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One enjoyable aspect of our A–Z series is that an issue often comprises a real assortment, such as occurs here. We have: citrulline, a non-essential amino acid; coenzyme Q10, a coenzyme that is part of the total antioxidant system; colostrum, the initial milk produced by mammals after giving birth, with bovine colostrum being a popular supplement among athletes because of its reported immune benefits; this is followed by conjugated linoleic acid, a series of structural and geometric isomers of linoleic acid which may play a role in optimising body composition; and, finally, copper, a mineral with multifunctional uses that may require monitoring in athletes at risk. We are grateful to our invited reviewers for their excellent contributions giving impartial advice on the value of these individual nutrients and supplements. They are establishing that, for some, performance evidence is limited or simply does not yet exist.
Citrulline is a non-essential α-amino acid (C6H13N3O3) found in protein-rich foods of both animal and plant origin. The body uses it to synthesise arginine, a precursor of nitric oxide (NO) production. In fact, citrulline supplementation appears to be more effective in increasing plasma arginine concentration than the ingestion of arginine itself.1 Citrulline is also an intermediate in the urea cycle, an important pathway that removes ammonia from muscle and liver cells.
Despite some reported benefits of citrulline supplementation in clinical populations, few studies have investigated its potential benefits for athletic performance. Surprisingly, one study found a potentially blunted NO production and a reduction in treadmill time to exhaustion after ingestion by healthy volunteers of 3 and 9 g L-citrulline over 24 h before testing.2 However, a field study showed that ingesting 6 g citrulline malate before a cycling event increased post-race NO concentration in polymorphonuclear neutrophils …