Supplementation of a suboptimal protein dose with leucine or essential amino acids: effects on myofibrillar protein synthesis at rest and following resistance exercise in men

Abstract

Leucine is a nutrient regulator of muscle protein synthesis by activating mTOR and possibly other proteins in this pathway. The purpose of this study was to examine the role of leucine in the regulation of human myofibrillar protein synthesis (MPS). Twenty-four males completed an acute bout of unilateral resistance exercise prior to consuming either: a dose (25 g) of whey protein (WHEY); 6.25 g whey protein with total leucine equivalent to WHEY (LEU); or 6.25 g whey protein with total essential amino acids (EAAs) equivalent to WHEY for all EAAs except leucine (EAA-LEU). Measures of MPS, signalling through mTOR, and amino acid transporter (AAT) mRNA abundance were made while fasted (FAST), and following feeding under rested (FED) and post-exercise (EX-FED) conditions. Leucinaemia was equivalent between WHEY and LEU and elevated compared to EAA-LEU (P=0.001). MPS was increased above FAST at 1–3 h post-exercise in both FED (P <0.001) and EX-FED (P <0.001) conditions with no treatment effect.At 3–5 h, only WHEY remained significantly elevated above FAST in EX-FED(WHEY 184% vs. LEU 55% and EAA-LEU 35%; P =0.036). AAT mRNA abundance was increased above FAST after feeding and exercise with no effect of leucinaemia. In summary, a low dose of whey protein supplemented with leucine or all other essential amino acids was as effective as a complete protein (WHEY) in stimulating postprandial MPS; however only WHEY was able to sustain increased rates of MPS post-exercise and may therefore be most suited to increase exercise-induced muscle protein accretion.

Trial registration: ClinicalTrials.gov NCT01492010.

Figures

Figure 1. Schematic diagram of the experimental protocol

Study participants consumed either EAA-LEU, LEU, or WHEY (see Methods) in single-blinded fashion (n = 8 per treatment group) immediately following resistance exercise. Exercise consisted of 4 sets each of unilateral seated knee extension and leg press. Asterisk indicates blood sample; single upward arrow indicates unilateral biopsy; double upward arrow indicates bilateral biopsy.

Figure 2. Mean (±SEM) blood concentrations (μmol l−1) of leucine (A), branched chain amino acids (BCAAs) (B), essential amino acids (EAAs) (C) and total amino acids (D) following EAA-LEU, LEU and WHEY treatments

Inset shows the area under the curve (AUC). Upward arrow indicates time of treatment administration. *Significantly greater than EAA-LEU (P < 0.05); +significantly greater than LEU (P < 0.05); ‡significantly greater than WHEY (P < 0.05).

Figure 3. Mean (±SEM) fractional synthetic rate (FSR) (% h−1) calculated during FAST, and over both early (1–3 h), and late (3–5 h) time periods of post-exercise recovery in both FED (A) and EX-FED (B) conditions after EAA-LEU, LEU and WHEY treatments

Times with different letters are significantly different from each other within that treatment and condition. *Significantly greater than EAA-LEU within that time and condition (P < 0.05); +significantly greater than LEU within that time and condition (P < 0.05); †significantly greater than FED condition at that time point (P < 0.05).

Figure 4. Mean (±SEM) intracellular concentrations (μmol l−1) of leucine (A and B), branched chain amino acids (BCAAs) (C and D) and essential amino acids (EAAs) (E and F) measured during FAST and at 1, 3 and 5 h post-exercise recovery in both FED and EX-FED conditions following EAA-LEU, LEU and WHEY treatments

Times with different letters are significantly different from each other within that treatment and condition. *Significantly greater than EAA-LEU within that time and condition (P < 0.05); +significantly greater than LEU within that time and condition (P < 0.05); ‡significantly greater than WHEY within that time and condition (P < 0.05); †significantly greater than EX-FED condition at that time-point (P < 0.05).

Figure 5. Mean (±SEM) mRNA expression of CD98 (SLC3A2) (A and B), LAT1 (SLC7A5) (C and D), and PAT1 (SLC36A1) (E and F) (expressed as fold-difference from FAST) at 1, 3 and 5 h post-exercise recovery in both FED and EX-FED conditions following EAA-LEU, LEU and WHEY treatments

Times with different letters are significantly different from each other within that treatment and condition. *Significantly greater than EAA-LEU within that time and condition (P < 0.05); †significantly greater than EX-FED condition at that time point (P < 0.05).

Figure 6. Mean (±SEM) phosphorylation status of AktSer473 (A and B), mTORSer2448 (C and D) and p70S6kThr389 (E and F) (expressed as fold-difference from FAST) at 1, 3 and 5 h post-exercise recovery in both FED (top panel) and EX-FED (bottom panel) conditions following EAA-LEU, LEU and WHEY treatments

Times with different letters are significantly different from eachother within that treatment and condition. *Significantly greater than EAA-LEU within that time and condition (P < 0.05); +significantly greater than LEU within that time and condition (P < 0.05); ‡significantly greater than WHEY within that time and condition (P < 0.05).