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http://www.sportsnutritionsupplementguide.com/supplementation/item/1528-the-power-of-leucine

Leucine could be considered the most important supplement you should add to your regimen as you age, especially if you are a competitor. Leucine breaks down much faster during exercise than the other two branched-chain amino acids (isoleucine and valine), and is also unique in that it plays a critical role in intramuscular protein production through anabolic signaling and mediation of insulin secretion, so it certainly makes sense to ensure that you are getting sufficient amounts of leucine in your diet.

If you are over the age of 35 or 40, leucine is depleted even more rapidly, so replenishing this amino acid through leucine rich food or through leucine supplements can be an insurance policy to guard against age-related muscle loss.

Leucine crosses the blood-brain barrier, and once in the brain, it sends satiety signals to the hypothalamus, thus guarding against overeating.1 It also promotes glucose tolerance and insulin sensitivity by optimizing glucose metabolism. Another important function which leucine has in the brain is the stimulation of glutamate production and release. Since glutamate is an important neurotransmitter in the brain which is involved in memory and learning, the presence of leucine exerts protective effects on cognitive function.

One study by Donato et al discovered that leucine supplementation while on a calorie restricted diet resulted in more fat loss overall, as well as increased protein synthesis in muscle tissue and the liver. 2 The human requirement for leucine is the highest of all the essential amino acids, at approximately 40 milligrams per kilogram of body weight in sedentary adults. However, this requirement does not take into account the muscle preserving needs of individuals who are engaged in intense resistance training. Such individuals benefit greatly from supplementing with up to 200 milligrams of leucine per kilogram of body weight. 3

If you are considering adding leucine to your regimen, be sure to time it so that you ingest it when you take in a protein-rich meal. If you prefer actual food sources of leucine, you can consume turkey breast (over 2 grams of leucine in a 3 ounce serving), fish, dairy products, seeds, soy or nuts. Leucine powder is VERY bitter, so make sure to mix it with a tasty protein powder, or take it in capsule form with your whole food meals. This will optimize protein synthesis in muscle tissue and enhance the anabolic effect.

REFERENCES

1. Potier M, Darcel N, Tome (2009) Protein, amino acids and the control of food intake. Curr Opin Clin Nutr Metab Care 12(1):54-58.

2. Donato J Jr, Pedrosa RG, Cruzat VF, Pires IS, Tirapegui J (2006) Effects of leucine supplementation on the body composition and protein status of rats submitted to food restriction. Nutrition 22 : 520 –527,2006

3. WHO/FAO/UNU (2007) Protein and amino acid requirements in human nutrition. Report of a joint WHO/FAO/UNU expert consultation. World Health Organ Tech Rep Ser. 1–265.