What does leucine do?

What is leucine and what does it do?

Leucine is one of the nine essential amino acids, the building blocks of protein. Your body can’t make them, so they must come from food. It’s also one of the three branched-chain amino acids (BCAAs), named for their shape, with valine and isoleucine.

Its big job is as a signal. After a protein meal, muscle cells sense leucine and switch on their muscle-building machinery.

Think of leucine as the foreman who shouts “start”. The other eight essential amino acids are the bricks.

That’s the something to build with. Protein researcher Robert Wolfe says that without them from food, they have to come from muscle your body breaks down. So a shout with no bricks gets little built.

Your next move: count the protein in your meals, not the leucine in a tub.

How much leucine per meal?

The International Society of Sports Nutrition (ISSN), a sports science group, suggests 700 to 3,000 mg of leucine per meal. That’s 0.7 to 3 g. It pairs this with 20 to 40 g of protein.

You rarely need to count it. 25 g of whey protein holds 2.7 g of leucine. A serving of eggs gives about 0.5 g.

So hit 20 to 40 g of good protein at a meal and you’re usually there. For your daily total, see how much protein per day.

Which foods are high in leucine?

Whey and casein, the proteins in milk, are among the richest in leucine per gram of protein. Plant proteins have less on average, with some exceptions.

These are lab tests of protein powders, not cooked meals. The point still holds for food: plant eaters need a bit more protein for the same leucine.

Plant-based? Plan about 38 g of pea or 40 g of soy protein where a whey eater has 25 g. See plant protein for strength training.

Do leucine supplements build muscle?

Over a few hours, extra leucine raises muscle building. Over months, trials in older people found no extra muscle.

The short study was part-run by Nestlé researchers. Wolfe, quoted above, holds shares in an amino acid supplement company. The 3-month trial was the one in the hook: 7.5 g a day added no muscle or strength.

The ISSN agrees: leucine supplements should not be leucine alone. It says all nine essential amino acids give the biggest rise.

Want a powder anyway? A full protein or an EAA (all nine essential amino acids) product beats leucine alone. Compare them in EAAs vs BCAAs and protein to build muscle.

Is a leucine supplement safe?

Small studies have looked for the most leucine a day that’s safe. Their limits sit far above the 0.7 to 3 g a meal the ISSN suggests.

In 5 young men, blood ammonia, a waste product, rose once intake went above 500 mg per kg a day. The researchers said more than about 39 g a day may be a risk to health. A study in 6 older men suggested a similar limit could apply, about 35 g a day for someone weighing 70 kg.

A more cautious reading gives about 24.5 g a day for an older person of 70 kg. These were tiny, short studies, in men only.

Why batch-tested? UK Anti-Doping says labels can be inaccurate. The British Dietetic Association adds that supplements will not make up for poor food choices.

So spend on protein foods first, then decide if you need a powder.

When to speak to a GP

Speak to a GP or your specialist team if:

• You have kidney disease. The ISSN says there’s not enough data on amino acid supplements when the kidneys don’t work well.
• You have MSUD (maple syrup urine disease), a rare inherited condition that stops the normal breakdown of protein. The ISSN says amino acid supplements could cause a bad reaction.
• You keep losing weight without changing your diet or exercise routine. The NHS says to see a GP.

Over 40 and worried about losing muscle? See protein after 40.