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 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 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 specific dose of L-citrulline for 10 days doesn’t change how your body uses sugar or handles lactic acid when you bike really hard, so it doesn’t help or hurt your energy or fatigue during intense exercise.

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 for 10 days might help young women bike harder for a little longer, but the results weren’t strong enough to be sure — so scientists think it might work differently for women than for men, and more research is needed.