Leucine: How It Works, Benefits, Clinical Studies And Side Effects

bodybuilder flexing bicep

What is Leucine?


Abbreviation: L or Leu

It is the most popular amino acid among the three BCAAs because it plays a major role in muscle building. It is also the most researched and studied BCAA.

Function: Leucine is responsible for triggering mechanisms that promote the synthesis of proteins in muscles.

Leucine is a branched chain alpha-amino acid which is utilized by the body in the biosynthesis of proteins. It is an essential amino acid – meaning that the human body is incapable of producing leucine, therefore it has to be obtained from other sources such as diet or supplements. Leucine is naturally synthesized by plants and other microorganisms. It is actively utilized in the adipose (fat) and muscular tissues.

Benefits of Leucine for Bodybuilding

Following benefits of leucine are backed by research:

  • It promotes formation of proteins
  • It spares muscles during weight loss
  • It targets fat for cutting down weight
  • Increased muscle growth
  • Enhanced amino acid uptake
  • Prevents muscle breakdown

How Does It Work?

Leucine directly stimulates the synthesis of muscle proteins. The mTOR (mechanistic target of rapamycin) pathway is one of the most widely researched muscle building pathway. Leucine activates the mTOR complex, unlocking the pathways for muscle building. mTOR gets disabled in low leucine levels. When sufficient leucine is present, the protein synthesis in the muscles is switched on which causes muscle growth, and also prevents muscle degradation.

Other Benefits

Leucine has also been shown to regulate cholesterol in rats. In clinical studies conducted on animals, it was found that the rats that were given leucine supplements had increased HDL (high density lipoprotein) levels – the good cholesterol.

Sourcessalmon fishes

Leucine can be obtained from diet to get the required amount of leucine for your body. However, if you are planning on building muscle, you will have to turn towards leucine supplements for help.

Some natural food sources of leucine are beef, eggs, chicken, milk, cheese, yoghurt, fish, etc.

Clinical Studies

Eight male participants completed a three-trial research study in which they performed lower body workout for 45 minutes. The three trials were spaced at one week intervals. The trials focused on three separate diets:

  1. carbohydrates
  2. carbohydrates and protein
  3. carbohydrate, protein and leucine

The results from the trial including leucine reported a significant insulin response in the patients. Increased insulin response leads to increased uptake of amino acids and decreased breakdown of muscle proteins. Both these effects increase the rate of protein synthesis in muscles.

Another study (source) was performed in older men, in which the participants were provided a combination of whey protein, and leucine or milk protein. Both the groups consumed the same number of calories but it was found that the group consuming leucine experienced increased rates of muscle protein synthesis.


The upper limit of the dose for leucine is 35g/day or 500mg/kg/day for adult healthy men.

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Side Effects

  • If taken over 30 gm/day by normal adults, leucine can interfere with the absorption of vitamins B6 and B3. This can lead to deficiency diseases.
  • It can lower blood sugar levels by interfering with insulin levels in diabetics. People on diabetic medications should monitor their blood glucose regularly if they wish to start on leucine supplements.

A word of caution

If taken in excess, leucine may cause pellagra, which has the following symptoms:

  • Diarrhoea
  • Dermatitis
  • Dementia
  • Death

If a dose greater than 500 mg/kg/day is taken, then it causes hyperammonemia.

Leucine toxicity can also occur as a consequence of high doses of leucine, as seen in decompensated maple syrup urine disease (MSUD), which can be a life-threatening situation.

How to Maximize Leucine Benefits?

Because leucine is proven to promote protein synthesis and muscle growth, you can use leucine to supplement your training workout sessions at the gym. Follow the tips below to maximize the benefits of leucine:

  • Take 2.5gm of leucine in the first 30 minutes of exercise training. This will act as an anabolic stimulus
  • After work out, add an additional 5gm of leucine in your protein shake
  • Before going to bed, take 5gm of leucine for the recovery and strength of your muscles in response to intense workout
  • To enhance your protein synthesis, take leucine in-between meals to accelerate muscle recovery

Conclusionbodybuilder flexing right bicep

Leucine is one of the essential amino acids which cannot be produced by our body, but nonetheless, is very important for muscular growth and protein synthesis. Research has backed the effects of leucine for promoting and stimulating the synthesis of proteins in the muscles. If you are planning to supplement your daily workout with leucine supplements, then you will experience increased rates of muscular growth as well as increased muscular strength. Just make sure to consult your doctor before starting on leucine supplements so as to avoid any unforeseen health complications.