What Is Leucine? The Leucine Threshold, Explained

Leucine comes up whenever anyone talks about protein and muscle, and the "leucine threshold" is the reason. The idea fits in one line: muscle appears to respond fully to a meal only once the protein in it delivers a certain amount of leucine. What sits behind that line is more useful than the slogan. Here is what leucine is, where the threshold idea came from, the two numbers people confuse, how much leucine ordinary foods carry, and one position a British reader should know about: leucine itself has no authorised health claim in the UK or EU. Protein, the thing you actually eat, does.

What Is Leucine?

Leucine is one of the twenty amino acids the body uses to build protein, and one of the nine essential ones it cannot make and has to get from food.⊕ It is also one of three branched-chain amino acids, or BCAAs, alongside isoleucine and valine. "Branched-chain" describes the shape of the molecule and nothing more. The "L" in L-leucine marks the form found in food and in the body. BCAAs are sold as a group, but in the research they do not behave as one, and leucine is the member researchers keep coming back to.

What Does Leucine Do in the Body?

Like every amino acid, leucine is a building block. What sets it apart is a second job. After a protein meal, the rise in leucine in the blood is sensed by a pathway inside muscle cells called mTORC1, and in studies that pathway is associated with the increase in muscle protein synthesis, the rate at which muscle builds new protein, over the hours after eating.⊕ In plain English, in those studies leucine is the signal that tells muscle protein has arrived.

A small share of leucine is converted into a compound called HMB, which has a research literature of its own. In one small trial in young men, both leucine and HMB raised muscle protein synthesis.⊕ HMB is a subject in its own right. For regulatory purposes it sits where leucine does: EFSA reviewed the proposed claims for it in 2011 and found none established.⊕

What Is the Leucine Threshold?

The idea came out of research in the 2000s on how much protein a meal needs to produce the full muscle protein synthesis response. The response tracked the rise in blood leucine. Below a certain amount per meal it was smaller than it could be; give more and it rose; keep giving more and it levelled off. Researchers called this the "leucine trigger hypothesis", and the amount where the response plateaued became known as the leucine threshold.

In one study, 20 g of whey produced the full response in young men, in both rested and exercised muscle, and doubling the dose to 40 g added only a little on top.⊕ Whey is about 11 per cent leucine, so 20 g carries roughly 2.2 g.⊕

A 2021 systematic review tested the hypothesis across 29 published trials. Sixteen supported it. The support was strongest when isolated protein sources such as whey were compared against each other, and in older adults, and weaker for whole foods and mixed meals, where fat, fibre and the rest of the plate change how quickly amino acids arrive.⊕ So the threshold is a working model with evidence behind it and limits around it. Muscle does not stay switched off until a number is reached.

How Much Leucine Per Meal, and How Much Per Day?

There are two numbers, and they answer different questions.

The daily requirement. The World Health Organization's reference figure for leucine is 39 mg per kilogram of body weight per day,⊕ about 2.7 g for a 70 kg adult. That is the amount needed to avoid running short. Anyone eating the European reference intake for protein, 0.83 g per kg per day,⊕ from ordinary foods clears it comfortably.

The per-meal amount. The threshold research asks something different: how much leucine in a single meal is associated with the full response. In younger adults the studies put that at roughly 2 to 3 g, which is what you get from about 20 to 30 g of high-quality protein.⊕ In older adults it was higher. The 2021 review notes a proposed threshold of around 3 g for that age group,⊕ and the PROT-AGE expert group cites 25 to 30 g of protein per meal, carrying around 2.5 to 2.8 g of leucine.⊕ This is the number that describes a portion on a plate. Rounded to about 3 g per meal, three times a day, it covers younger and older adults alike, and it is where the PROT-AGE group's 25 to 30 g of protein per meal lands.

The bridge between the two numbers is protein, and this is where the regulatory position earns its place in the article. Leucine has no authorised health claim. Protein does: "Protein contributes to a growth in muscle mass" and "Protein contributes to the maintenance of muscle mass".⊕ Leucine explains the mechanism. Protein is the instruction.

Does the Leucine Threshold Change With Age?

