L-leucine is one of the eight essential amino acids, which means the body does not produce it naturally.  So this protein building block must come from the diet.  Plant and animal proteins that contain L-leucine include lentils, peanuts, and other nuts and seeds, beef, chicken, egg yolk, fish and milk.

In one study, 26 untrained healthy men, ages 20 - 36, took 4,000 mg of L-leucine per day or a placebo.  Twice per week during the study, professional trainers supervised the men in eight standard resistance-training machine exercises.  Researchers tested strength before and after the study, measuring the maximum weight each participant could resist in five machine repetitions.  After 12 weeks, the placebo group had increased its maximum five repetition weight resistance ability by 31% while the leucine group increased its ability by 41%.  (International Journal of sports physiology and performance; 2001, Vol. 6, No. 1, 38-50)