, glucose-6-phosphate, and intracellular glucose concentrations were measured using carbon-13 and phosphorous-31 nuclear magnetic resonance spectroscopy in seven healthy subjects before and after a hyperinsulinemic-euglycemic clamp following a five-hour infusion of either lipid/heparin or glycerol/heparin. I...

To examine the mechanism by which metformin lowers endogenous glucose production in type 2 diabetic patients, we studied seven type 2 diabetic subjects, with fasting hyperglycemia (15.5 ± 1.3 mmol/l), before and after 3 months of metformin treatment. Seven healthy subjects, matched for sex, age, and BMI, served as control subjects. Rates of net hepatic glycogenolysis, estimated by 13 C nuclear magnetic resonance spectroscopy, were combined with estimates of contributions to glucose production of gluconeogenesis and glycogenolysis, measured by labeling of blood glucose by 2 H from ingested 2 H 2 O. Glucose production was measured using [6,6-2 H 2 ]glucose. The rate of glucose production was twice as high in the diabetic subjects as in control subjects (0.70 ± 0.05 vs. 0.36 ± 0.03 mmol · m -2 · min -1 , P < 0.0001). Metformin reduced that rate by 24% (to 0.53 ± 0.03 mmol · m -2 · min -1 , P = 0.0009) and fasting plasma glucose concentration by 30% (to 10.8 ± 0.9 mmol/l, P = 0.0002). The rate of gluconeogenesis was three times higher in the diabetic subjects than in the control subjects (0.59 ± 0.03 vs. 0.18 ± 0.03 mmol · m -2 · min -1 ) and metformin reduced that rate by 36% (to 0.38 ± 0.03 mmol · m -2 · min -1 , P = 0.01). By the 2 H 2 O method, there was a twofold increase in rates of gluconeogenesis in diabetic subjects (0.42 ± 0.04 mmol · m -2 · min -1 ), which decreased by 33% after metformin treatment (0.28 ± 0.03 mmol · m -2 · min -1 , P = 0.0002). There was no glycogen cycling in the control subjects, but in the diabetic subjects, glycogen cycling contributed to 25% of glucose production and explains the differences between the two methods used. In conclusion, patients with poorly controlled type 2 diabetes have increased rates of endogenous glucose production, which can be attributed to increased rates of gluconeogenesis. Metformin lowered the rate of glucose production in these patients through a reduction in gluconeogenesis. A lthough it is generally agreed that metformin reduces fasting plasma glucose concentrations by reducing rates of hepatic glucose production (1,2), its effect on the relative contributions of hepatic glycogenolysis and gluconeogenesis remains controversial. Some studies conclude that metformin works mostly by reducing rates of gluconeogenesis (3); others, that it works by reducing rates of hepatic glycogenolysis (4,5).Because of limitations of the methods used in the previous studies to assess gluconeogenesis and glycogenolysis, we used two independent and complementary methods to assess these processes in patients with poorly controlled type 2 diabetes before and after 3 months of metformin therapy. 13C nuclear magnetic resonance (NMR) spectroscopy was used to directly measure rates of net hepatic glycogenolysis, in combination with [6,6-2 H 2 ]glucose administration, to calculate the rates of endogenous glucose production (6). Rates of gluconeogenesis were estimated by subtracting the rates of net hepatic glycogenolysis from the rates of endogenous glucose production. In addition...

No abstract

Obesity and dysfunctional energy partitioning can lead to the development of insulin resistance and type 2 diabetes. The antidiabetic thiazolidinediones shift the energy balance toward storage, leading to an increase in whole-body adiposity. These studies examine the effects of pioglitazone (Pio) on adipose tissue physiology, accumulation, and distribution in female Zucker (fa/fa) rats. Pio treatment (up to 28 days) decreased the insulin-resistant and hyperlipidemic states and increased food consumption and whole-body adiposity. Magnetic resonance imaging (MRI) analysis and weights of fat pads demonstrated that the increase in adiposity was not only limited to the major fat depots but also to fat deposition throughout the body. Adipocyte sizing profiles, fat pad histology, and DNA content show that Pio treatment increased the number of small adipocytes because of both the appearance of new adipocytes and the shrinkage and/or disappearance of existing mature adipocytes. The remodeling was time dependent, with new small adipocytes appearing in clusters throughout the fat pad, and accompanied by a three- to fourfold increase in citrate synthase and fatty acid synthase activity. The appearance of new fat cells and the increase in fat mass were depot specific, with a rank order of responsiveness of ovarian > retroperitoneal > subcutaneous. This differential depot effect resulted in a redistribution of the fat mass in the abdominal region such that there was an increase in the visceral:subcutaneous ratio, as confirmed by MRI analysis. Although the increased adiposity is paradoxical to an improvement in insulin sensitivity, the quantitative increase of adipose mass should be viewed in context of the qualitative changes in adipose tissue, including the remodeling of adipocytes to a smaller size with higher lipid storage potential. This shift in energy balance is likely to result in lower circulating free fatty acid levels, ultimately improving insulin sensitivity and the metabolic state.

Sarcopenia, the age-related skeletal muscle decline, is associated with relevant clinical and socioeconomic negative outcomes in older persons. The study of this phenomenon and the development of preventive/therapeutic strategies represent public health priorities. The present document reports the results of a recent meeting of the International Working Group on Sarcopenia (a task force consisting of geriatricians and scientists from academia and industry) held on June 7–8, 2011 in Toulouse (France). The meeting was specifically focused at gaining knowledge on the currently available biomarkers (functional, biological, or imaging-related) that could be utilized in clinical trials of sarcopenia and considered the most reliable and promising to evaluate age-related modifications of skeletal muscle. Specific recommendations about the assessment of aging skeletal muscle in older people and the optimal methodological design of studies on sarcopenia were also discussed and finalized. Although the study of skeletal muscle decline is still in a very preliminary phase, the potential great benefits derived from a better understanding and treatment of this condition should encourage research on sarcopenia. However, the reasonable uncertainties (derived from exploring a novel field and the exponential acceleration of scientific progress) require the adoption of a cautious and comprehensive approach to the subject.