Amino Acid Supplement Supports Peak Performance L-Arginine and L-Citrulline are two amino acids bundled into a single powerful supplement. L-Arginine is an important factor in muscle metabolism and is a precursor for nitric oxide, which promotes increased circulation by relaxing blood vessels. L-Citrulline helps the body rid itself of ammonia, a by-product of exercise. This clearing enables the body to recover after a workout.