In the research, yes. One dose-response analysis compared younger and older men and found the protein dose associated with the maximal response was around 0.24 g per kg of body weight per meal in the younger group and around 0.40 g per kg in the older group.⊕ For a 70 kg adult that is roughly 17 g of protein against 28 g. A separate trial gave older adults a small dose of essential amino acids and found that raising the proportion of leucine in it brought the response back up to what younger adults showed.⊕ The signal needs to be louder with age. Muscle loss with age is a subject in its own right rather than a paragraph here.

Which Foods Contain Leucine, and How Much?

Leucine is in every protein food. The useful way to think about it is as a percentage of the protein, because that works for any label. In one laboratory comparison, whey was the richest at 11 per cent leucine, milk protein was 9 per cent, casein 8 per cent and egg 7 per cent. Animal proteins averaged just under 9 per cent; plant protein isolates averaged about 7 per cent, with a wide spread from hemp at the bottom to corn, unusually, above whey.⊕

Here are the main leucine foods in UK portions, rounded, from a published food composition table (values per 100 g of food, raw weight unless stated):⊕

• Whey protein. 25 g of protein (about one scoop): roughly 2.7 g of leucine.
• Chicken breast, skinless. 120 g portion, about 28 g of protein: roughly 2.3 g of leucine.
• Turkey breast, skinless. 120 g portion, about 29 g of protein: roughly 2.4 g of leucine.
• Beef, lean rump. 120 g portion, about 26 g of protein: roughly 2.3 g of leucine.
• Tuna, tinned, drained. 100 g, about 25 g of protein: roughly 2.0 g of leucine.
• Salmon. 120 g portion, about 22 g of protein: roughly 1.8 g of leucine.
• Eggs. 2 large (about 110 g), about 14 g of protein: roughly 1.1 g of leucine.
• Lentils, boiled. 200 g portion, about 14 g of protein: roughly 1.1 g of leucine.
• Chickpeas, boiled. 200 g portion, about 14 g of protein: roughly 1.1 g of leucine.
• Greek yoghurt, low-fat. 170 g pot, about 15 g of protein: roughly 1.2 g of leucine.
• Semi-skimmed milk. 250 ml glass, about 9 g of protein: roughly 0.9 g of leucine.
• Parmesan. 30 g, about 10 g of protein: roughly 0.9 g of leucine.

The whey figure comes from the laboratory comparison above, which measured 2.7 g of leucine in 25 g of whey protein.⊕ The rest are from the food table; cooked meat and fish lose water, so a 120 g raw portion is nearer 90 g on the plate.

A palm-sized portion of chicken, turkey, beef or tuna, or a scoop of whey, reaches the 2 to 3 g range on its own. Most other foods get there in combination: eggs with a glass of milk, yoghurt with a handful of nuts, lentils with something dairy alongside. One large egg carries about 0.5 to 0.6 g, which is why "how many eggs for 3 g" comes out at five or six, and why people combine foods instead.

Look at the plant rows and the gap is obvious: 200 g of lentils or chickpeas carries about a third of what a chicken breast does. For plant-based athletes the threshold is worth knowing about, because from plant foods alone it takes larger portions and a mix of sources, planned at each meal rather than assumed.

Can You Reach the Leucine Threshold on Plant Protein?

Yes, but it takes larger portions, a mix of sources, and more deliberate planning than most people expect. If a plant protein averages 7 per cent leucine and whey is 11 per cent, a plant-based meal needs roughly half as much protein again to carry the same leucine; against milk protein at 9 per cent the gap is nearer a quarter.⊕ A review of plant versus animal protein lists the obvious fixes: eat a little more of it, or blend sources so one covers another's gaps.⊕ A 200 g portion of lentils with a 170 g pot of yoghurt lands in the same range as a chicken breast; lentils on their own, in a portion most people would call generous, do not. Plant-based protein for people who train is a fuller subject of its own.

Why Spreading Protein Across the Day Matters More Than Any Single Meal

If the threshold is a per-meal idea, the obvious question is how many meals. The physiology explains why it has to be asked. Passing the threshold produces a rise in muscle protein synthesis, but the rise is transient: in one study it peaked within about two hours of a protein meal and then returned to baseline even though amino acid levels in the blood were still raised.⊕ Between meals the balance tips back towards muscle breakdown, which is how the body keeps a steady pool of amino acids and nitrogen available for its other functions. Each meal that passes the threshold buys a window of building, not a permanent state, and the research on how many windows a day has a fairly clear answer.

