The short version
- Muscle protein synthesis after resistance exercise was maximally stimulated at 20 g of whole egg protein in young men, with leucine oxidation rising significantly at 20 and 40 g.
- A 6.25 g whey drink topped up with leucine matched 25 g of whey for muscle protein synthesis from one to three hours post-exercise, but only the full 25 g dose was still elevated at three to five hours (184% versus 55%).
- Essential amino acid content is 21% for oat, 21% for lupin and 22% for wheat isolates, against 43% for whey, 39% for milk and 38% for human muscle protein.
- Leucine content ranges from 5.1% in hemp protein to 13.5% in corn protein, compared with 9.0% in milk, 7.0% in egg and 7.6% in human muscle protein.
- Topping a 6.25 g whey dose up to 3.0 g of total leucine was not enough to match a 25 g dose, while topping it to 5.0 g produced around 220% versus 267% for the full dose.
Two scoops of protein powder can contain the same 25 grams and produce measurably different responses in muscle. That is not a manufacturing defect. It is the amino acid profile, and the single amino acid that does most of the explaining is leucine.
The threshold, and what sits on the other side of it
Start with the dose-response. Moore and colleagues fed six young men 0, 5, 10, 20 or 40 g of whole egg protein after leg resistance exercise and measured muscle protein synthesis with a leucine infusion. Synthesis rose with dose and was maximally stimulated at 20 g. Above that, the extra protein did not add to synthesis — it added to oxidation, which increased significantly at both 20 and 40 g.
So there is a per-meal ceiling, and also a per-meal floor: a feeding has to be big enough to trigger the response at all. The trigger is where leucine comes in, and the cleanest demonstration of both its power and its limits is a 2012 trial.
Leucine pulls the trigger. It does not hold it.
Churchward-Venne and colleagues gave 24 men one of three drinks after single-leg resistance exercise: 25 g of whey; 6.25 g of whey topped up with leucine to match whey's total leucine; or 6.25 g of whey topped up with every essential amino acid except leucine to match whey's amounts.
From one to three hours post-exercise, all three raised muscle protein synthesis equally — the small dose plus leucine matched the full 25 g. Then the curves separated. From three to five hours, only the full whey drink remained significantly elevated above fasting in the exercised leg: whey 184%, versus 55% for the leucine-supplemented drink and 35% for the one missing leucine.
That is the whole mechanism in one experiment. Leucine initiates the response. The rest of the essential amino acids are the raw material that sustains it. A trigger with nothing behind it fires once.
A follow-up in 2014 found you can push a small dose further, but it takes more leucine than people assume. Forty men received 25 g of whey (3.0 g leucine), 6.25 g of whey (0.75 g leucine), or 6.25 g of whey topped up to 3.0 or 5.0 g total leucine. Over the 1.5 to 4.5 hour window, synthesis was greatest after the 25 g whey drink at around 267% and the high-leucine 6.25 g drink at around 220%. Topping up to 3.0 g of leucine was not enough; 5.0 g got close.
Why this lands hardest on plant protein
Gorissen and colleagues measured the essential amino acid content and full amino acid profile of a large set of commercial plant isolates against animal proteins and against human skeletal muscle. The essential amino acid contents are not close: oat 21%, lupin 21% and wheat 22%, against whey 43%, milk 39%, casein 34% and egg 32%. Human muscle protein itself is 38%.
The leucine content varies even more widely between plant sources — from 5.1% in hemp to 13.5% in corn — compared with 9.0% for milk, 7.0% for egg and 7.6% for muscle protein. Methionine and lysine are also typically lower in plant proteins, at 1.0% and 3.6% on average, against 2.5% and 7.0% in animal proteins.
Put those two findings together and the practical conclusion is arithmetic rather than ideology. If your protein source is roughly half as rich in essential amino acids, a scoop that hits the threshold has to be roughly twice the size.
