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Amino acids help your body recover, build muscle, and break down food. And one amino acid, in particular, is a favorite in the health space for its benefits related to athletic performance and muscle growth: leucine. Water Soluble Hydrolyzed Pea Protein
Here, performance nutrition experts share the main benefits of leucine for muscle mass, weight loss, and more. They also cover how much leucine you need daily, and how to ensure all your amino acid bases are covered—without driving yourself crazy.
Leucine is a branched-chain amino acid that is an essential building block of protein.
There are 20 amino acids1 in total. Eleven are “non-essential,” meaning your body synthesizes them sufficiently. The other nine are “essential,” meaning your body doesn’t produce enough of them and they need to be obtained through diet.
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Among the nine essential amino acids, there are three branched-chain amino acids (BCAAs): leucine, isoleucine, and valine. BCAAs receive lots of attention in the fitness and scientific research worlds for their role in muscle protein synthesis (MPS)2 and recovery following exercise3 .
“Leucine is considered a major stimulus of the mTOR signaling pathway, which is important for muscle hypertrophy,” says David Church, Ph.D., an assistant professor at the University of Arkansas for Medical Sciences Institute on Aging. This is important because the mTOR signaling pathway helps regulate growth in the body and stimulates the production of new muscle proteins4 .
“Consuming a protein source that contains all essential amino acids with a high proportion of leucine will result in a substantial anabolic response,” adds Church, meaning it can tip the scales towards muscle growth and all the health benefits that come with it.
Leucine is an essential nutrient that supports many bodily functions and impacts health in the following key ways:
Leucine and the other BCAAs are broken down in muscle tissue2 instead of the liver like other essential amino acids. Of the three BCAAs, research shows leucine functions as a regulator of MPS5 to promote muscle growth. Having adequate muscle and lean tissue is beneficial for your cognitive health6 , blood sugar7 , and inflammatory response8 . It also helps protect you from bone fractures9 that become more common—and dangerous10 —as you age.
The research is mixed when it comes to leucine and increased muscle mass.
When combined with strength training, regular consumption of leucine has been shown to improve muscle growth11 in animal trials.
In a recent human study12 , participants completed a strength training routine over 12 weeks, with half of participants taking a leucine-rich protein supplement in addition to the workouts. The group that took the supplement in addition to strength training had significantly greater lean muscle and skeletal muscle mass when compared to the group who just did the strength training.
However, the results may be different without the strength training component. A recent review13 found that isolated leucine supplementation had no effect on muscle mass in older adults. It did find that when leucine supplementation included vitamin D, there was a significant improvement in handgrip strength.
“Leucine supplementation can improve endurance and strength in athletes when paired with a training program,” James McKendry, Ph.D., and Changhyun Lim, Ph.D., postdoctoral research fellows of the Protein Metabolism Research Laboratory at McMaster University, tell mindbodygreen.
Taking leucine-enriched essential amino acids14 after resistance training can improve recovery and promote healthy muscle function, allowing you to recover faster and work out again sooner. Whole-food protein sources15 can also be consumed to the same effect.
When eating in a caloric deficit, l-leucine supplementation16 may increase fat loss, decrease fat accumulation during aging, and help prevent obesity. McKendry and Lim add, “Leucine may help preserve muscle mass during weight loss, which can help improve body composition.”
It is possible that leucine supplementation may also increase leptin (a satiety hormone)16 , thus promoting weight loss. Some studies have also found that leucine supplementation improves glucose tolerance16 .
High protein diets have also been studied for weight loss, with clinical trials finding that these diets can reduce body weight and preserve fat-free mass17 . If fat loss is your goal, you'll want to eat at a caloric deficit and combine a nutritious high-protein diet with regular cardio and strength training exercises and limit your intake of sugar and other refined carbs.
Finally, research shows supplementing l-leucine can improve glucose homeostasis16 to help prevent blood sugar spiking.
Interestingly, this is where whole protein sources may not be as effective on blood glucose as leucine supplements. A recent review found that a high protein diet did not significantly improve blood pressure18 or glycemic control. However, researchers did find that a high-protein diet lowered triglycerides, LDL, total cholesterol, and insulin resistance in diabetes patients.
As Gabrielle Lyon, D.O., a functional medicine physician and the founder of the Institute for Muscle-Centric Medicine, previously explained to mindbodygreen, it takes about 2.5 grams of leucine to "turn on" your body's muscle-building process for five to six hours, so this is a threshold you should aim to hit during each meal.
While there’s no one-size-fits-all approach to nutrition and fitness, you generally require more leucine if you’re physically active or an older adult.
That's why McKendry and Lim recommend consuming about 2.5 to 3 grams of leucine per meal if you're a younger adult (under 60), and 4 to 5 grams per meal if you're an older adult. "This works out to roughly 7.5 to 9 grams and 12 to 15 grams of leucine per day for younger and older individuals, respectively,” they say.
