Sarcopenia is the name given to the characteristic loss of muscle mass and strength that takes place with advancing age. A surprisingly large fraction of this loss is self inflicted: few people undertake the necessary exercise and strength training to maintain muscle in later life. But the rest of the losses are to some degree inevitable, a consequence of damage and disarray in muscle stem cells, neuromuscular junctions, and various processes necessary to muscle tissue maintenance. There is evidence for one those issues to be a growing inability to process leucine, an essential amino acid. Leucine supplementation may thus slow the onset of sarcopenia, even while being a compensatory approach that in no way addresses the underlying causes of this form of age-related degeneration.
One of the main ways in which sarcopenia contributes to disease is that it alters muscular turnover and metabolism. Moreover, older adults exhibit a decreased anabolic response to protein feeding, which is a mechanism underpinning the loss of muscle mass in sarcopenic individuals. Compared to younger adults, those aged over 65 years require ∼70% more protein per meal to maximally stimulate muscle protein synthesis. Furthermore, at a global level, only 40% of older adults meet the recommended daily allowance for protein (0.8 g/kg/day) and 10% of older women do not even meet the estimated average requirement of 0.66 g/kg/day.
One strategy to increase the muscle protein synthesis that has been investigated is the supplementation of diets with leucine, an essential branched-chain amino acid with important regulatory actions in muscles, which are at least partially mediated by the mammalian target of the rapamycin pathway. Leucine modifies protein turnover in skeletal muscles, by decreasing proteolysis and by increasing protein synthesis. Physiological research reports have shown that leucine can enhance muscle protein-synthesis. Furthermore, leucine can stimulate insulin release by pancreatic cells, showing that besides its beneficial effect in enhancing skeletal muscle glucose uptake, it is also an important anabolic signal in skeletal muscle.
Based on the above, administration of leucine-containing supplements is therefore a promising approach for treating sarcopenia. We took a systematic approach to analysing the current scientific evidence in this area, and to ascertaining whether the administration of leucine-containing supplements is effective in the treatment of sarcopenia. We also included interventions that used whey protein as a supplement, because these contain large amounts of leucine (approximately 13 g leucine/100 g protein) and the consumption of whey protein appears to be the most effective at increasing muscle protein synthesis.
In overall terms, published study results show that administration of leucine or leucine-enriched proteins (in a range of 1.2 g to 6 g leucine/day) is well-tolerated and significantly improves sarcopenia in elderly individuals, mainly by improving lean muscle-mass content and in this case most protocols also include vitamin D co-administration. The effect of muscular strength showed mixed results, however, and the effect on physical performance has seldom been studied.