Leucine: Everything You Need to Know
Getty Images / Olena Ruban
Medically reviewed by Elizabeth Barnes, RDN
Leucine is an essential branched-chain amino acid (BCAA) important for protein synthesis and muscle growth. It is sold as a dietary supplement and is found naturally in foods like chicken, turkey, and cheese.
Some research suggests leucine may help improve muscle mass, athletic recovery, inflammation, and blood sugar levels. However, further studies are needed to confirm many of these benefits.
This article discusses the potential benefits and uses of leucine. It also covers the risk factors of a leucine deficiency and the side effects of taking supplements.
Supplement Facts
Active ingredient(s): Leucine
Alternate name(s): L-leucine, (S)-2-Amino-4-methylpentanoic acid, (S)-Leucine
Suggested dose: Varies depending on the health condition.
Safety considerations: It may not be safe for people who are pregnant, breastfeeding, or have certain health conditions, such as maple syrup urine disease.
Uses of Leucine
Although further research is needed, people generally use leucine supplements to build muscle mass and improve exercise performance.
Explainer
Supplement use should be individualized and vetted by a healthcare professional, such as a registered dietitian, pharmacist, or healthcare provider. No supplement is intended to treat, cure, or prevent disease.
Muscle Health
Studies have looked at leucine's role in supporting muscle growth and recovery with mixed results.
Muscle Growth
Leucine is popular among bodybuilders due to its potential to support muscle growth. Branched-chain amino acids, especially leucine, have been shown to activate the mTOR signaling pathway, which promotes muscle growth.
However, the evidence is mixed. Leucine seems to offer the most health benefits to older adults—especially those with age-related muscle loss (sarcopenia)—when combined with strength training.
For example, one 2022 randomized control trial looked at the effects of leucine supplementation on body composition and physical function in healthy Korean adults 50 years and older. All participants completed a 12-week strength-training routine, with or without leucine supplements.
At the end of the study, the group that combined the leucine supplement with strength training experienced significant improvements in lean muscle mass compared to those who only did strength training.
However, a recent review of 17 randomized control trials found no improvements in lean muscle mass and strength in older adults who took an isolated leucine supplement without strength training.
Similarly, another small randomized control trial found no improvements in muscle mass or strength with high-dose leucine supplementation in resistance-trained men who consumed adequate dietary protein.
Takeaway
Most studies to date have been short-term and have reported improvements in muscle protein synthesis but not necessarily in muscle mass. Long-term human studies are needed to confirm the relationship between leucine and muscle growth.
Muscle Recovery
According to studies, leucine supplementation may help improve muscle recovery when combined with resistance training. It appears to be most beneficial when combined with other essential amino acids.
One recent study found that leucine-enriched essential amino acids can help reduce muscle damage in the days following an acute bout of resistance exercise. However, no significant improvements in muscle protein synthesis were observed during the study.
Another study examining leucine supplementation's effects on muscle recovery from resistance exercise in young adults found no significant improvements.
May Reduce Inflammation
While inflammation is a normal part of your body's healing process, chronic inflammation can play a role in the development of many diseases, including heart disease, autoimmune disease, cancer, and diabetes.
One study in adolescents and young adults with cerebral palsy found that participants who were given leucine supplements for 10 weeks experienced significant improvements in markers of inflammation. Improvements in muscle strength, muscle soreness, mood, muscle mass, and overall well-being were also observed.
However, the study was small, including only 21 participants.
May Stabilize Blood Sugar
Human and animal studies show that supplementing with leucine, especially alongside glucose (sugar), may help stimulate insulin secretion, which may help control blood sugar.
In addition, a 2014 test-tube study noted that leucine could help improve insulin sensitivity and promote glucose uptake to help keep blood sugar levels under control.
However, further human studies are needed to confirm these effects.
Getty Images / Olena Ruban
What Are the Side Effects of Leucine?
Side effects are unlikely from consuming leucine-rich foods. However, consuming a leucine supplement may have potential side effects.
Common Side Effects
No severe side effects have been reported with leucine supplements when taken at recommended doses. However, taking high doses of leucine supplements may lead to some side effects, including:
Gastrointestinal (GI) issues: When combined with high doses of whey protein, some users have reported gas, nausea, and diarrhea.
Imbalances of other amino acids, including valine and isoleucine: This may lead to a short-term positive effect on muscle protein synthesis but not long-term muscle growth.
Low blood sugar: High doses of leucine may cause blood sugar to drop too low.
Severe Side Effects
High doses of leucine may interfere with the production of niacin from tryptophan, which can worsen symptoms of pellagra, a disease caused by a deficiency in niacin or tryptophan.
Studies have shown that doses over 500 milligrams (mg) per kilogram (kg) of body weight daily can increase ammonia in the blood. This can lead to elevated ammonia concentrations in the brain, which can cause neurological damage and confusion.
However, one study noted that blood ammonia levels returned to normal the day after the supplement was discontinued, causing no serious health consequences.
If you experience confusion, severe gastrointestinal discomfort, or other severe symptoms, stop taking leucine and contact your healthcare provider.
Precautions
People with existing health conditions, such as the following, should be aware of possible safety concerns when taking leucine supplements:
Diabetes: Because leucine may cause low blood sugar, people with diabetes taking medications to control their blood sugar may be at a greater risk for hypoglycemia (low blood sugar).
Maple syrup urine disease (MSUD): People with MSUD are unable to break down branched-chain amino acids like leucine, causing them to accumulate in the blood. This can lead to neurological symptoms, including lethargy and irritability.
Pregnancy and breastfeeding: Due to a lack of research, pregnant or breastfeeding people should avoid using leucine supplements unless otherwise recommended by a healthcare provider.
