Build muscles but eat little protein? Use L-citrulline
Athletes who for whatever reason don't eat much protein might benefit from L-citrulline [structural formula below]. We draw this tentative conclusion from a human study that researchers at the Mayo Clinic in the US published in Clinical Nutrition. If the results are to be believed, L-citrulline has anabolic qualities in combination with a low-protein diet.
Animal studies have shown that L-citrulline supplementation protects against muscle mass breakdown while on a low-calorie diet. This is why the researchers wondered whether L-citrulline would have the same effect when combined with a low-protein diet.
So they did a small experiment with 8 subjects, who ate a low-protein diet for three days. They were given just under 0.7 g protein per kg bodyweight daily.
On the fourth day the subjects were given extra L-citrulline in soluble form on an empty stomach. Over a period of 8 hours they drank 11-24 g L-citrulline in total. The dose of L-citrulline depended on their lean body mass: the higher this was, the more L-citrulline they were given.
On another occasion the subjects were given a similar quantity of amino acids, which – like L-citrulline – were not essential.
The figure below shows that L-citrulline boosted the synthesis of new muscle protein [FSR]. In non-muscle tissue L-citrulline had no effect on protein synthesis.
The researchers did not measure the breakdown of muscle tissue. If L-citrulline boosts breakdown considerably, there would be no point taking L-citrulline. But if L-citrulline inhibits muscle breakdown, L-citrulline and related substances may be even more interesting than they already are.
The researchers measured the activity of a number of genes in the muscle cells, but were unable to work out exactly how L-citrulline works. They didn't detect any effect on hormones such as insulin and IGF-1 either.
"The current pilot study, the first performed in humans, demonstrates that oral ingestion of citrulline stimulated muscle protein synthesis in healthy participants while on a short-term low-protein diet", the researchers write.
"This anabolic action of citrulline is independent of insulin action and is specific of muscle. Overall, this novel finding opens the potential opportunity for clinical application of citrulline in situations where muscle anabolism is diminished."
Clin Nutr. 2015;34(3):449-56.
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