In one small crossover study, eight adults ate the same total protein either as about 30 g at each of breakfast, lunch and dinner, or skewed towards dinner. The even pattern was associated with 25 per cent higher muscle protein synthesis over 24 hours.⊕ A trial after resistance exercise compared 80 g of whey taken as two, four or eight doses over twelve hours. Four doses of 20 g produced the greatest response.⊕

The caveat comes from a 2023 study in which 100 g of protein in one sitting produced a bigger and longer response than 25 g, with no upper limit found within the twelve hours measured.⊕ So the old idea that anything over 30 g in a meal is "wasted" does not hold. A big protein dinner is still used. It just does not undo a breakfast that had almost none.

Put together, the research points to a plain habit: a proper protein portion at breakfast, lunch and dinner. The authorised claim is about the protein itself, protein contributes to the maintenance of muscle mass; the distribution studies above are the research on how to arrange it across the day. "When to take leucine" has the same answer: with meals, across the day.

Do You Need a Leucine Supplement?

For someone already eating deliberate protein portions at each meal, no, and the reason is in the food list. The honest caveat is that "deliberate" is doing real work in that sentence. Reaching the amounts used in the studies takes a planned, often large portion of protein at every meal, not a token amount, and a toast-and-coffee breakfast does not get close. The studies that compared isolated leucine with whole protein explain why. When researchers topped up a small dose of whey with leucine, the response at rest matched a full 25 g dose. After exercise, only the full dose of protein kept the response going into the third to fifth hour.⊕ Leucine is the signal; the other essential amino acids are the material.

The regulatory position matches. EFSA assessed the proposed BCAA claims in 2010, covering muscle mass, muscle power, recovery, cognitive function after exercise, perceived exertion and immune function, and found none substantiated.⊕ No claim for leucine or BCAAs is authorised in the UK or EU. Creatine, by contrast, does carry an authorised claim at 3 g a day,⊕ and what creatine does and why it is so well studied is covered separately. A product can list leucine and state the amount, but not what that amount does.

None of this makes leucine powder pointless to study. It makes it unnecessary for someone already eating enough protein in decent portions. Research has looked at other single amino acids in the same way, among them glutamine, the body's most abundant amino acid.

Is Leucine Safe?

In protein foods, entirely. For isolated leucine, a set of short dose-finding studies in young men proposed a cautious upper intake of 500 mg per kg of body weight per day, around 35 g a day, far above anything a supplement label suggests.⊕ One exception matters: people with maple syrup urine disease, a rare inherited condition in which the body cannot break down BCAAs, manage leucine intake under specialist care.⊕ Anyone with diabetes or kidney disease considering a leucine or BCAA supplement should speak to their GP first.

Leucine FAQs

Is Leucine an Essential Amino Acid?

Yes, one of the nine the body cannot make, so it has to come from protein foods.

What Is L-Leucine?

The same thing. The "L" marks the natural form, not a stronger one.

Is Leucine the Same as BCAA?

No. There are three BCAAs: leucine, isoleucine and valine, and all three are essential amino acids. Leucine is the one studied as a signal.

How Much Leucine Is in Whey Protein?

About 11 g per 100 g of protein, so 25 g of whey protein carries roughly 2.7 g.

How Many Eggs for 3 g of Leucine?

Five or six, at roughly 0.5 to 0.6 g per large egg. Two eggs with dairy alongside gets there more easily.

When Should You Take Leucine?

With meals, across the day. Timing tricks are not supported by the research.

What Is HMB?

A derivative of the amino acid leucine: the body makes it from a small share of its leucine. It is also involved in muscle metabolism and has an extensive research base of its own.

Does Leucine Build Muscle on Its Own?

No. Leucine is a signal; the other amino acids are the material. The authorised claim belongs to protein: "Protein contributes to a growth in muscle mass".

The Bottom Line

Leucine is the amino acid that, in the research, signals to muscle that protein has arrived. The threshold is the per-meal amount, roughly 2 to 3 g and a little more with age, that studies associate with the full response. It is a working model, not a switch, and nobody needs to count leucine, but the portions have to be deliberate. A palm-sized portion of meat, fish or dairy, or a noticeably larger and better-planned plant-based one, at each of three meals does the counting for you. The authorised claim belongs to the protein on that plate: protein contributes to the maintenance of muscle mass. Leucine is the part of it researchers study as the signal. Protein is what you eat, as part of a varied, balanced diet and a healthy lifestyle.