Leucine Facts | Benefits | Dosage | Toxicity |


What is Leucine?

L-leucine is a branched-chain amino acid necessary for muscle development and recovery. It is classified as an essential amino acid because the body cannot make it, and thus it has to be obtained through diet or supplementation. Many protein sources contain it, such as meat, cheese, and plant-based protein foods. When combined with resistance training, leucine promotes muscle development and aids in strength building. Supplementing leucine may also improve endurance, as it replenishes leucine levels lost during exercise. Because of its numerous performance and skeletal muscle benefits, leucine has gained a loyal following among bodybuilders and athletes.

Benefits of Leucine

Leucine is perhaps one of the most studied and researched amino acids in the arena of training, performance and muscle building. For a good reason, it is the most popular of the branched-chain amino acids, as it is the most anabolic of the 3 BCAA and yields the most benefits for athletes and bodybuilders. Leucine is the strongest out of the three BCAA and has the most benefits for muscles and performance; next to leucine would be isoleucine, and last would be valine.

The following is a breakdown of the main benefits of leucine.

muscle growth

Leucine is classified as anabolic due to its potent ability to stimulate protein synthesis and muscle growth. Leucine is such a potent stimulator of protein synthesis that it can stimulate muscle growth without the user even training or working out. [1] The mechanism of action that leucine uses to stimulate muscle growth is through the mammalian target of rapamycin (mTOR). mTOR is the mediator of hypertrophy within skeletal muscle.

When leucine activates mTOR, it turns off catabolic processes and turns on anabolic processes such as protein synthesis. [2][3][4][5] The thing to remember with leucine’s ability to stimulate muscle growth via increased protein synthesis is that it might turn on the anabolic process of building muscle. However, the body still requires the other amino acids to maintain the anabolic process going. Your body cannot build muscle with just leucine; it needs essential and non-essential amino acids. That is why it is recommended to take leucine with a high-protein meal. One study showed that adding leucine to your post-workout protein shake boosted the anabolic effect by 16%. [8] 

Stops muscle loss (Sarcopenia)

As we age and get older, it becomes harder to maintain lean muscle mass, and in the elderly, the progressive loss of skeletal muscle is a medical condition called Sarcopenia. Leucine has proven itself time and again as an effective treatment option against this muscle atrophy condition. One study showed that just adding four grams of leucine to each meal of test subjects in their seventies increased their rate of protein synthesis. [10]

Researchers speculate that the body becomes desensitized to anabolic signalling of certain amino acids as we age, so it requires more of those signalling amino acids such as leucine to stimulate the body into an anabolic muscle-building environment. [11][12] This could explain why simply supplementing with extra leucine with meals can increase muscle mass in the elderly. The extra leucine is enough to overcome the desensitized body and stimulate anabolism. So if you are elderly or an ageing athlete or bodybuilder, it is important to ensure you take enough leucine to trigger the anabolic switches. [13]

Improves performance and recovery

Leucine increases muscle growth via mTOR and protein synthesis and improves training performance and speed post-workout recovery. These things ultimately help with muscle growth as the better and harder you can train, the more stimulus you are giving the muscle to grow. Also, the harder you train, the longer it takes to recover before you can train that hard again. Still, with the recovery benefits from leucine, your muscle can recover faster, leading to more training and at higher intensities, which should lead to better results from your training and workouts.

One study done using rats demonstrated leucine’s recovery benefits. The study measured the rat’s muscle glycogen as an indicator of recovery post-workout. The results showed that the muscle glycogen levels were much higher when given leucine, leading the researchers to conclude that leucine helps enhance post-workout recovery. [6] another interesting study used competitive canoeists, and the researchers gave one group leucine and another group a placebo. The study results after six weeks showed that the leucine group had significantly better endurance and upper body power than the placebo group. [7]

Fat Burning

Fat loss or the ability to reduce adipose tissue is one of the benefits of leucine that not many people know. Everyone seems to know that leucine can help build muscle, but very few take the supplement to help burn fat. Maybe people should start, based on the studies we have found. one rat study found that low doses of leucine administered for six weeks caused a reduction in fat mass. [14]

Another study that appears in the journal Nutrients confirmed that oral leucine supplementation could help to decrease body adiposity. The researchers were not exactly sure how it achieved this. [15] Some researchers believe that leucine might activate AMPK, which stands for adenosine 5′ monophosphate-activated protein kinase. AMPK is an energy sensor in the body, and when activated, it ramps up energy production, which could be how leucine helps with fat burning. A study using leucine combined with resveratrol showed that the combination stimulated AMPK and increased fat burning. [16]

Glucose uptake

Leucine is a very potent stimulator of insulin, with many studies showing its ability to affect insulin secretion and glucose uptake. One study compared the insulin level secreted following a post-workout protein shake. The researchers had three different protein shakes, one with just protein, one with protein and carbohydrate and the last with protein, carbohydrate and leucine. The biggest insulin release was seen in the protein shake with leucine added to it and carbohydrates. [8] The result of that study is not surprising since it showed that leucine helps pancreatic β cells function better; these are the cells that secrete insulin from the pancreas. [9]

Chart showing leucine's ability to stimulate insulin release
Chart showing leucine’s ability to stimulate insulin release

Side Effects of Leucine

Dietary leucine intake is safe and is highly unlikely to cause any side effects, but if you are supplementing with leucine above what you obtain in your diet, you can get some side effects. 

