Increased protein intake derived from leucine-enriched protein enhances the integrated myofibrillar protein synthetic response to short-term resistance training in untrained men and women: a 4-day randomized controlled trial

Leucine is a critical amino acid stimulating myofibrillar protein synthesis (MyoPS). The consumption of higher leucine-containing drinks stimulates MyoPS, but we know less about higher leucine solid foods. Here, we examined the effect of short-term resistance exercise training (STRT) combined with supplementation of a protein and leucine-enriched bar, compared with STRT alone, on integrated (%/day) rates of MyoPS and anabolic protein signaling. In a nonblinded, randomized crossover trial, eight young adults performed four sessions of STRT without or while consuming the study bar (STRT+Leu, 16 g of protein containing ∼3 g of leucine) for two 4-day phases, separated by 2 days nonexercise (Rest) washout. In combination with serial muscle biopsies, deuterated water permitted the measurement of MyoPS and protein signaling phosphorylation. MyoPS during STRT (1.43 ± 0.06%/day) and STRT+Leu (1.53 ± 0.06%/day) were greater than Rest (1.31 ± 0.05%/day), and MyoPS during STRT+Leu (1.53 ± 0.06%/day) was greater than STRT alone (1.43 ± 0.06%/day). STRT+Leu increased the ratio of phosphorylated to total mechanistic target of rapamycin and 4EBP1 compared to Rest. Engaging in STRT increased integrated MyoPS and protein signaling in young adults and was enhanced with increased protein intake derived from a leucine-enriched protein bar. This study was registered at clinicaltrials.gov as NCT03796897.