Leucine supplementation of a low-protein mixed macronutrient beverage enhances myofibrillar protein synthesis in young men: a double-blind, randomized trial.
Level 2 - randomized trial
Double-blind randomized controlled trial in humans
PubMed 24284442 · doi:10.3945/ajcn.113.068775
What was done
Forty healthy young men (aged 21 ± 1 years) completed unilateral knee-extensor resistance exercise before ingesting one of five mixed macronutrient beverages in a parallel group design: 25 g whey protein (3.0 g leucine), 6.25 g whey protein (0.75 g leucine), 6.25 g whey protein supplemented with leucine to 3.0 g total leucine, 6.25 g whey protein supplemented with leucine to 5.0 g total leucine, or 6.25 g whey protein supplemented with leucine, isoleucine, and valine to 5.0 g total leucine. Myofibrillar protein synthesis (MPS) was measured using primed continuous L-[ring-13C6]phenylalanine infusions with serial muscle biopsies at baseline (fasted) and postprandially (0-1.5 h and 1.5-4.5 h) in both rested and exercised legs.
What was found
The area under the blood leucine curve was greatest for the 6.25 g whey + 5.0 g leucine group compared with the 6.25 g whey and 6.25 g whey + 3.0 g leucine groups (P < 0.001). In the postprandial period, MPS rates increased above baseline over 0-1.5 h in all treatment groups. Over 1.5-4.5 h, MPS remained elevated above baseline across all treatments but was significantly greatest after the 25 g whey (~267%) and 6.25 g whey + 5.0 g leucine (~220%) treatments (P = 0.002).
Why it matters
This study demonstrates that fortifying a low dose of protein with leucine (5.0 g total) can stimulate sustained myofibrillar protein synthesis to an extent comparable to a standard 25 g high-protein dose, which informs protein beverage formulation.
Limits
The sample size is small (n = 40 total, across five parallel groups). The study only measured acute fractional synthetic rates over 4.5 hours and did not assess chronic training-induced muscle hypertrophy. The cohort was restricted entirely to young, healthy men, limiting generalizability to females and older populations.
Cited by
- supports Adding supplemental leucine to a suboptimal or low-protein dose stimulates muscle protein synthesis to a level comparable to a higher-protein dose.