L-Citrulline Reduces Time to Exhaustion and Insulin Response to a Graded Exercise Test

Abstract

It can be concluded that, contrary to the hypothesized improvement in treadmill time following L-citrulline ingestion, there is a reduction in treadmill time following L-citrulline ingestion over the 24 h prior to testing. The normal response of increased plasma insulin following high-intensity exercise is also not present in the L-citrulline condition, indicating that L-citrulline ingestion may reduce nitric oxide-mediated pancreatic insulin secretion or increased insulin clearance.

l-Citrulline supplementation improves O2 uptake kinetics and high-intensity exercise performance in humans

“…These findings differ from previous research demonstrating that acute Cit supplementation lowers plasma [NOx] and compromises exercise tolerance (26) but are consistent with studies showing that short-term (15 days) supplementation with L-citrulline malate can positively affect skeletal muscle power output and metabolic responses (7). These findings are important, since they suggest that Cit might be responsible for the positive effects previously reported following L-citrulline malate supplementation and offer new insights into the mechanisms by which Cit supplementation might be ergogenic.…”

“…While L-citrulline malate supplementation has been shown to increase muscle force production (7,19), improve muscle contractile efficiency (19), and prevent the decline in muscle force production with endotoxemia (18), studies investigating the effects of Cit on exercise performance or muscle fatigue resistance are limited and equivocal. Of the two studies investigating the influence of Cit supplementation on exercise performance to date, one reported improved exercise tolerance in mice performing swimming exercise (56), while the other showed that Cit compromised incremental exercise performance in humans (26). It is unclear why our findings contrast with those of Hickner et al (26).…”

“…However, there is also a suggestion that Cit ingestion tends to lower NO production, as inferred from plasma [NOx] (26). Plasma [NO 2 Ϫ ] better reflects human NOS activity than plasma [NOx] (34) and is likely to provide a more accurate assessment of NOS-derived NO.…”

“…Of the two studies investigating the influence of Cit supplementation on exercise performance to date, one reported improved exercise tolerance in mice performing swimming exercise (56), while the other showed that Cit compromised incremental exercise performance in humans (26). It is unclear why our findings contrast with those of Hickner et al (26). While Hickner et al designed an experiment to assess the influence of acute ingestion of 3 or 9 g of Cit on incremental treadmill exercise performance, we investigated the effects of 7 days of Cit supplementation at 6 g/day on cycling exercise tolerance and performance.…”

“…However, since Cit was administered as L-citrulline malate in these experiments and since malate is an important tricarboxylic acid cycle intermediate that might itself influence muscle function (61), it is unclear whether these beneficial effects can be attributed to Cit, per se. Hickner et al (26) reported compromised endurance performance in concert with a lower plasma concentration of nitrate ϩ NO 2 Ϫ ([NOx]) in humans following the acute ingestion of pure Cit. In contrast, 7 days of supplementation with pure Cit has been shown to improve endurance exercise performance in mice (56).…”