Early responses of insulin signaling to high-carbohydrate and high-fat overfeeding

<p>Abstract</p> <p>Background</p> <p>Early molecular changes of nutritionally-induced insulin resistance are still enigmatic. It is also unclear if acute overnutrition alone can alter insulin signaling in humans or if the macronutrient composition of the diet can modulate such effects.</p> <p>Methods</p> <p>To investigate the molecular correlates of metabolic adaptation to either high-carbohydrate (HC) or high-fat (HF) overfeeding, we conducted overfeeding studies in 21 healthy lean (BMI < 25) individuals (10 women, 11 men), age 20-45, with normal glucose metabolism and no family history of diabetes. Subjects were studied first following a 5-day eucaloric (EC) diet (30% fat, 50% CHO, 20% protein) and then in a counter balanced manner after 5 days of 40% overfeeding of both a HC (20% fat, 60% CHO) diet and a HF (50% fat, 30% CHO) diet. At the end of each diet phase, <it>in vivo </it>insulin sensitivity was assessed using the hyperinsulinemic-euglycemic clamp technique. <it>Ex vivo </it>insulin action was measured from skeletal muscle tissue samples obtained 15 minutes after insulin infusion was initiated.</p> <p>Results</p> <p>Overall there was no change in whole-body insulin sensitivity as measured by glucose disposal rate (GDR, EC: 12.1 ± 4.7; HC: 10.9 ± 2.7; HF: 10.8 ± 3.4). Assessment of skeletal muscle insulin signaling demonstrated increased tyrosine phosphorylation of IRS-1 (p < 0.001) and increased IRS-1-associated phosphatidylinositol 3 (PI 3)-kinase activity (p < 0.001) following HC overfeeding. In contrast, HF overfeeding increased skeletal muscle serine phosophorylation of IRS-1 (p < 0.001) and increased total expression of p85α (P < 0.001).</p> <p>Conclusion</p> <p>We conclude that acute bouts of overnutrition lead to changes at the cellular level before whole-body insulin sensitivity is altered. On a signaling level, HC overfeeding resulted in changes compatible with increased insulin sensitivity. In contrast, molecular changes in HF overfeeding were compatible with a reduced insulin sensitivity.</p

Increasing Dietary Fat Elicits Similar Changes in Fat Oxidation and Markers of Muscle Oxidative Capacity in Lean and Obese Humans

In lean humans, increasing dietary fat intake causes an increase in whole-body fat oxidation and changes in genes that regulate fat oxidation in skeletal muscle, but whether this occurs in obese humans is not known. We compared changes in whole-body fat oxidation and markers of muscle oxidative capacity differ in lean (LN) and obese (OB) adults exposed to a 2-day high-fat (HF) diet. Ten LN (BMI = 22.5±2.5 kg/m2, age = 30±8 yrs) and nine OB (BMI = 35.9±4.93 kg/m2, 38±5 yrs, Mean±SD) were studied in a room calorimeter for 24hr while consuming isocaloric low-fat (LF, 20% of energy) and HF (50% of energy) diets. A muscle biopsy was obtained the next morning following an overnight fast. 24h respiratory quotient (RQ) did not significantly differ between groups (LN: 0.91±0.01; OB: 0.92±0.01) during LF, and similarly decreased during HF in LN (0.86±0.01) and OB (0.85±0.01). The expression of pyruvate dehydrogenase kinase 4 (PDK4) and the fatty acid transporter CD36 increased in both LN and OB during HF. No other changes in mRNA or protein were observed. However, in both LN and OB, the amounts of acetylated peroxisome proliferator-activated receptor γ coactivator-1-α (PGC1-α) significantly decreased and phosphorylated 5-AMP-activated protein kinase (AMPK) significantly increased. In response to an isoenergetic increase in dietary fat, whole-body fat oxidation similarly increases in LN and OB, in association with a shift towards oxidative metabolism in skeletal muscle, suggesting that the ability to adapt to an acute increase in dietary fat is not impaired in obesity

A Unique Control Mechanism in the Regulation of Insulin Secretion Secretagogue-induced Somatostatin Receptor Recruitment

In this study, we have correlated the translocation of somatostatin (SRIF) receptors from the cell interior to the plasma membrane with the ability of SRIF to inhibit insulin release

Multiple abnormalities of myocardial insulin signaling in a porcine model of diet-induced obesity

Heightened cardiovascular risk among patients with systemic insulin resistance is not fully explained by the extent of atherosclerosis. It is unknown whether myocardial insulin resistance accompanies systemic insulin resistance and contributes to increased cardiovascular risk. This study utilized a porcine model of diet-induced obesity to determine if myocardial insulin resistance develops in parallel with systemic insulin resistance and investigated potential mechanisms for such changes. Micropigs (n = 16) were assigned to control (low fat, no added sugars) or intervention (25% wt/wt coconut oil and 20% high-fructose corn syrup) diet for 7 mo. Intervention diet resulted in obesity, hypertension, and dyslipidemia. Systemic insulin resistance was manifest by elevated fasting glucose and insulin, abnormal response to intravenous glucose tolerance testing, and blunted skeletal muscle phosphatidylinositol-3-kinase (PI 3-kinase) activation and protein kinase B (Akt) phosphorylation in response to insulin. In myocardium, insulin-stimulated glucose uptake, PI 3-kinase activation, and Akt phosphorylation were also blunted in the intervention diet group. These findings were explained by increased myocardial content of p85α (regulatory subunit of PI 3-kinase), diminished association of PI 3-kinase with insulin receptor substrate (IRS)-1 in response to insulin, and increased serine-307 phosphorylation of IRS-1. Thus, in a porcine model of diet-induced obesity that recapitulates many characteristics of insulin-resistant patients, myocardial insulin resistance develops along with systemic insulin resistance and is associated with multiple abnormalities of insulin signaling

Room calorimeter results (mean±SE).

1<p>OB > LN.</p>2<p>LF > HF.</p>3<p>LF < HF.</p>4<p>PAL  = 24 h EE/resting metabolic rate.</p><p>RQ = respiratory quotient.</p><p>FFM = fat free mass.</p><p>FM = fat free mass.</p

Plasma FFA (± SE; Left) and triglycerides (± SE; Right) during LF (Top) and HF (Middle) diets in LN (N = 10; dashed line), and OB (N = 9; solid line).

<p>24 h FFA and triglycerides areas under the curve (AUC) are presented in the bottom graphs.</p

Mean (± SE) Protein (Top), carbohydrate (Middle), and fat (Bottom) oxidation (g/kg FFM/day) measured using room calorimetry during LF (black bars) and HF (grey bars) in LN (N = 10) and OB (N = 9).

<p>* LF significantly different from HF (P<0.05).</p

Mean (± SE) total (Top) and phosphorylated (Bottom) AMPK protein during LF (black bars) and HF (grey bars) in LN (N = 5) and OB (N = 6).

<p>Representative blots are show above the graphs. * LF significantly different from HF (P<0.05).</p