People in their seventies who take an extra 4 g leucine with every meal synthesise more muscle mass. Nutritionists at the University of Texas discovered this when they did a small study of eight healthy elderly people.
Leucine is an amino acid that gives muscle cells' anabolic machinery a boost. Leucine – or one of its metabolites – probably helps muscle cells to 'see' that they are being supplied with amino acids and that they need to store these in the form of muscle protein. That's the rationale behind leucine-based bodybuilding supplements – and some functional foods intended to help patients to retain muscle mass.
But there are also studies which show that leucine doesn't work. Two disappointing studies were done in the Netherlands. [Am J Clin Nutr. 2009 May; 89(5): 1468-75.] [J Nutr. 2011 Jun; 141(6): 1070-6.] In both of these studies elderly subjects were given 2.5 g leucine at every meal, in one for three months and in the other for six months. In addition the subjects in the studies consumed 1 g protein/kg bodyweight/day.
Nutritionists in Texas will soon publish in Clinical Nutrition the results of a study in which elderly people were also given extra leucine with their meals. But instead of 2.5 g the Texans gave their subjects 4 g leucine at each meal. That meant the subjects got a total of 12 g leucine each day.
The subjects in the Texan experiment were given 0.8 g protein per kg bodyweight daily, so less than in the Dutch studies. And the Texan experiment only lasted two weeks.
This may be relevant: supplements users' experiences indicate that the effects of amino acid supplementation decline as time wears on, partly because the amino acid uptake decreases, and partly because the body' speeds up the metabolism of the supplement.
The Texan study was a success. Their subjects' mixed muscle fractional synthesis rate [FSR] increased – the build-up of muscle proteins – probably because anabolic signal molecules like mTOR and p70S6K were activated in the muscle cells. Both the FSR and the number of activated signal molecules increased throughout the 14 days of the study.
The researchers didn't measure the effect on the subjects' muscle mass, but believe there is a positive effect. "The magnitude of the increase we observed in basal and postabsorptive muscle protein synthesis during this period should have mathematically increased muscle mass by approximately 4 percent - conservatively assuming no change in muscle protein breakdown."