Picture a building site. The bricks, sand and cement have all been delivered. The workers are there too. But nobody has started yet.
They're waiting for one person: the foreman to shout "go".
Inside your muscles, one amino acid plays exactly that foreman's role. Its name is leucine.
In this article
Who is leucine?
Leucine is one of the 9 essential amino acids, meaning your body can't make it and has to get it from food.
It belongs to a small group of three siblings called branched-chain amino acids, or BCAAs. The other two are isoleucine and valine. They're called "branched chain" because their chemical structure has a side branch, like a twig growing sideways.
Of the three siblings, leucine gets the most attention. Because it isn't just a brick. It's also a signal.
How does the "switch" work?
Inside muscle cells there's a control system for building protein, called mTOR. You can think of it as the site office.
When leucine levels in your blood rise after a meal, it sends a signal to mTOR: "The materials are here, start building". Muscle protein synthesis switches on.
But leucine only does the shouting. To actually build, your body still needs enough of all the other amino acids. Without bricks, the foreman can shout all day and no house gets built.
How much leucine is "enough to flip the switch"?
Many researchers talk about a "leucine threshold". The idea is that each meal needs a big enough dose of leucine for the switch to clearly turn on.
The number most often mentioned is about 2 to 3g of leucine per meal for adults. The International Society of Sports Nutrition (ISSN) gives a wider range, from 0.7 to 3g per protein serving, depending on the person. This is a reference point, not a hard line. Older adults may need more, because their muscles respond less strongly.
Good news: you don't need to track leucine separately. A meal with about 20 to 40g of protein from meat, fish, eggs or dairy usually reaches this threshold.
Which foods have leucine?
Leucine is in every kind of protein. The difference is the proportion. Reference figures from the USDA, rounded:
| Portion | Protein, about | Leucine, about |
|---|---|---|
| 100g cooked chicken breast | 31g | 2.5g |
| 100g cooked salmon | 22g | 1.8g |
| 2 chicken eggs | 12.6g | 1.1g |
| 1 glass of cow's milk, 250ml | 8g | 0.8g |
| 1 block of tofu (about 100g) | 8 to 11g | 0.6 to 0.9g |
| 1 bowl of white rice | 4g | 0.3g |
Looking at the table, a palm-sized piece of chicken breast already hits the threshold. A breakfast of just two eggs falls short. Add a glass of milk or a few slices of meat and you're there.
Does plant protein lose out?
Plant proteins usually have a slightly lower share of leucine than dairy, egg or meat protein. Soy protein does fairly well. Corn is actually quite high in leucine, but low in lysine.
Making up the difference isn't hard:
- Eat a bigger portion. Add half a block of tofu or a handful of edamame.
- Combine several sources. Beans with grains, like rice with tofu.
- Choose soy. Tofu, soy milk and edamame are reliable picks.
See more in Complete protein and plant protein.
Can BCAA drinks replace a meal?
This is where many people get the wrong idea.
Remember the building site? BCAAs are like sending three people: the foreman and two assistants. But no bricks. The switch may flip, but your body lacks the other six essential amino acids to actually build.
In 2017, researcher Robert Wolfe reviewed the evidence and concluded: BCAAs on their own aren't enough to meaningfully increase muscle protein synthesis. The body needs complete protein.
In other words: a piece of chicken breast, a pot of yogurt or a plate of tofu all do a better job than a glass of plain BCAA drink.
Inside the kOBE kitchen
Our Sous Vide Chicken Breast has 31g of protein per 100g of cooked meat. Going by the reference figures above, one 100g portion already reaches the leucine threshold for a meal.
We brine it only in sea salt, cook it sous vide at 60°C for 90 minutes, then chill it quickly and freeze it. The 90 to 120g packs are just right for one serving.
For plant protein fans: our Edamame has 11.9g of protein per 100g. A handful of edamame with brown rice and a few slices of tofu makes it easier for a vegetarian meal to reach the threshold.
Frequently asked questions
Should I take leucine pills?
Should older adults pay more attention to leucine?
Does leucine build muscle without exercise?
Keep reading
Sources
- Norton L.E., Layman D.K. (2006). Leucine regulates translation initiation of protein synthesis in skeletal muscle after exercise. Journal of Nutrition 136(2):533S to 537S.
- Wolfe R.R. (2017). Branched-chain amino acids and muscle protein synthesis in humans: myth or reality? Journal of the International Society of Sports Nutrition 14:30.
- Jäger R. et al. (2017). International Society of Sports Nutrition Position Stand: protein and exercise. Journal of the International Society of Sports Nutrition 14:20.
- USDA FoodData Central: amino acid content of chicken breast, salmon, eggs, milk, tofu and white rice.
- Further reading: Campbell B., Sports Nutrition, chapter 5; Understanding Nutrition 16th ed., chapter 6.
