Amino acid · C6H13NO2
The amino acid that starts muscle building. Pooled across nine trials in older people it lifted the synthesis rate and left lean mass exactly where it was.
L-leucine; free leucine; often called the anabolic trigger amino acid. One of the three branched-chain amino acids and one of the nine essential amino acids. Its metabolites are sold as separate products with their own literature: HMB (beta-hydroxy-beta-methylbutyrate) and KIC (alpha-ketoisocaproate, one of the branched-chain ketoacids).
Doses actually tested. Long-term training: 3.0 g a day as a single post-training dose across eight weeks in untrained young adults, which changed nothing (Aguiar 2017). Acute muscle protein synthesis: 3.0 to 5.0 g of total leucine in a single drink, where topping a small 6.25 g whey drink up to 5.0 g of leucine matched the response to 25 g of whey (Churchward-Venne 2014). For scale, 25 g of whey already contains 3.0 g of leucine (Churchward-Venne 2014).
35 g a day in young adults and 30 g a day in older adults, from a dose-finding programme run by the International Council on Amino Acid Science, an industry-funded nonprofit; no national or international standards body has adopted these figures (Elango 2023). The underlying dose-response study in young men found leucine oxidation reached its maximum at 550 mg per kg per day and plasma ammonia started climbing above 500 mg per kg per day, with no gut intolerance up to 1,250 mg per kg per day, which is 25 times the average requirement (Pencharz 2012).
Blood leucine peaks about 30 minutes after a drink containing it, measured on a branched-chain amino acid mixture rather than on leucine alone.
Free leucine powder and capsules, sold on its own. Inside every branched-chain amino acid (BCAA) and essential amino acid (EAA) product, where it is usually the largest single component. Whey protein. A 25 g serving supplies 3.0 g of leucine. Chicken breast. About 1.9 g of leucine per 100 g raw. Whole egg. About 1.1 g per 100 g. Milk. About 0.3 g per 100 g. Sold separately as its metabolite HMB, a different compound with a separate evidence base.
Leucine is the switch that turns muscle building on, so hitting your leucine threshold at every meal is what decides whether you grow.
Leucine really does start the response. Pooled across nine trials in older people it raised the muscle protein fractional synthetic rate substantially, a standardised mean difference of 1.08 with a confidence interval running from 0.50 to 1.67, and topping a small 6.25 g whey drink up to 5.0 g of total leucine made that drink match a 25 g whey drink for four and a half hours. What it does not do is finish the job. The same meta-analysis found no change in lean body mass, 0.18 with an interval from minus 0.18 to 0.54, or in leg lean mass. Three grams a day of free leucine across eight weeks of resistance training, in people already eating adequate protein, produced the same strength and the same quadriceps cross-sectional area as cornstarch.
Leucine is a switch, and the marketing stops there. Inside one meta-analysis the synthesis measure moved and the size measure did not, which is the distance between flipping a switch more often and having more to build with.
Trials of isolated branched-chain amino acids used single doses of 5.6 to 6 g. A hundred grams of raw chicken breast carries 4.1 g of them, and whole egg 2.6 g.
A food-composition database, not a study: it says how much is in what, and nothing about what these amino acids do. Values are averages across sampled products and shift with cut, brand and cooking loss. The figures are per 100 g, not per portion.
Healthy young men were fed leucine up to 25 times the daily requirement. Their capacity to burn it off topped out at 550 mg per kilogram a day; ammonia rose above 500.
The abstract never says how many men took part, which is the first thing you would want from a dose-response study. The adverse signal is a blood chemistry change, not a symptom anyone reported. Healthy young men, fed briefly, so nothing here reaches long-term intake.
Across nine trials in older people, leucine raised the rate of muscle protein synthesis substantially and moved lean body mass not at all.
Nine trials is a small pool and the searches ended in December 2013, so a decade of later work is missing. The two outcomes are measured on different clocks: synthesis takes hours, lean mass takes weeks to months, so part of the contrast is a contrast of study durations. Older adults only.
A 6.25 g whey drink topped up to 5.0 g of total leucine held muscle protein synthesis about as high as a 25 g whey drink for three hours. Eight men per arm.
Eight men per arm, one acute measurement, nothing past four and a half hours and nothing on muscle size. Part-funded and co-authored with a commercial protein-beverage research centre, a direct interest in a result about beverage formulation. A fifth arm added isoleucine and valine, and its value was never published.
No health agency has set an upper intake level for any amino acid. The figures quoted, 35 g a day for young adults and 30 g for older ones, come from that programme.
These numbers come from a programme run under the International Council on Amino Acid Science, a nonprofit funded by the amino acid industry. The review says so itself. Short-term trials in healthy adults only, so nothing about long-term intake or about people with liver or kidney disease.
No findings tagged to that goal.
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