Leucine

Leucine is the amino acid that tells your cells protein has arrived and it is time to build. That makes it central to muscle, but whether it is really the switch for muscle building is disputed, and in cell and mouse studies the same signal, kept high all day, pushes the immune cells in artery walls toward plaque.

Get leucine from whole protein, not supplements: about 0.4 g of protein per kilogram of body weight per meal (25–30 g for most adults) supplies roughly 2.5–3.0 g of it. Don't routinely go far past that at several meals a day unless you are training around it, and skip branched-chain amino acid (BCAA) products.

What it is

Leucine is one of the essential amino acids, the ones the body cannot make and has to get from food. With isoleucine and valine it forms the branched-chain amino acids (BCAAs).

Its fame comes from a growth sensor inside cells called mTOR (mechanistic target of rapamycin). mTOR reads how much amino acid is available, and leucine is the single strongest nutrient trigger for it.^[1] When mTOR is on, the cell builds protein and new ribosomes and switches off autophagy, its recycling-and-cleanup program.^[2] That one switch is why leucine matters for both muscle and arteries.

The muscle side: a useful marker, a disputed trigger

In older men, muscle building from a single meal plateaued at about 0.4 g of high-quality protein per kilogram of body weight, about two-thirds more than the 0.24 g/kg that did the same in young men.^[3] From whey or dairy, that dose carries roughly 2.5–3 g of leucine, and from mixed whole foods nearer 2–2.5 g, which is why leucine is usually named as the trigger.

The evidence for that is split. The review that tested the "leucine trigger" idea directly counted 16 study arms supporting it and 13 against, and limited it to older adults eating isolated proteins rather than whole food.^[4] A bedtime trial in 48 older men points the same way: 40 g of protein raised overnight muscle building, while 20 g did nothing, with or without extra leucine added.^[5] Topping up a small dose with leucine did not turn it into a large one.

Older adults do seem to need more of it. Measured directly in adults over 60, the daily leucine requirement came out at about 78 mg per kilogram, more than twice the 34–39 mg/kg currently recommended.^[6] Ageing muscle responds less to the same amount of protein, and enough total protein covers this.

The artery side: a careful mechanism, not a proven harm

The muscle-building dose sits close to the dose that switches on mTOR in macrophages, the immune cells that clear fat from artery walls. In one study, eating more than about 25 g of protein at a meal, or more than about 22% of daily calories as protein over time, pushed blood leucine high enough to do this.^[7]

The chain the authors describe: macrophages need autophagy to clear oxidised LDL (low-density lipoprotein) cholesterol from the artery wall; switching on mTOR shuts that autophagy off; macrophages that cannot clear their fat become foam cells, and foam cells become plaque. In male mice bred to develop atherosclerosis, adding leucine to a moderate-protein diet enlarged plaques, while adding the same amount of nitrogen as other amino acids did not.

The limits are real. The human groups were 14 and 9 people. What was measured was a signalling marker in blood immune cells, not plaque or heart attacks. The blood leucine range that switched the cells on was mapped in human macrophages in a dish, and most of the mice were male. It is a well-made mechanistic case, not a demonstrated clinical dose.

The one human outcome signal in this direction concerns total protein rather than leucine: in a large UK cohort, people eating 1.8 g of protein per kilogram a day or more had more cardiovascular events and deaths, and the extra risk of major cardiovascular events showed up only in the over-55s; the younger group was smaller, so its result is less certain.^[8] That study, and what it does and doesn't show, is covered under Protein.

Where leucine comes from

Among measured protein isolates, whey is one of the richest, at about 2.7 g of leucine per 25 g of protein; soy and pea isolates carry roughly 1.7–1.8 g.^[9] Collagen and gelatin carry very little, which is one reason they are poor for building muscle. Older adults who eat mostly plant protein can make up the difference by eating more of it and leaning on blends.

Why BCAA supplements don't work as sold

BCAA supplements deliver leucine, isoleucine and valine on their own. They trip the mTOR growth signal without supplying the other essential amino acids a muscle needs to build complete proteins, so the response is limited and can even draw on muscle breakdown to fill the gaps.^[10] Whole protein does the job better.

What to do

Count protein, not leucine. About 0.4 g of protein per kilogram per meal (25–30 g for most adults) brings the leucine with it.
Use whey or another high-leucine protein around training if you want a supplement, not BCAAs.
Don't routinely go far past 0.4 g/kg at several meals a day unless you are training around it; that is where the artery mechanism starts to apply.
Older adults: eat enough total protein. The higher leucine requirement past 60 is met by the protein targets on Protein, not by adding leucine to small meals.