Too much leucine can interfere with tryptophan and its function in Niacin formation. This could lead to Niacin deficiency and possibly to a condition called Pellagra. [18] Another possibly severe side effect is hyperammonemia, which is a build-up of ammonia in the body. [19] The chances are low that you would develop hyperammonemia as the researchers’ doses were very large. The study showed that a 70kg person’s safe upper limit is 35 grams per day. [19]

The above-mentioned severe side effects are rare and require very large doses taken daily for long periods. It is unlikely that the average user would reach these levels before experiencing gastrointestinal side effects that would prevent them from continuing with the large doses. The majority of users, if they are going to experience side effects, will experience minor gastrointestinal ones such as:

  • Nausea
  • Vomiting
  • Diarrhea

Suppose you have a genetic condition called maple syrup urine disease. In that case, you should not take any of the branched-chain amino acids in supplement form because you lack the required enzymes to break down the BCAA, and this could be dangerous.

Leucine Dosage

The most effective dosage for leucine is between 4 to 20 grams. Studies show that the dosage can range based on age and activity level. [8][17] Most users will adjust the dosage within the above range depending on their goals and how their body tolerates it. Some people experience gastrointestinal discomfort when taking higher doses.

When is the best time to take leucine?

The best time to take leucine is with your post-workout protein shake. studies show that this can increase the anabolic effect by 16%. [6][8] If you are looking to boost anabolism and protein synthesis throughout the day, you should take leucine with each protein-rich protein-rich meal. For the prevention of catabolism and stimulation of anabolism, some users take it before their training or workouts, especially for long strenuous workouts.

Dietary sources of leucine

  • Red meat
  • Chicken
  • Dairy
  • Fish
  • Eggs
  • Oats
  • Spirulina
  • Lentils
  • Hemp seeds
  • Sesame seeds
  • Peanuts

Summary of Leucine

Leucine is a fantastic amino acid supplement and has many useful benefits for athletes and bodybuilders. It is a supplement that almost everyone should have in their supplement arsenal. The real question becomes should you take leucine on its own or in combination with the other BCAA amino acids isoleucine and valine. You will have to answer yourself after testing which works best for you and your specific training goals.


  1. Double-blind, placebo-controlled pilot trial of L-Leucine-enriched amino-acid mixtures on body composition and physical performance in men and women aged 65-75 years – PubMed (nih.gov)
  2. Leucine stimulates translation initiation in skeletal muscle of postabsorptive rats via a rapamycin-sensitive pathway – PubMed (nih.gov)
  3. Rapamycin administration in humans blocks the contraction-induced increase in skeletal muscle protein synthesis – PubMed (nih.gov)
  4. Influence of leucine infusion on intracellular amino acids in humans – PubMed (nih.gov)
  5. Leucine as a regulator of whole body and skeletal muscle protein metabolism in humans – PubMed (nih.gov)
  6. Leucine Supplementation Enhances Skeletal Muscle Recovery in Rats Following Exercise | The Journal of Nutrition | Oxford Academic (oup.com)
  7. Effects of dietary leucine supplementation on exercise performance – PubMed (nih.gov)
  8. Combined ingestion of protein and free leucine with carbohydrate increases postexercise muscle protein synthesis in vivo in male subjects | American Journal of Physiology-Endocrinology and Metabolism
  9. Leucine metabolism in regulation of insulin secretion from pancreatic beta cells | Nutrition Reviews | Oxford Academic (oup.com)
  10. Leucine supplementation chronically improves muscle protein synthesis in older adults consuming the RDA for protein – PubMed (nih.gov)
  11. Amino acids and muscle loss with aging – PubMed (nih.gov)
  12. A high proportion of leucine is required for optimal stimulation of the rate of muscle protein synthesis by essential amino acids in the elderly | American Journal of Physiology-Endocrinology and Metabolism
  13. Lean body mass change over 6 years is associated with dietary leucine intake in an older Danish population | British Journal of Nutrition | Cambridge Core
  14. Effects of leucine supplementation on the body composition and protein status of rats submitted to food restriction – PubMed (nih.gov)
  15. Reviewing the Effects of l-Leucine Supplementation in the Regulation of Food Intake, Energy Balance, and Glucose Homeostasis (nih.gov)
  16. Synergistic effects of leucine and resveratrol on insulin sensitivity and fat metabolism in adipocytes and mice – PubMed (nih.gov)
  17. Caffeinated chewing gum increases repeated sprint performance and augments increases in testosterone in competitive cyclists – PubMed (nih.gov)
  18. Effect of leucine on enzymes of the tryptophan–niacin metabolic pathway in rat liver and kidney | Biochemical Journal | Portland Press
  19. Determination of the safety of leucine supplementation in healthy elderly men – PubMed (nih.gov)

Always consult your doctor prior to taking any nutritional supplement. The information contain in this article is not intended to be taken as medical advice.