Leucine is a key amino acid involved in the regulation of skeletal muscle protein synthesis. Leucine triggers the anabolic mTOR pathway, which is essential for muscle growth; additionally, leucine has proven to build muscle, maintain lean mass when in a caloric deficit, improve endurance, and boost recovery. Researchers wanted to examine how adding leucine to a protein drink would affect muscle protein synthesis. Researchers assessed the effect on muscle protein synthesis at rest and after resistance exercise. The men completed unilateral knee extensor resistance exercise before the ingestion of various intakes of whey protein, BCAAs, and leucine:
The researchers found that muscle protein synthesis remained above baseline after all treatments but was greatest after 25 grams of whey and 6.25 grams whey protein supplemented with leucine to five grams total leucine (low-protein/high leucine group). The really interesting finding was the low protein (6.25 grams) mixed beverage was as effective as a high-protein whey (25 grams) shake at stimulating increased muscle protein synthesis rates when supplemented with a high amount of leucine (five grams). These results, indicating that high leucine can have enhanced muscle anabolism effects, are important for bodybuilders who are on a strict diet and cutting back on total calories.