Plasma concentrations of amino acids are frequently elevated in insulin-resistant states, and a protein-enriched diet can impair glucose metabolism. This study examined effects of short-term plasma amino acid (AA) elevation on whole-body glucose disposal and cellular insulin action in skeletal muscle. Seven healthy men were studied for 5.5 h during euglycemic (5.5 mmol/l), hyperinsulinemic (430 pmol/l), fasting glucagon (65 ng/l), and growth hormone (0.4 μg/l) somatostatin clamp tests in the presence of low (∼1.6 mmol/l) and increased (∼4.6 mmol/l) plasma AA concentrations. Glucose turnover was measured with d-[6,6-²H₂]glucose. Intramuscular concentrations of glycogen and glucose-6-phosphate (G6P) were monitored using ¹³C and ³¹P nuclear magnetic resonance spectroscopy, respectively. A ∼2.1-fold elevation of plasma AAs reduced whole-body glucose disposal by 25% (P < 0.01). Rates of muscle glycogen synthesis decreased by 64% (180–315 min, 24 ± 3; control, 67 ± 10 μmol · l⁻¹ · min⁻¹; P < 0.01), which was accompanied by a reduction in G6P starting at 130 min (ΔG6P_{260–300 min}, 18 ± 19; control, 103 ± 33 μmol/l; P < 0.05). In conclusion, plasma amino acid elevation induces skeletal muscle insulin resistance in humans by inhibition of glucose transport/phosphorylation, resulting in marked reduction of glycogen synthesis.

Decreased skeletal muscle glucose disposal and increased endogenous glucose production (EGP) contribute to postprandial hyperglycemia in type 2 diabetes, but the contribution of hepatic glycogen metabolism remains uncertain. Hepatic glycogen metabolism and EGP were monitored in type 2 diabetic patients and nondiabetic volunteer control subjects (CON) after mixed meal ingestion and during hyperglycemic-hyperinsulinemic-somatostatin clamps applying ¹³C nuclear magnetic resonance spectroscopy (NMRS) and variable infusion dual-tracer technique. Hepatocellular lipid (HCL) content was quantified by ¹H NMRS. Before dinner, hepatic glycogen was lower in type 2 diabetic patients (227 ± 6 vs. CON: 275 ± 10 mmol/l liver, P < 0.001). After meal ingestion, net synthetic rates were 0.76 ± 0.16 (type 2 diabetic patients) and 1.36 ± 0.15 mg · kg⁻¹ · min⁻¹ (CON, P < 0.02), resulting in peak concentrations of 283 ± 15 and 360 ± 11 mmol/l liver. Postprandial rates of EGP were ∼0.3 mg · kg⁻¹ · min⁻¹ (30–170 min; P < 0.05 vs. CON) higher in type 2 diabetic patients. Under clamp conditions, type 2 diabetic patients featured ∼54% lower (P < 0.03) net hepatic glycogen synthesis and ∼0.5 mg · kg⁻¹ · min⁻¹ higher (P < 0.02) EGP. Hepatic glucose storage negatively correlated with HCL content (R = −0.602, P < 0.05). Type 2 diabetic patients exhibit 1) reduction of postprandial hepatic glycogen synthesis, 2) temporarily impaired suppression of EGP, and 3) no normalization of these defects by controlled hyperglycemic hyperinsulinemia. Thus, impaired insulin sensitivity and/or chronic glucolipotoxicity in addition to the effects of an altered insulin-to-glucagon ratio or increased free fatty acids accounts for defective hepatic glycogen metabolism in type 2 diabetic patients.

The nutrient-sensitive kinase mammalian target of rapamycin (mTOR) and its downstream target S6 kinase (S6K) are involved in amino acid–induced insulin resistance. Whether the mTOR/S6K pathway directly modulates glucose metabolism in humans is unknown. We studied 11 healthy men (29 years old, BMI 23 kg/m2) twice in random order after oral administration of 6 mg rapamycin, a specific mTOR inhibitor, or placebo. An amino acid mixture was infused to activate mTOR, and somatostatin-insulin-glucose clamps created conditions of low peripheral hyperinsulinemia (∼100 pmol/l, 0–180 min) and prandial-like peripheral hyperinsulinemia (∼450 pmol/l, 180–360 min). Glucose turnover was assessed using d-[6,6-2H2]glucose infusion (n = 8). Skeletal muscle biopsies were performed at baseline and during prandial-like peripheral hyperinsulinemia (n = 3). At low peripheral hyperinsulinemia, whole-body glucose uptake was not affected by rapamycin. During prandial-like peripheral hyperinsulinemia, rapamycin increased glucose uptake compared with placebo by 17% (Rd|300–360 min, 75 ± 5 vs. 64 ± 5 μmol · kg−1 · min−1, P = 0.0008). Rapamycin affected endogenous glucose production neither at baseline nor during low or prandial-like peripheral hyperinsulinemia. Combined hyperaminoacidemia and prandial-like hyperinsulinemia increased S6K phosphorylation and inhibitory insulin receptor substrate-1 (IRS-1) phosphorylation at Ser312 and Ser636 in the placebo group. Rapamycin partially inhibited this increase in mTOR-mediated S6K phosphorylation and IRS-1 Ser312 and Ser636 phosphorylation. In conclusion, rapamycin stimulates insulin-mediated glucose uptake in man under conditions known to activate the mTOR/S6K pathway.

