Taking L-citrulline or L-arginine pills for eight days doesn’t boost the body’s nitric oxide levels in elite swimmers and triathletes during their normal training, so these supplements don’t seem to help improve blood flow in this situation.

In trained swimmers and triathletes, compare plasma NOx levels after 8 days of 8 g/day L-citrulline, 8 g/day L-arginine, and placebo, with washout periods between conditions, measuring NOx at rest and immediately after standardized short- to middle-distance swimming bouts.

Compare plasma NOx changes over 8 days in two groups: one receiving 8 g/day L-citrulline and another receiving 8 g/day L-arginine, versus a placebo group, all undergoing identical swimming training protocols, with NOx measured pre- and post-intervention.

Measure plasma NOx in the same athletes before and after 8 days of L-citrulline or L-arginine supplementation, under identical swimming conditions, to detect within-person changes in NOx response to exercise.

Track plasma NOx levels in trained swimmers and triathletes who self-select to take 8 g/day L-citrulline or L-arginine for 8 days during normal training, comparing changes to non-supplementing controls.

Observe changes in plasma NOx in a group of trained swimmers and triathletes taking 8 g/day L-citrulline or L-arginine for 8 days without a control group.