No major sex differences in muscle protein synthesis rates in the postabsorptive state and during hyperinsulinemia-hyperaminoacidemia in middle-aged adults

Men have more muscle than women, but most studies evaluating sex differences in muscle protein metabolism have been unable to discern sexual dimorphism in basal muscle protein turnover rates in young and middle-aged adults. We hypothesized that the anabolic response to nutritional stimuli (i.e., amino acids and insulin) would be greater in young/middle-aged men than women. We therefore measured the rates of muscle protein synthesis (MPS) in 16 healthy individuals [8 men and 8 women, matched for age (mean ± SE: 37.7 ± 1.5 yr) and body mass index (25.2 ± 0.7 kg/m²)] after an overnight fast (plasma insulin ∼5 μU/ml and plasma phenylalanine ∼60 μM) and during a hyperinsulinemichyperaminoacidemic-euglycemic clamp (plasma insulin ∼28 μU/ml; plasma phenylalanine ∼110 μM; plasma glucose ∼5.4 mM). The rates of MPS were not different between men and women (ANOVA main effect for sex; P = 0.49). During the clamp, the rate of MPS increased by ∼50% (P = 0.003) with no difference in the increases from basal values between men and women (+0.019 ± 0.004 vs. +0.018 ± 0.010%/h, respectively; P = 0.93). There were also no differences between men and women in the basal concentrations of muscle phosphorylated Akt^Ser473, Akt^Thr308, mTOR^Ser2448, and p70s6k^Thr389 or in the hyperinsulinemia-hyperaminoacidemia-induced increases in phosphorylation of those signaling elements (P ≥ 0.25). We conclude that there are no major differences in the rate of MPS and its intracellular control during basal conditions and during hyperinsulinemia-hyperaminoacidema between young and middle-aged adult men and women.