Children: Single amino acids may cause growth problems in children and should be avoided.
Dosage: How Much Leucine Should I Take?
Explainer
Always speak with a healthcare provider before taking a supplement to ensure that the supplement and dosage are appropriate for your individual needs.
There are no official dosage guidelines for leucine supplements. However, a common dose is 5 grams (g) per day.
According to the World Health Organization (WHO), the average healthy adult should consume 39 mg/kg of leucine daily.
This means a 176-pound (80 kg) individual would need around 3.1 g of dietary leucine daily.
However, studies suggest that older adults may need more than double the current recommendations. For example, a leucine intake of 3 g during three main meals, along with 25 to 30 g of protein, is commonly recommended for older adults to prevent or recover from muscle loss.
Explainer
Dietary supplements are not regulated like drugs in the United States, meaning the Food and Drug Administration (FDA) does not approve them for safety and effectiveness before products are marketed. When possible, choose a supplement tested by a trusted third party, such as U.S. Pharmacopeia (USP), ConsumerLab, or NSF.
However, even if supplements are third-party tested, they are not necessarily safe for all or effective in general. Therefore, it is important to talk to your healthcare provider about any supplements you plan to take and check in about potential interactions with other supplements or medications.
What Happens If I Take Too Much Leucine?
There is little information about leucine overdoses and toxicity in humans. However, to prevent toxicity and severe side effects, be aware of the appropriate dosage and keep the upper limit in mind.
According to recent research, adults should limit daily leucine intake to less than 500 mg/kg or around 35 g for a person weighing 154 pounds (70 kg).
As mentioned previously, doses over 500 mg/kg can lead to elevated ammonia concentrations in the brain.
If you experience severe gastrointestinal discomfort or confusion, you may have taken too much and should seek immediate medical attention.
Interactions
Use caution when taking leucine with the following medications:
Diabetes medications: Leucine may lower your blood sugar. This may lead to dangerously low blood sugar levels when combined with insulin or other diabetes medications.
Niacin (B3) and vitamin B6: Leucine can interfere with the production of niacin and vitamin B6.
Erectile dysfunction medications: Animal studies suggest leucine may increase the effects of PDE5 inhibitors, such as Viagra (sildenafil).
Parkinson's disease medications: Leucine may interfere with the absorption of levodopa, decreasing its effectiveness.
It is essential to carefully read a supplement's ingredients list and nutrition facts panel to learn which ingredients and how much of each ingredient is included. Please review this supplement label with your healthcare provider to discuss potential interactions with foods, other supplements, and medications.
How to Store Leucine
Storage instructions may vary by supplement brand. Carefully read the product label to ensure you store your supplement properly.
Generally, dietary supplements should be stored in a cool, dry area in their original packaging. They should be kept out of the reach of children or pets, preferably in a locked cabinet.
Supplements should be discarded after one year or as indicated on the product label. Ask your healthcare provider or pharmacist any questions you have about how to dispose of your supplements or medications.
Similar Supplements
Leucine may be used to reduce inflammation, regulate blood sugar, and support muscle health.
Other supplements with similar properties include:
Creatine: Creatine supplements have been shown to improve athletic performance and lean body mass during training. It may also help prevent age-related muscle and bone loss in older adults.
L-carnitine: An amino acid that is frequently used to improve athletic performance.
Whey protein: Whey protein is reasonably high in leucine and may help prevent muscle loss in older adults. For best results, it should be used in conjunction with resistance training.
Frequently Asked Questions
What does leucine do for you?
Leucine is a branched-chain amino acid that stimulates muscle protein synthesis. It may also support muscle health, exercise recovery, and healthy blood sugar levels.
Can I take leucine every day?
Unless otherwise recommended by a healthcare provider, leucine can be taken daily. However, consuming it alongside other amino acids is important to sustain muscle protein synthesis and optimal health.
Who should take a leucine supplement?
Leucine may be beneficial in improving muscle size and function in older adults, especially those who are frail and malnourished.
People who don't eat animal products may also benefit from a leucine supplement since the amino acid is commonly found in animal foods. However, there's not much evidence that it can benefit healthy young adults who are already consuming adequate amounts of leucine in their diet.
Sources of Leucine & What to Look For
Leucine can be found in both food and supplement forms. However, consuming a well-balanced diet that includes high-leucine foods is best.
Food Sources of Leucine
Unfortunately, most food guidelines do not list the amount of leucine in foods. This makes it difficult to build a dietary plan based on current guidelines.
Here is a look at the amount of leucine in 100 g of the following foods:
Baked ham: 1.7 g
Skinless chicken breast: 2 g
Pork steak without visible fat: 1.7 g
Skinless turkey breast: 2 g
Asiago cheese: 2.8 g
Feta cheese: 1.5 g
Parmesan cheese: 2.9 g
Mozzarella cheese: 1.4 g
Navy beans: 0.6 g
Lentils: 0.6 g
Leucine Supplements
Leucine supplements are commonly sold in capsules or powder. Vegetarian and vegan options are also available. You can purchase leucine supplements online or in certain grocery, retail, or health food stores.
Follow a healthcare provider's recommendations or label directions to ensure proper dosage and avoid unwanted side effects from leucine supplements.
Summary
Leucine is a branched-chain amino acid that is important for rebuilding and repairing muscle tissue. There is some evidence that it may help improve muscle mass, recovery, inflammation, and blood sugar levels. It seems to be most beneficial for improving muscle function and counteracting the loss of muscle mass in older adults.
Before incorporating leucine supplements into your diet, talk with a healthcare provider to ensure it is appropriate for your health needs.
