There were no differences with respect to submaximal oxygen consumption, RER, or HR between L-citrulline and placebo conditions (data not shown).
The primary finding of this placebo-controlled, double-blind, counterbalanced study was that, in contrast to our hypothesis, there was a decrease in performance during this incremental graded treadmill exercise test to exhaustion in 12 of the 17 participants. Of potentially great importance with respect to investigations of insulin secretion and clearance is the additional finding that the previously reported increase in plasma insulin in response to high-intensity exercise (30) is blunted following L-citrulline ingestion. These are the first reports of L-citrulline ingestion resulting in reduced exercise capacity and reduced insulin response in healthy volunteers.
The original hypothesis that L-citrulline ingestion would result in higher NO production was not supported, in that plasma NOx was not higher in the L-citrulline than the placebo condition prior to exercise. In fact, preexercise plasma NOx was lower in 12 of 17 subjects following L-citrulline compared with placebo ingestion (P = not significant). Furthermore, there was an increase in plasma NOx in response to exercise only following placebo ingestion (Fig. 4A). It is possible that the L-citrulline supplementation resulted in end-product inhibition of the L-arginine/NO reaction and resulted in a lower NO production. Although there is no direct support for this hypothesis in the present investigation, it has been reported that the NO donor sodium nitroprusside, but not L-citrulline, inhibits neuronal NO synthase (29). The inhibitory effect of the NO donor was prevented by an NO scavenger in that study, indicating that NO can reduce NO synthase activity and subsequent NO production (29). There were no direct measures of NO or L-arginine in the present study; however, the plasma NOx data in combination with the small reduction in treadmill time with L-citrulline ingestion suggest that L-citrulline may reduce exercise-induced NO release or increase NO degradation, thereby limiting performance of the graded exercise test to exhaustion.
Previous treatments attenuating NO production during intense exercise are those in eNOS knockout mice (18) and a study of L-NAME infusion in humans (11): maximal exercise capacity was reduced in these treatments. In the human study, Jones et al. (11) ascribed the 15-s reduction in incremental cycle ergometry time to exhaustion to an approximately 5% reduction in HRmax and O2max, suggesting a reduced oxygen delivery during maximal exercise. In the present study, the slightly lower HR and O2max with L-citrulline ingestion observed did not reach statistical significance; however, there was a positive correlation between the difference in O2peak from L-citrulline to placebo conditions and the difference in treadmill time to exhaustion. The objective measure of reduced time to exhaustion in the L-citrulline condition was further supported by a higher RPE during exercise in the L-citrulline compared with placebo condition, indicating the participants' subjective opinion of the exercise test was that of a more difficult exercise session.
It can be concluded that, in healthy young men and women, contrary to the hypothesized improvement in exercise time, there was a reduction in treadmill time following L-citrulline ingestion over the 24 h prior to performance of a graded treadmill exercise test to exhaustion. There was also a reduction in insulin response to this high-intensity exercise following L-citrulline ingestion that may reflect a reduced NO-mediated insulin secretion or increased insulin clearance. The question remains as to whether L-arginine (precursor of NO production) ingestion would have the opposing action of L-citrulline ingestion and result in improved exercise performance and insulin response to high-intensity exercise in young, healthy individuals.
This study was funded by a contract from Experimental and Applied Sciences (EAS), a distributor of nutritional products.
Conflict of interest disclosure: R. Hickner is a member of the EAS Scientific Advisory Board.
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