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.

A study where healthy trained men (n≥20) are randomly assigned to receive either L-citrulline (2.4g/day for 7 days + 2.4g pre-exercise) or placebo in two separate 7-day phases with a 14-day washout, then perform a standardized 4-km cycling time trial on a lab ergometer; power output and oxygen consumption (via indirect calorimetry) are measured and compared within subjects.

A study where healthy trained men (n≥40) are randomly assigned to either L-citrulline (2.4g/day for 7 days + 2.4g pre-exercise) or placebo, with all participants completing a 4-km cycling time trial on a lab ergometer after the 7-day period; power output and oxygen consumption are measured and compared between groups.

A study where healthy trained men (n≥15) undergo two 7-day supplementation phases (L-citrulline vs. placebo) in random order, with strict dietary control and no other supplements, followed by a 4-km cycling time trial with power and oxygen consumption measured; differences in efficiency are calculated from pre- and post-supplementation data.

A study where healthy trained men (n≥30) self-administer L-citrulline (2.4g/day for 7 days + 2.4g pre-exercise) and perform a 4-km cycling time trial before and after supplementation, with power output and oxygen consumption measured to detect changes.

A study where a small group of healthy trained men (n=5–10) receive L-citrulline (2.4g/day for 7 days + 2.4g pre-exercise) and undergo a 4-km cycling time trial with power and oxygen consumption measured before and after to observe trends.