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As usual, the alphabet throws together a mixture of supplements with different levels of popularity and scientific support. Part 20 covers some rarely reported, studied and/or little used supplements in sport: glycine, histidine and inosine. The majority of human studies of supplementation with the essential amino acid histidine has involved clinical work. In terms of athletic performance, there is current interest in supplementation strategies to increase muscle content of the histidine-containing dipeptide (HCD), carnosine. Despite some interest in the use of a chicken breast extract (CBEX) described in this article, most of the focus in this area involves β-alanine supplementation (covered in part 5). There was some interest in inosine as an ergogenic aid in the 1990s but it appears not to have been studied since then. Meanwhile, there appears little role for glycine supplementation in sport although some interest in glycine-containing compounds is possible. β-Hydroxymethyl β-butyrate (HMB) is much more well known, with marketing usually targeting bodybuilders.
Glycine is the smallest amino acid; it is non-essential and can be synthesised from serine. Glycine is present in most proteins and is particularly highly concentrated in collagen. Consequently, one of the highest food sources of glycine is gelatin. Glycine is also one of the three amino acid components of glutathione, which is a key component of the body's defences against oxidative stress; however, it is thought that glycine availability is not the limiting step in glutathione synthesis. Glycine ingestion increases plasma concentrations of insulin in a similar way to other amino acids.1 Glycine is also an inhibitory neurotransmitter.
There is little research on supplementation with glycine. Research has looked at its potential role in decreasing inflammation.2 Sport specific research has focused on combining glycine with other nutrients. Glycine-propionyl-L-carnitine (GPLC) has been shown to influence exercise performance,3 decrease oxidative …
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