Leucine Benefits, Sources + How To Get Enough Of It (Even If You're Plant-Based)

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.

Leucine is a branched-chain amino acid that is an essential building block of protein.

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.

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 the participants taking a leucine-rich protein supplement in addition to doing 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.

"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.

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.

Finally, research shows supplementing L-leucine can improve glucose homeostasis16 to help prevent blood sugar spiking.

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."

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 upward 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."

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.

Leucine currently has no official upper tolerable limit, and you don't have to worry about getting too much of it from whole-food 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.