“Few studies have determined the required amount of leucine based on activity level," McKendry and Lim add, but they also recommend upping your leucine intake if you're physically active.
It’s important to note that getting enough leucine is essential for everyone, not just athletes and older adults. Consistently low leucine levels can result in several side effects, including lack of appetite, lethargy, poor growth and development, weight loss, skin rashes, and hair loss.
Church adds, “Without sufficient leucine intake, your muscles will break down to provide higher priority organs (like the brain, liver, kidneys, and heart) with their respective leucine requirements.”
Speak with your doctor or a registered dietitian about how much leucine you should take to support good health and take your fitness to the next level.
Diligently counting your daily leucine intake is a tall order. The easiest way to ensure you're consuming enough is to prioritize eating a nutritionally balanced, diverse diet containing various whole foods.
Most animal-based protein sources such as meat, milk, eggs, and fish have a higher essential amino acid content19 , particularly leucine. Include a variety of these foods in your weekly meal rotation to help guarantee you’re getting enough of the amino acid.
While leucine is vital for protein synthesis, consuming it alongside other amino acids is critical. “Though leucine stimulates muscle protein synthesis, the full complement of amino acids is required to support the sustained elevation of protein synthesis,” says McKendry and Lim. Without other amino acids, muscle protein synthesis slows down, they explain, so getting adequate complete protein is key. (Most people should aim to consume upwards of 100 grams of protein per day, depending on their weight.)
“The food-first approach can provide all the leucine and amino acids you need since they come packed with an array of other nutrients, which is excellent for overall health,” say McKendry and Lim. “However, there may be situations where a protein powder or leucine supplement may be beneficial. For example, individuals struggling to meet their daily protein goals (e.g., athletes, older adults) can easily meet their daily protein needs with a protein shake or leucine supplement.”
If you're going this route, choose high-quality protein powders that deliver 2 to 3 grams of leucine per serving to supplement your amino acid intake and help ensure you’re getting enough.
Research shows that plant-based proteins have a lower anabolic effect19 than animal proteins due to their lower digestibility and reduced essential amino acid content (especially leucine).
"In whey protein, leucine is about 12%, so 23 grams of whey protein isolate will trigger [the 2.5 threshold]," leading amino acid requirements researcher Don Layman, Ph.D. previously told mindbodygreen. "Whereas in soy protein isolate, it's about 7.8%, so now you need 33 or 34 grams. So all proteins aren't equal."
However, getting enough leucine is doable for plant-based eaters. Eating a wide range of various whole foods is the way to go if you're a plant-based eater looking to hit your leucine target. All plant foods contain all essential amino acids; however, not all plant-based protein sources contain adequate amounts of essential amino acids, so regularly consuming complementary protein sources is key.
This means pairing plant-based protein foods low in one amino acid with another high in the lacking amino acid (e.g., brown rice and beans, whole wheat bread with peanut butter). Foods like tofu, pumpkin seeds, and hemp are all high in protein and contain reasonable amounts of leucine. From there, investing in a high-quality plant protein powder can help you fill any gaps that exist.
Whether it’s breakfast, lunch, or dinner, include more of these high-leucine foods in your diet to help ensure you’re getting enough of the essential amino acid.
Leucine currently has no official upper tolerable limit, and you don't have to worry about getting too much of it from whole foods sources. However, there are some side effects of taking isolated leucine supplements in excess. These include hypoglycemia (low blood sugar), skin lesions, and hair loss. In addition, those who are pregnant or breastfeeding should avoid taking leucine supplements.
“As with the consumption of any nutritional supplement, [taking leucine] runs the risk of gastrointestinal issues,” McKendry and Lim add.
Eating whole foods rich in leucine and supplementing with a high-leucine protein powder before taking leucine supplements in isolation is a smart strategy if you're looking to avoid side effects.
Animal-based proteins, such as fish, beef, and chicken, are the richest sources of leucine. Other good leucine sources include soy products, seeds, hemp, spirulina, rice, wheat, potatoes, and corn.
Absolutely. It's best to meet your daily leucine needs through diet first (whether whole foods or a good-quality, high-leucine protein powder with a robust amino acid profile that contains at least 2 to 3 grams of leucine per serving) before supplementing leucine individually.
Leucine is the primary amino acid responsible for stimulating muscle protein synthesis. It also plays an important role in other metabolic processes, such as fat loss, muscle recovery and repair, and blood sugar control. Eating a diverse, whole-food diet is a great way to meet your daily leucine requirements without stressing yourself out.
Adam Meyer is a health writer, certified holistic nutritionist, and 100 percent plant-based athlete. He graduated from the NutraPhoria School of Holistic Nutrition in 2019 and has since founded Pillars Nutrition. His work has been featured on EatingWell, Eat This Not That!, The Beet, Verywell Fit, The Healthy, Livestrong, Alive, Best Life and others.
Adam lives in British Columbia, Canada, with his wife, two kids and an Australian shepherd. That's where you can find him running mountain trails, working out in his home gym, or writing in a coffee shop.
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