BACKGROUND: Growth differentiation factor-15 (GDF-15) is a stress-responsive cytokine linked to obesity comorbidities such as cardiovascular disease, inflammation, and cancer. GDF-15 also has adipokine properties and recently emerged as a prognostic biomarker for cardiovascular events. METHODS: We evaluated the relationship of plasma GDF-15 concentrations with parameters of obesity, inflammation, and glucose and lipid metabolism in a cohort of 118 morbidly obese patients [mean (SD) age 37.2 (12) years, 89 females, 29 males] and 30 age- and sex-matched healthy lean individuals. All study participants underwent a 75-g oral glucose tolerance test; 28 patients were studied before and 1 year after Roux-en-Y gastric bypass surgery. RESULTS: Obese individuals displayed increased plasma GDF-15 concentrations (P < 0.001), with highest concentrations observed in patients with type 2 diabetes. GDF-15 was positively correlated with age, waist-to-height ratio, mean arterial blood pressure, triglycerides, creatinine, glucose, insulin, C-peptide, hemoglobin A_1c, and homeostatic model assessment insulin resistance index and negatively correlated with oral glucose insulin sensitivity. Age, homeostatic model assessment index, oral glucose insulin sensitivity, and creatinine were independent predictors of GDF-15 concentrations. Roux-en-Y gastric bypass led to a significant reduction in weight, leptin, insulin, and insulin resistance, but further increased GDF-15 concentrations (P < 0.001). CONCLUSIONS: The associations between circulating GDF-15 concentrations and age, insulin resistance, and creatinine might account for the additional cardiovascular predictive information of GDF-15 compared to traditional risk factors. Nevertheless, GDF-15 changes following bariatric surgery suggest an indirect relationship between GDF-15 and insulin resistance. The clinical utility of GDF-15 as a biomarker might be limited until the pathways directly controlling GDF-15 concentrations are better understood.

Insulin resistance is frequently associated with increased lipid content in muscle and liver. Insulin excess stimulates tissue lipid accumulation. To examine the effects of insulin and improved glycemia on insulin sensitivity and intracellular lipids, we performed stepped (1, 2, and 4 mU · min⁻¹ · kg⁻¹) hyperinsulinemic-euglycemic clamps in eight type 2 diabetic and six nondiabetic control subjects at baseline and after 12 and 67 h of insulin-mediated near-normoglycemia (118 ± 7 mg/dl). Intrahepatocellular lipids (IHCLs) and intramyocellular lipids (IMCLs) of soleus (IMCL-S) and tibialis anterior muscle (IMCL-TA) were measured with ¹H nuclear magnetic resonance spectroscopy. At baseline, nondiabetic subjects had an approximate twofold higher insulin sensitivity (P < 0.02) and lower IHCLs than diabetic patients (5.8 ± 1.2 vs. 18.3 ± 4.2%, P < 0.03), in whom IMCL-TA negatively correlated with insulin sensitivity (r = −0.969, P < 0.001). After a 67-h insulin infusion in diabetic patients, IMCL-S and IHCLs were increased (P < 0.05) by ∼36 and ∼18%, respectively, and correlated positively with insulin sensitivity (IMCL-S: r = 0.982, P < 0.0005; IHCL: r = 0.865, P < 0.03), whereas fasting glucose production, measured with d-[6,6-²H₂]glucose, decreased by ∼10% (P < 0.04). In conclusion, these results indicate that IMCLs relate to insulin resistance in type 2 diabetic patients at baseline and that insulin-mediated near-normoglycemia for ∼3 days reduces fasting glucose production but stimulates lipid accumulation in liver and muscle without affecting insulin sensitivity.

BACKGROUND: The triglyceride-to–HDL cholesterol (TG/HDL-C) ratio was introduced as a tool to estimate insulin resistance, because circulating lipid measurements are available in routine settings. Insulin, C-peptide, and free fatty acids are components of other insulin-sensitivity indices but their measurement is expensive. Easier and more affordable tools are of interest for both pediatric and adult patients.METHODS: Study participants from the Relationship Between Insulin Sensitivity and Cardiovascular Disease [43.9 (8.3) years, n = 1260] as well as the Beta-Cell Function in Juvenile Diabetes and Obesity study cohorts [15 (1.9) years, n = 29] underwent oral-glucose-tolerance tests and euglycemic clamp tests for estimation of whole-body insulin sensitivity and calculation of insulin sensitivity indices. To refine the TG/HDL ratio, mathematical modeling was applied including body mass index (BMI), fasting TG, and HDL cholesterol and compared to the clamp-derived M-value as an estimate of insulin sensitivity. Each modeling result was scored by identifying insulin resistance and correlation coefficient. The Single Point Insulin Sensitivity Estimator (SPISE) was compared to traditional insulin sensitivity indices using area under the ROC curve (aROC) analysis and χ2 test.RESULTS: The novel formula for SPISE was computed as follows: SPISE = 600 × HDL-C0.185/(TG0.2 × BMI1.338), with fasting HDL-C (mg/dL), fasting TG concentrations (mg/dL), and BMI (kg/m2). A cutoff value of 6.61 corresponds to an M-value smaller than 4.7 mg · kg−1 · min−1 (aROC, M:0.797). SPISE showed a significantly better aROC than the TG/HDL-C ratio. SPISE aROC was comparable to the Matsuda ISI (insulin sensitivity index) and equal to the QUICKI (quantitative insulin sensitivity check index) and HOMA-IR (homeostasis model assessment–insulin resistance) when calculated with M-values.CONCLUSIONS: The SPISE seems well suited to surrogate whole-body insulin sensitivity from inexpensive fasting single-point blood draw and BMI in white adolescents and adults.