Impact of prolonged overfeeding on skeletal muscle mitochondria in healthy individuals.

AIMS/HYPOTHESES: Reduced mitochondrial capacity in skeletal muscle has been observed in obesity and type 2 diabetes. In humans, the aetiology of this abnormality is not well understood but the possibility that it is secondary to the stress of nutrient overload has been suggested. To test this hypothesis, we examined whether sustained overfeeding decreases skeletal muscle mitochondrial content or impairs function.

METHODS: Twenty-six healthy volunteers (21 men, 5 women, age 25.3 ± 4.5 years, BMI 25.5 ± 2.4 kg/m) underwent a supervised protocol consisting of 8 weeks of high-fat overfeeding (40% over baseline energy requirements). Before and after overfeeding, we measured systemic fuel oxidation by indirect calorimetry and performed skeletal muscle biopsies to measure mitochondrial gene expression, content and function in vitro. Mitochondrial function in vivo was measured by P NMR spectroscopy.

RESULTS: With overfeeding, volunteers gained 7.7 ± 1.8 kg (% change 9.8 ± 2.3). Overfeeding increased fasting NEFA, LDL-cholesterol and insulin concentrations. Indirect calorimetry showed a shift towards greater reliance on lipid oxidation. In skeletal muscle tissue, overfeeding increased ceramide content, lipid droplet content and perilipin-2 mRNA expression. Phosphorylation of AMP-activated protein kinase was decreased. Overfeeding increased mRNA expression of certain genes coding for mitochondrial proteins (CS, OGDH, CPT1B, UCP3, ANT1). Despite the stress of nutrient overload, mitochondrial content and mitochondrial respiration in muscle did not change after overfeeding. Similarly, overfeeding had no effect on either the emission of reactive oxygen species or on mitochondrial function in vivo.

CONCLUSIONS/INTERPRETATION: Skeletal muscle mitochondria are significantly resilient to nutrient overload. The lower skeletal muscle mitochondrial oxidative capacity in human obesity is likely to be caused by reasons other than nutrient overload per se.

TRIAL REGISTRATION: ClinicalTrials.gov NCT01672632.