L-arginine is a semi-essential amino acid. Therefore, supplementation is sometimes needed, especially in times of stress.
L-arginine dilates blood vessels and increases blood flow, there fore building muscle.
L-arginine is used to treat erectile dysfunction by improving blood flow into the penis.
Since L-arginine can improve blood flow, it may improve exercise tolerance.
Citrulline is a non-essential amino acid that is produced in the body and is converted into L-arginine and L-ornithine. These three amino acids are part of the urea cycle which processes ammonia, a metabolic waste product.
L-citrulline increases L-arginine which increases nitric oxide. Increases in nitric oxide can increase blood flow to muscle, which can reduce fatigue, reduce energy spent, and increase exercise endurance.
L-ornithine is part of the urea cycle, along with L-arginine and L-citrulline. Together these amino acids help buffer ammonia in the blood. Ammonia activates the enzyme phosphofructokinase, which increases the production of lactate. This causes a decrease in muscle cell pH, a decrease in calcium release, and therefore a decrease in muscle contractility.
Ornithine and arginine promote the synthesis of growth hormone, which is expected to increase protein synthesis and muscle mass.