Leucine is utilized in the liver, adipose tissue, and muscle tissue. In adipose and muscle tissue, leucine is used in the formation of sterols, and the combined usage of leucine in these two tissues is seven times greater than its use in the liver.
Leucine is the only dietary amino acid that has the capacity to stimulate muscle protein synthesis. As a dietary supplement, leucine has been found to slow the degradation of muscle tissue by increasing the synthesis of muscle proteins in aged rats.However, results of comparative studies are conflicted. Long-term leucine supplementation does not increase muscle mass or strength in healthy elderly men. More studies are needed, preferably those which utilize an objective, random sample of society. Factors such as lifestyle choices, age, gender, diet, exercise, etc. must be factored into the analyses in order to isolate the effects of supplemental leucine as a standalone, or if taken with other branched chain amino acids (BCAAs). Until then, dietary supplemental leucine cannot be associated as the prime reason for muscular growth or optimal maintenance for the entire population.
While once seen as an important part of the three branch chained amino acids in sports supplements, leucine has since earned more attention on its own as a catalyst for muscle growth and muscular insurance. Supplement companies once marketed the "ideal" 2:1:1 ratio of leucine, isoleucine and valine; but with furthered evidence that leucine is the most important amino acid for muscle building, it has become much more popular as the primary ingredient in dietary supplements with a 4:1:1 ratio.
Leucine potently activates the mammalian target of rapamycin kinase that regulates cell growth. Infusion of leucine into the rat brain has been shown to decrease food intake and body weight via activation of the mTOR pathway.
Leucine toxicity, as seen in decompensated maple syrup urine disease (MSUD), causes delirium and neurologic compromise, and can be life-threatening. More studies need to be done on liver overactivity due to taking supplemental leucine.
Leucine is a branched-chain α-amino acid with the chemical formula HO₂CCHCH₂CH(CH₃)₂. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain.
Leucine is a tad different from the other two BCAAs Isoleucine and Valine as leucine seems to have a fair bit of testing on the amino acid in isolation rather than in a BCAA mixture, whereas the other two BCAAs are not as well studied.
The studies assessing leucine mostly look at muscle protein synthesis when additional leucine is added to the diet or to a test meal, and it appears that leucine is able to reliably increase muscle protein synthesis after test meals. Whether this results in more lean mass over a period of time is somewhat less reliable though, and leucine appears to be more effective at promoting gains in muscle in people with lower dietary protein intake and in the elderly (who tend to have impaired muscle protein synthesis in response to the diet).
The interactions of leucine on glucose are not clear, to be honest. Leucine possesses both blood sugar reducing properties (can release insulin from the pancreas, can directly stimulate glucose uptake into a cell without insulin) but also the opposite (via stimulating S6K, it can inhibit insulin-stimulated glucose uptake). In a cell culture, leucine stimulates glucose uptake for up to 45 minutes and then hinders itself while in living systems acute doses of leucine do not appear to do anything remarkable (some limited evidence that leucine can be rehabilitative in diabetes, but this is preliminary).
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