If you're trying to max out your muscle gains or bulking up with bodybuilding, leucine (or l-leucine) is one of the most important nutrients you can eat.
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That's because leucine plays an enormous role in telling your muscle fibers to use the protein and other amino acids you eat to grow, explains Gabriel Wilson, PhD, CSCS, a certified strength and conditioning specialist at the Applied Science and Performance Institute. Leucine works as an anabolic trigger, and is metabolized directly in muscle tissue.
Here's everything you need to know about the importance of leucine for muscle growth and how to feed your muscles what they need.
What Is L-Leucine?
Leucine is one of 20 amino acids that are the building blocks of proteins, says Albert Matheny, RD, CSCS, registered dietitian and co-founder of Soho Strength Lab. FYI, every protein in your body — and every protein you eat — is made up of some combination of these amino acids.
Of these 20 amino acids, l-leucine is one of three essential aminos known as branched-chain amino acids (BCAAs). They're called essential because your body can't produce them on its own. You have to get them from foods or supplements, Wilson says.
These three BCAAs — which include leucine, isoleucine and valine — are key to muscle protein synthesis (aka muscle growth or hypertrophy). "They're rate-limiting, so without enough of them, your recovery from working out is hindered," Matheny says.
How Leucine Boosts Muscle Growth
All BCAAs help your muscles bounce back and grow bigger and stronger, but "leucine, in particular, is unique in its ability to stimulate muscle protein growth," Wilson says. Basically, without leucine, muscle growth can't start.
The theory is that when levels of the amino acid in your blood increase, they flip the switch on a signaling chain called the mTOR (mechanistic target of rapamycin) pathway. That mTOR pathway is in charge of telling your body to shift into a state of growth, also called an anabolic state, Wilson explains.
Because it acts as the green light signaling your body to build muscle, leucine can ultimately help increase your overall muscle growth and lean body mass, according to the University of Rochester Medical Center. It can also help athletes who are injured avoid declines in muscle mass, according to a 2020 review in Nutrients.
How Much Leucine Do You Need Per Day?
The recommended daily intake (RDI) for leucine is 17.7 milligrams per pound of your body weight. So a person weighing 180 pounds should get around 3,186 milligrams, or 3.186 grams, of leucine per day.
That said, if you're bodybuilding or actively trying to maximize your muscle growth, you will likely need much more. For instance, a June 2017 Journal of the International Society of Sports Nutrition position stand states that athletes need 700 - 3,000 milligrams (up to 3 grams) of leucine every 3 to 4 hours throughout the day. That is several times the current RDI.
That said, more leucine doesn't always equal more muscle. "There appears to be a 'leucine threshold,'" according to Wilson, which means that past a certain leucine dosage, you may not reap additional muscle growth.
For example, a 2014 American Journal of Clinical Nutrition study compared the muscle-growth responses in men who drank different combinations of whey protein and a leucine supplement after strength training. Results showed that eating 6.25 grams of whey protein supplemented with leucine is no more effective in promoting muscle growth than 25 grams of whey protein alone.
Are You Getting Enough Leucine From Foods?
Luckily, there are plenty of leucine-rich foods out there, according to Wilson. Because foods high in protein generally contain the amino acid, you're probably getting enough of this essential amino acid — if you're getting enough quality protein, that is — according to Matheny. Supplements, specifically whey protein and BCAA powders, are also a popular way to make sure you're meeting your needs.
The American College of Sports Medicine recommends that people wanting to build muscle get 1.2 to 1.7 grams of protein per kilogram, or 0.5 to 0.8 grams per pound of body weight per day. For someone who weighs 150 pounds, that's between 75 and 120 grams of protein per day.
However, that's only the case if you eat animal proteins (like beef, fish, chicken, milk and eggs). Animal proteins are the best food sources of l-leucine. People who do not animal-based foods may need closer to 120 to 150 grams of daily protein to get that same amount of leucine, Wilson says.
Should You Take Leucine Supplements?
As you've read, if you're not eating animal-based proteins, you may need to eat more protein to cover your daily leucine dosage needs through foods alone. And to get 40 to 50 grams of protein at every meal, you would have to eat a lot of food to eat per meal and per day. It could potentially be more food than is feasible, comfortable or healthy for you.
That's where supplementing with an l-leucine, BCAA or protein powder comes in handy.
But plant eaters aren't the only ones who might want to supplement with leucine, either. If you're into fasting (or at least exercise fasted, like first thing in the morning before having breakfast), you may need to increase your intake to get the full benefits of leucine.
"Since leucine and other amino acids play other roles in your body besides building muscle, if you're low on them because of fasting, your body breaks down muscle to use those amino acids for other priorities," Matheny says.
Super active people like avid CrossFit competitors who train hard may also need more leucine (and other aminos) than the average gym-goer because they regularly put greater stress on their muscles, he adds.
If you’re not sure whether you’re meeting your leucine — or overall protein needs — working with a registered dietitian can help you take a closer look at your diet and make any adjustments needed to reach your health, fitness and muscle goals.
How and When to Take Leucine for Muscle Growth
As mentioned, getting 30 to 40 grams of high-quality protein three to four times daily is typically going to give your body the leucine you need to effectively build muscle.
But if you need to incorporate supplements to hit these marks, go for it. Leucine and BCAA supplements are also easier to digest than protein because the amino acids in them are already broken down and ready for your body to use, Matheny says. Just make sure the supplement you choose is third-party tested by an independent lab like USP or NSF and the ingredient list is as short as possible.
Take 2.5 grams of leucine or 5 grams of total branch chain amino acids 30 minutes or less after training to encourage an anabolic stimulus.
Immediately after a workout is the best time to take leucine. Taking a supplement 15 to 30 minutes post-workout will help your body to start repairing your muscles, he says. This is especially helpful if you work out several times per week on back-to-back days.
However, in some cases, you might also want to incorporate a leucine-rich supplement during your workouts. By keeping your muscles fueled with leucine and the other BCAAs, you can minimize the muscle breakdown that often occurs during long bouts of exercise, say while long-distance running or cycling.
Taking leucine during exercise can also be beneficial if you're working out after hours without eating, like if you're intermittent fasting and bodybuilding or just don't have time to eat before hitting the gym, Matheny says.
If you want a BCAA supplement, opt for one that contains a leucine dosage of at least 2.5 grams per serving, Wilson suggests.
If you prefer to stick with a protein powder, whey will be richest source of the amino acid. You can get enough from plant-based protein powders, too. Just check the label to see how much leucine a serving offers.
5 Leucine Supplements for Muscle Growth
- Nutricost Pure L-Leucine Powder (Amazon.com; $19.95)
- NutraBio 100% Pure L-Leucine 400mg Caps (Amazon.com; $15.99)
- Naked Nutrition BCAAs (NakedNutrition.com; $34.99)
- Optimum Nutrition BCAA 1000 Caps (Amazon.com; 27.46)
- PROMIX Madagascar Vanilla Grass-Fed Native Irish Whey Protein (PromixNutrition.com; $39.99)
- American Journal of Clinical Nutrition: "Leucine Supplementation of a Low-Protein Mixed Macronutrient Beverage Enhances Myofibrillar Protein Synthesis in Young Men: A Double-Blind, Randomized Trial"
- University of Rochester Medical Center: "Leucine"
- American College of Sports Medicine: "Protein Intake for Optimal Muscle Maintenance"
- Journal of the International Society of Sports Nutrition: "International Society of Sports Nutrition Position Stand: protein and exercise"