Leucine is famous for its muscle-building properties and is a core component of BCAA supplements and bodybuilding diets. As leucine is not synthesized in the human body, it must be obtained from a diet of protein-rich foods.
Leucine is best known in bodybuilding circles as an essential amino acid, which is needed to stimulate muscle growth. This is because leucine provides muscles with nutrients and activates the pathways required to promote muscle protein synthesis.
10 Major Sources of Leucine
- Navy Beans
- Ricotta Cheese
- Squash and Pumpkin Seeds
Leucine is found in animal protein, egg whites, and plant protein sources such as soy. Leucine’s ability to stimulate muscle protein synthesis is ten times more powerful than any other branch chain amino acid (BCAA).
Leucine and Muscle Growth
The health and growth of skeletal muscles are essential to everyone, and adding leucine to your diet can help your muscles grow and thrive. A study by the University of Illinois at Urbana-Champaign found that the addition of leucine in the diets of people who eat foods that are low in protein helped to increase skeletal muscle growth.
Leucine does this by activating a muscle-building pathway called mTOR (Mechanistic Target of Rapamycin) to stimulate muscle growth. When there is not enough leucine in the body, mTOR is disabled. This is why it’s important to reach the proper leucine threshold to ensure your muscles grow and develop.
The Leucine Threshold
For your body to utilize the muscle-building capabilities of leucine, you must first consume enough leucine. Remarkably, the human body absorbs a full 100% of the leucine that it consumes. However, only a small percentage of leucine goes towards building muscle.
Other organs and muscles in the human body also require amino acid support. When a person eats protein, amino acids are distributed throughout the digestive tract to the liver, kidneys, cardiac and smooth muscles, and even tissues like the skin.
To reach the leucine threshold where amino acids are used to build and support muscles instead of being delegated to the rest of the body, you need to make sure you consume enough protein with a high leucine content.
One study, reported in the Nutrition Journal, discovered that older people needed to consume higher amounts of leucine to receive leucine’s benefits. In younger populations, the mTOR pathway is more sensitive, and lower amounts of leucine are required to promote muscle protein synthesis compared to older adults.
Furthermore, once a person reaches the leucine threshold, there is a “refractory period” where the body must use the amino acids it has before it can begin to utilize additional amino acids that enter the body.
For a person to achieve this threshold and ensure they are getting the most out of their diet’s nutrients, it’s best to eat a large amount of protein in one meal and then take additional protein after four to five hours instead of eating small amounts of protein throughout the day.
According to the website Nutrition Data, the amount of leucine in a protein meal needed to achieve the leucine threshold is between 26% and 41% leucine (or 3.2 grams to 4.4 grams). As a point of reference, a 4.7 oz steak would provide a person with this amount of leucine, as would five large eggs.
Getting the right amount of nutrients and amino acids can be difficult. However, it’s important to choose a healthy diet with ample fruits, vegetables, and protein to reach the necessary leucine threshold and maintain a healthy body.
Safety and Side Effects
Leucine is an amino acid found in protein-rich foods. As a result, leucine is safe to consume and an essential part of a healthy diet.
Research published in the Journal of Nutrition states that the tolerable upper limit for leucine consumption is 500 mg per kg of bodyweight per day.
Exceeding this upper limit of 500 mg can lead to an increase in blood ammonia concentrations.
Where single doses have been studied, consumption of as little as 2.5 grams of leucine is enough to stimulate muscle protein synthesis. In long-term studies, 8 grams (or more) leucine per day is recommended in divided doses, so that at least 2.5 grams of leucine are consumed during each meal.
The best food sources of leucine include proteins contain all the essential amino acids. The protein supplement source with the highest leucine content is whey protein, which contains about 10% leucine (or 10 grams per 100 grams of protein).
If a person drinks a whey protein shake with about 25 grams of protein, they would be consuming approximately 2.5 grams of leucine. Other protein supplements like casein and soy contain less leucine (at about 8%).
Leucine can also be taken as a single amino acid or as a BCAA combination formula. Since most BCAA formulas contain about 40%–50% leucine, a dose of 5–6 grams of BCAAs provides about 2.5 grams of leucine.
Effectiveness of Leucine
Supplementing with leucine alone will not result in optimal effects if your diet is low in protein. This is because other BCAAs (like isoleucine and valine) are given priority when oxidized, which leads to a BCAA imbalance that compromises anabolism.
Therefore, if your diet’s total protein intake is low, it’s best to supplement with all three BCAAs. That being said, leucine is a critical amino acid when it comes to building muscle and strength.
However, it’s best to get your daily leucine intake from protein-rich foods (such as chicken, beef, pork, eggs, tofu, milk, ricotta cheese, and tuna) because the amino acid profile is much richer and more complete in whole foods as opposed to individual supplements.