The review, published in the British Journal of Nutrition, looked at nine randomised control trials with the primary outcome of protein synthesis - or fractional synthetic rate - and the secondary outcomes of lean body and leg lean mass.
The researchers from the Zhejiang University in China found that muscle protein fractional synthetic rate after intervention significantly increased in the leucine group compared with the control group. However, no difference was found between the groups in relation to lean body mass or leg lean mass.
“In conclusion, we found that ingestion of leucine significantly increased the muscle protein fractional synthetic rate in elderly individuals, and thus may be of benefit to address sarcopenia [the degenerative loss of skeletal muscle mass] in this population,” they wrote.
The researchers recommended long-term or acute supplementation of the amino acid leucine to achieve these results, suggesting this could be an effective way of compensating for the blunted response to amino acid ingestion in the elderly and normalising the postprandial response of muscle protein synthesis.
Leucine can have an impact on muscle mass in several ways including by acting as a building block for protein synthesis and as a nutritional signal which stimulates protein synthesis.
“The effect of leucine on muscle protein metabolism is complex, as it influences both muscle protein synthesis and degradation, and this complexity may confound study findings,” the researchers wrote.
For example, one study not included showed leucine (0.14 grams per kilograms of body weight over a seven hour period) in healthy subjects resulted in decreased protein degradation by about 35%. Several other studies have shown a significant increase of around 35–50% in this rates in healthy males after the administration of amino acids including leucine. Meanwhile others have not detected an effect at all.
The researchers said results could differ according to the amount of leucine administered, the time of administration and the population studied.
Five of the studies included in the review used long-term dosing (ten days) and four had short-term dosing (eight hours). It has been suggested that how long leucine is administered for could influence results, but since one of the short-term studies found an improvement in the muscle protein fractional synthesis rate they suggested that this was not the only influencing factor.
The longest study included in the meta-analysis was three months, and the researchers said the long-term effect of leucine on muscle mass were still not clear.
The researchers said the different effects of leucine supplementation in elderly subjects could be explained in several ways.
“One idea is consistent with the ‘anabolic threshold concept’, suggesting that in elderly people, changes in key signalling pathways and changes in catabolic factors and oxidative stress may have negative effects on amino acid or insulin signalling pathways that play a role in the stimulation of muscle anabolism after food intake.”
These changes may lead to ‘anabolic resistance’ of muscle, meaning a higher level of stimulation is needed to retain protein.
Another idea, not mutually exclusive, was that the muscle becomes ‘full’ when exposed to persistent levels of amino acid.
Source: British Journal of Nutrition
113, pp 25-34. doi:10.1017/S0007114514002475
“The effectiveness of leucine on muscle protein synthesis, lean body mass and leg lean mass accretion in older people: a systematic review and meta-analysis”
Authors: Z. Xu, Z. Tan, Q. Zhang, Q. Gui and Y. Yang