Progesterone does not modulate myofibrillar protein synthesis at rest or following acute resistance exercise in premenopausal females.

Progesterone has been proposed to modulate skeletal muscle protein turnover, although direct support for this hypothesis is lacking. We examined whether acute elevation of progesterone alters myofibrillar protein synthesis (MyoPS) at rest and following resistance exercise (RE) in naturally menstruating females. Twenty premenopausal females (21 ± 3 years of age) were randomised in a double-blind, placebo-controlled trial to receive oral micronized progesterone (400 mg) or matching placebo at -34 and -10 h before a stable isotope infusion on days 1-5 of their menstrual cycle. One leg performed RE while the contralateral leg remained rested, followed by ingestion of 30 g whey protein. MyoPS was quantified using the l-[ring- 1 3 C 6 ]-phenylalanine precursor-product

Method: across fasted-rested, fed-rested and fed-exercised conditions. Progesterone elevated serum concentrations well above the luteal-phase range (185 ± 105 vs. 1.0 ± 0.2 ng mL -1 ). Despite this, we detected no difference in MyoPS groups in any conditions, including the exercise-induced increment (treatment × leg interaction, P = 0.945). Both protein feeding (P = 0.00459) and RE (P -1 ) during the early follicular phase. Despite this, MyoPS did not differ between progesterone and placebo in fasted-rested, fed-rested or fed-exercised conditions. Resistance exercise robustly elevated MyoPS above the fed-rested conditions, confirming model sensitivity. Neither postprandial amino acid availability nor individual progesterone exposure was associated with MyoPS. These

Findings: challenge the characterisation of progesterone as pro-catabolic/anti-anabolic to skeletal muscle and indicate that menstrual cycle phase-based training and nutrition recommendations predicated on progesterone-driven effects on skeletal muscle are not supported by direct mechanistic evidence.