In healthy young adults consuming adequate daily protein (≥1.6 g/kg/day), supplemental branched-chain amino acids or leucine provide no additional benefit to muscle strength or hypertrophy during resistance training, as demonstrated by multiple longitudinal studies showing no significant differences in lean mass, muscle thickness, or 1RM strength compared to placebo.

When a person eats enough protein, their muscles already have all the building blocks needed to grow. Adding extra leucine does not make more muscle because the other amino acids required to build proteins are already present in sufficient amounts. Leucine can turn on the signal to make muscle proteins, but without all the other amino acids, the body cannot build and keep new muscle tissue.