Taking a specific amino acid called L-citrulline every day for a week, plus a little more before a bike race, helps men who are already fit ride harder for 4 kilometers without getting more out of breath — meaning their bodies use energy more efficiently.

Taking a supplement called L-citrulline every day for a week, plus a little more right before riding a bike for 4 km, makes healthy, fit men feel less tired and more focused right after they finish the ride.

Taking L-citrulline pills for 10 days doesn’t help healthy, moderately active young adults ride harder or longer on a bike at a tough pace.

Taking a specific amino acid supplement called L-citrulline every day for a week, then taking one more dose an hour before a short, intense bike ride, helps trained male cyclists finish the ride 1.5% faster.

Taking a specific supplement called L-citrulline before and during a tough bike workout may cause your body to use up more of certain amino acids (BCAAs) for energy, which means less of them are left floating in your blood.

Taking a specific dose of L-citrulline for 10 days doesn’t make you bike harder, faster, or feel less tired—even though some people think it might help. Your heart, lungs, and muscles act the same as if you didn’t take it.