Myofibrillar Protein Synthesis Rates Do not Differ With Low and High Estradiol Concentrations Across the Menstrual Cycle

Context Skeletal muscle can respond and adapt to sex hormones; however, the degree to which fluctuations in endogenous estradiol across the menstrual cycle (MC) influences rates of myofibrillar protein synthesis (MyoPS) is not clear. Objective We compared MyoPS in postabsorptive and postprandial postexercise states, during the early follicular (EF; low estradiol) and late follicular (LF; high estradiol) phases of the MC. Methods Seventeen healthy women (age: 28 ± 7 y; body mass index: 24 ± 3 kg.m²), participated in a randomized, crossover trial, during the EF (day 4 ± 1; estradiol, 183 ± 78 pmol.L⁻¹) and LF (day 15 ± 3; estradiol, 855 ± 571 pmol.L⁻¹) phases. Following a resistance exercise bout, participants ingested an amino acid (AA) drink. Blood and muscle samples were collected, pre and post exercise and AA ingestion. Following primed-continuous infusion of L-[ring-²H₅]-phenylalanine, MyoPS was measured prior to and during a 4 hour postprandial postexercise period. Results MyoPS increased above postabsorptive rates between 0-2 hours to 0.111 ± 0.049 and 0.117 ± 0.058%.h⁻¹ (P <.001) but not between 2-4 hours (P =.522), for EF and LF, respectively, with no interactions observed (P =.971). Moderate correlations were shown between total and free testosterone and 0-4 hour MyoPS (r = 0.364, P =.048; r = 0.369, P =.045, respectively). Expression of several genes associated with protein synthesis, muscle remodeling, and inflammation were increased in LF vs EF (P <.050), whereas protein breakdown genes were decreased. Conclusion Despite a gene expression profile consistent with muscle growth, MyoPS did not differ with elevated estradiol concentrations. Hence, estradiol does not seem to be important for acutely regulating muscle mass in eumenorrheic women.