Athletes feature greater rates of muscle glucose transport and glycogen synthesis during lipid infusion

Abstract

BACKGROUND. Insulin resistance results from impaired skeletal muscle glucose transport/ phosphorylation, linked to augmented lipid availability. Despite greater intramuscular lipids, athletes are highly insulin sensitive, which could result from higher rates of insulin-stimulated glycogen synthesis or glucose transport/phosphorylation and oxidation. Thus, we examined the time course of muscle glycogen and glucose-6-phosphate concentrations during low and high systemic lipid availability. METHODS. Eight endurance-Trained and 9 sedentary humans (VO2 peak: 56 ± 2 vs. 33 ± 2 mL/ kg/min, P 0.05) underwent 6-hour hyperinsulinemic-isoglycemic clamp tests with infusions of triglycerides or saline in a randomized crossover design. Glycogen and glucose-6-phosphate concentrations were monitored in vastus lateralis muscles using 13C/31P magnetic resonance spectroscopy. RESULTS. Athletes displayed a 25% greater (P 0.05) insulin-stimulated glucose disposal rate (Rd) than sedentary participants. During Intralipid infusion, insulin sensitivity remained higher in the athletes (δRd: 25 ± 3 vs. 17 ± 3 umol/kg/min, P 0.05), supported by higher glucose transporter type 4 protein expression than in sedentary humans. Compared to saline infusion, AUC of glucose-6-phosphate remained unchanged during Intralipid infusion in athletes (1.6 ± 0.2 mmol/L vs. 1.4 ± 0.2 [mmol/L] × h, P = n.s.) but tended to decrease by 36% in sedentary humans (1.7 ± 0.4 vs. 1.1 ± 0.1 [mmol/L] × h, P 0.059). This drop was accompanied by a 72% higher rate of net glycogen synthesis in the athletes upon Intralipid infusion (47 ± 9 vs. 13 ± 3 umol/kg/min, P 0.05). CONCLUSION. Athletes feature higher skeletal muscle glucose disposal and glycogen synthesis during increased lipid availability, which primarily results from maintained insulin-stimulated glucose transport with increased myocellular glucose-6-phosphate levels for subsequent glycogen synthesis.