Optimal adaptation to resistance exercise likely requires a maximal rate of post-exercise myofibrillar protein synthesis (MyoPS), which is thought to be partly regulated by sex hormones. In postmenopausal women, supplementation with testosterone and progesterone but not oestrogen has been shown to increase postabsorptive muscle protein synthesis. However, limited data are available on how naturally fluctuating sex hormones across the menstrual cycle regulate post-exercise MyoPS in younger females. This study investigated the relationship between sex hormone concentrations and MyoPS following resistance exercise and protein ingestion in young females.
Seventeen, healthy, eumenorrheic females (age: 27±7 y; BMI: 24±3 kg/m2) participated in this randomised cross-over study during the early follicular (4±1d following menses) and late follicular (2±2 d before luteinising hormone surge) phases of the menstrual cycle. On each visit, a blood sample was collected to measure serum sex hormone concentrations (oestradiol, free oestradiol index, progesterone, oestrogen to progesterone ratio, testosterone, and free testosterone), participants then received a primed continuous infusion of L-[ring–2H5]phenylalanine for 7.5 h. Following a bout of resistance exercise, participants ingested a protein beverage. Muscle biopsies were collected before and during a 4 h post-exercise postprandial period to assess MyoPS. This study was approved by the Sport and Health Sciences Ethics Committee of the University of Exeter, in accordance with the standards for human research as outlined in the Declaration of Helsinki. Sex hormone concentrations between menstrual cycle phases were analysed using a paired t test. The relationship between sex hormones and MyoPS were assessed using Pearson’s product moment correlation. Data are expressed as means±SD.
Serum oestradiol (855±571 vs. 183±78 pmol/L; P<0.001), progesterone (4.0±5.1 vs. 1.1±0.7 nmol/L; P=0.041), free oestradiol index (13.1±7.1 vs. 3.3±2.7 pmol/nmol; P<0.0001), oestradiol to progesterone ratio (659±866 vs. 225±154 pmol/nmol; P=0.046), total testosterone (1.3±0.6 vs. 1.2±0.6 nmol/L; P=0.023), and free testosterone (18±15 vs. 16±15 pmol/L; P=0.097) were all greater during the late follicular phase compared to during the early follicular phase. A moderate correlation was observed between free testosterone and postexercise MyoPS from 0-4 h (r=0.475, P=0.008), and this was trending to be correlated under the basal and early postexercise 0-2 h period (r=0.334, P=0.072; r=0.355, P=0.054; respectively). Total testosterone also moderately correlated with postexercise MyoPS from 0-4 h (r=0.418, P=0.022), but the remainder of the sex hormones (oestradiol, r=0.003, P=0.987; free oestradiol index, r=0.275, P=0.141; progesterone r=-0.078, P=0.682; oestrogen to progesterone ratio r=0.105, P=0.583) did not significantly correlate with MyoPS over 0-4 h or at any other time point (P>0.050).
Post-exercise MyoPS correlated with total and free testosterone yet did not correlate with any other sex hormone, despite a wide range of concentrations measured across the early and late follicular phases. These findings suggest that testosterone, both total and free, may play an important role in the regulation of muscle mass in females.