The part that is not only about leucine
van Vliet, Burd and van Loon's review is a useful corrective to treating this as a single-nutrient story. Plant proteins may produce a smaller muscle protein synthetic response for several reasons at once: lower digestibility, greater splanchnic extraction and subsequent urea synthesis of the amino acids that do get through, a relative lack of specific essential amino acids, and typically low leucine. Their proposed strategies are the honest list — fortify with methionine, lysine and/or leucine; selectively breed better profiles; eat greater amounts; or combine multiple protein sources for a more balanced profile. They also note the efficacy of those strategies still needs studying.
What we'd actually tell you
Aim for around 20 to 25 g of a complete protein per feeding, four times a day. If you are eating plant protein, either take a larger serving or blend sources — a soy-pea-rice mix covers more of the profile than any of the three alone, and Gorissen's data shows the profiles differ enormously between isolates.
What we would not do is buy leucine on its own to sprinkle on a small dose. The 2012 trial shows exactly what that buys: a normal early response and a truncated later one. Two spare grams of leucine is a cheaper purchase than ten more grams of protein and a worse one. That is also, in miniature, the argument against BCAAs — three amino acids can start something that nine amino acids have to finish — and it is why protein timing turns out to matter less than protein distribution.
Good questions
What is the leucine threshold?
It is the idea that a protein feeding has to deliver enough leucine to switch on muscle protein synthesis at all, rather than the total grams alone determining the response. In one trial, a 6.25 g whey dose topped up to 5.0 g of leucine produced around 220% of baseline synthesis while the same dose with 0.75 g of leucine did much less. Leucine starts the process; the other essential amino acids sustain it.
Why do plant proteins need a bigger scoop?
Because there is less of the relevant material per gram. Oat, lupin and wheat isolates measured around 21 to 22% essential amino acids against 43% for whey, and their leucine content varies from 5.1% in hemp to 13.5% in corn. Plant proteins are also generally lower in methionine and lysine and less digestible. A larger serving, or a blend of sources, closes most of that gap.
Should I buy a leucine supplement instead of more protein?
No, and there is a trial that shows exactly why not. Adding leucine to a small whey dose matched a full 25 g dose for the first three hours after exercise, then fell away: at three to five hours only the full dose was still significantly elevated, 184% against 55%. Leucine fires the trigger, the full amino acid profile provides the material. Spare leucine with nothing behind it gives you a short response.
Is 20 g of protein per meal really the ceiling?
It was in young men eating whole egg protein after leg training, where synthesis maxed out at 20 g and extra protein went to oxidation instead. Treat it as a practical target rather than a hard biological limit — the study used six participants, one protein source and one exercise protocol. Four feedings of 20 to 25 g across the day is a defensible way to apply it.
Can I hit the threshold on a fully plant-based diet?
Yes, with larger servings or a blend. The research group that measured these profiles suggested exactly that: combine multiple plant isolates for a more balanced amino acid profile, fortify with the amino acids that are short, or simply consume greater amounts. They also note those strategies have not been fully tested for their effect on muscle protein synthesis, so the honest answer is that the arithmetic is clear and the trials are still catching up.
Sources
- Moore DR, Robinson MJ, Fry JL, et al. Ingested protein dose response of muscle and albumin protein synthesis after resistance exercise in young men. Am J Clin Nutr, 2009. View study
- Churchward-Venne TA, Burd NA, Mitchell CJ, et al. Supplementation of a suboptimal protein dose with leucine or essential amino acids: effects on myofibrillar protein synthesis at rest and following resistance exercise in men. J Physiol, 2012. View study
- Churchward-Venne TA, Breen L, Di Donato DM, et al. Leucine supplementation of a low-protein mixed macronutrient beverage enhances myofibrillar protein synthesis in young men: a double-blind, randomized trial. Am J Clin Nutr, 2014. View study
- Gorissen SHM, Crombag JJR, Senden JMG, et al. Protein content and amino acid composition of commercially available plant-based protein isolates. Amino Acids, 2018. View study
- van Vliet S, Burd NA, van Loon LJ. The Skeletal Muscle Anabolic Response to Plant- versus Animal-Based Protein Consumption. J Nutr, 2015. View study
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