Supraphysiological hyperinsulinaemia is necessary to stimulate skeletal muscle protein anabolism in older adults: evidence of a true age-related insulin resistance of muscle protein metabolism

Aims/hypothesis The physiological increase in muscle protein anabolism induced by insulin is blunted in healthy, glucose-tolerant older adults. We hypothesised that the age-related defect in muscle protein anabolism is a true insulin resistance state and can be overridden by supraphysiological hyperinsulinaemia. Methods We used dye dilution, stable isotopic and immunoblotting techniques to measure leg blood flow, muscle protein synthesis, protein kinase B/mammalian target of rapamycin (Akt/mTOR) signalling, and amino acid kinetics in 14 healthy, glucose-tolerant older volunteers at baseline, and during an insulin infusion at postprandial (PD, 0.15 mU min⁻¹ 100 ml⁻¹) or supraphysiologically high (HD, 0.30 mU min⁻¹ 100 ml⁻¹) doses. Results Leg blood flow, muscle protein synthesis, and Akt/mTOR signalling were not different at baseline. During hyperinsulinaemia, leg blood flow (p < 0.01) and muscle protein synthesis increased in the HD group only (PD [%/h]: from 0.063 ± 0.006 to 0.060 ± 0.005; HD [%/h]: from 0.061 ± 0.007 to 0.098 ± 0.007; p < 0.01). Muscle Akt phosphorylation increased in both groups, but the increase tended to be greater in the HD group (p = 0.07). The level of p70 ribosomal S6 kinase 1 (S6K1) phosphorylation increased in the HD group only (p < 0.05). Net amino acid balance across the leg improved in both groups, but a net anabolic effect was observed only in the HD group (p < 0.05). Conclusions/interpretation We conclude that supraphysiological hyperinsulinaemia is necessary to stimulate muscle protein synthesis and anabolic signalling in healthy older individuals, suggesting the existence of a true age-related insulin resistance of muscle protein metabolism.