Leucine is an essential amino acid, meaning the human body cannot produce it and therefore must be attained from dietary sources. Leucine is the most abundant of the three branched chain amino acids (BCAAs) in muscles (the other two are isoleucine and valine – see nutrient profiles for each). BCAAs are unique because they are the only amino acids burned by muscles as fuel; thus, both blood and muscle levels of BCAAs decrease during and after exercise.
Leucine stimulates protein synthesis to build muscle. Some studies suggest preloading leucine to a workout is very beneficial in acutely increasing protein synthesis (references 3, 4).
A series of studies has clearly shown that leucine directly activates a critical compound in muscle called the mTOR (mammalian target of rapamycin). mTOR acts like a molecular “switch” that turns on the process of utilizing muscle proteins, and leucine is one of the major activators of mTOR. In other words, leucine activates mTOR to “switch on” muscle building (references 1, 2). Even when an overabundance of amino acids are available to provide the building materials for new muscle, adding extra leucine augments protein synthesis rates further.
Leucine also helps spare muscle when losing weight during a diet. Two University studies examined weight loss diets containing 10 grams of leucine per day and 125 grams total of protein per day with a minimum of 2.5 grams of leucine at each meal. In two separate studies, the groups consuming high leucine diets had greater weight loss, greater fat loss and better preservation of lean body mass. In these studies, the high leucine diets also resulted in better glucose control (reference 6).
One study reported that men taking more than 39 grams of supplemental leucine per day had an increased likelihood of experiencing harmful side effects. These included vitamin B-3 and vitamin B-6 deficiencies since leucine interferes with the body's ability to synthesize these nutrients. Not having enough vitamin B-3 may cause vomiting, depression and indigestion and may increase your risk of Alzheimer's disease and high blood cholesterol. A lack of vitamin B-6, or pyridoxine, can result in memory loss, irritability and depression. Consumption of leucine with a source of glucose, increases insulin levels, therefore people with diabetes should be careful to monitor their leucine intake.
Supplementing with just leucine alone will not result in optimal effects if a person’s diet is low in protein. This is because other BCAAs (isoleucine and valine) will be preferentially oxidized and lead to a BCAA imbalance that compromises anabolism.
The upper limit for leucine in healthy adult men is suggested to be 500 mg/kg per day as a cautious estimate (reference 5). When single doses have been studied, intake of as little as 2.5 grams of leucine stimulated protein synthesis. In long-term studies, leucine intakes equivalent to 8 or more grams per day are recommended in divided doses so that at least 2.5 grams of leucine are consumed at each meal.
1. Nair KS, Schwartz RG, Welle S Leucine as a regulator of whole body and skeletal muscle protein metabolism in humans . Am J Physiol. (1992)
2. Alvestrand A, et al Influence of leucine infusion on intracellular amino acids in humans . Eur J Clin Invest. (1990)
3. Tipton KD, et al Stimulation of net muscle protein synthesis by whey protein ingestion before and after exercise . Am J Physiol Endocrinol Metab. (2007)
4. Tipton KD, et al Timing of amino acid-carbohydrate ingestion alters anabolic response of muscle to resistance exercise . Am J Physiol Endocrinol Metab. (2001)
5. Am J Clin Nutr. 2012 Oct;96(4):759-67. Epub 2012 Sep 5. - Determination of the tolerable upper intake level of leucine in acute dietary studies in young men.