Leucine is an essential amino acid, needed in the diet; the human body cannot synthesize it from simpler metabolites. Young adults need about 31 mg of this amino acid per day per kilogram (14 mg per lb) of body weight. Leucine can be degraded into simpler compounds by the enzymes of the body. Leucine contributes to the structure of proteins into which it has been incorporated by the tendency of its side chain to participate in hydrophobic interactions. Leucine was isolated from cheese in an impure form in 1819 and from muscle and wool in the crystalline state in 1820. It was named after the Greek word leukos [white], evidently because at that time the purification of a subtance from nature to a white, crystalline state was considered noteworthy. The stucture of leucine was established by laboratory synthesis in 1891.
It’s needed not only for protein synthesis but also for a sound immune system. Some infants can’t metabolize leucine (or Isoleucine or Valine), which causes a condition known as maple syrup urine disease. This is a very rare condition, however, and is not a general concern as it relates to leucine supplementation. Leucine supplementation should not be employed in these individuals.
Leucine, in conjunction with two other amino acids, isoleucine and valine, appear to be quite helpful in treating and in some cases even reversing hepatic encephalopathy, a form of liver damage in alcoholics. They also help curb muscle wasting in this disease and through their actions on brain neurotransmitters, help prevent some adverse neurological effects of chronic liver disease.
This is one of the most important amino acids for hard training body builders. Leucine and the other branched chain amino acids (BCAAs), isoleucine, and valine, escape liver metabolism and can directly and significantly influence muscle-protein metabolism. Dietary leucine serves as a substrate for muscle metabolism during periods of cellular energy depletion, there by sparing critical contractile and enzyme muscle protein from degradation to supply leucine requirements.
Because leucine contributes to gluterine synthesis, taking supplemental leucine before and after intense training and between meals can help normalize glutimine levels in both the serum and muscle, thereby promoting anticatabolic muscle metabolism as well as supporting immune function.
Leucine appears to be the most important BCAA for athletes, as it can affect various anabolic hormones, and have an effect on preventing protein degradation. HMB is a metabolite of Leucine.
Researchers have expended a considerable amount of effort on evaluating the effects of supplementation of branched-chain amino acids (BCAAs: leucine, isoleucine, and valine) on physiological and psychological responses to exercise. There are two primary hypotheses regarding the ergogenic value of supplementation with these amino acids.
First, BCAA supplementation has been reported to decrease exercise-induced protein degradation and/or muscle enzyme release (an indicator of muscle damage) possibly by promoting an anti-catabolic hormonal profile. Theoretically, BCAA supplementation during intense training may help minimize protein degradation and thereby lead to greater gains in fat-free mass. Although several studies support this hypothesis, additional research is necessary to determine the long-term effects of BCAA supplementation during training on markers of catabolism, body composition, and strength.
Second, the availability of BCAA during exercise has been theorized to contribute to central fatigue. During endurance exercise, BCAAs are taken up by the muscles rather than the liver in order to contribute to oxidative metabolism. The source of BCAAs for muscular oxidative metabolism during exercise is the plasma BCAA pool, which is replenished through the catabolism of whole body proteins during endurance exercise.