Leucine: Definition, Classification, Functions, Supplement Uses, Side Effects, and Dosage

It is part of the group of nine essential amino acids that are part of the structure in humans.

Of the total amino acids in the body’s protein structures, leucine constitutes 80%. It is not produced by the body and must be provided by food in the daily diet.

Leucine is available in many foods and deficiency is rare. When this happens it can lead to muscle wasting , muscle weakness, depressive states, low energy levels, and changes in blood sugar levels.

When the daily diet is balanced, an excess of this amino acid is not produced, but when high doses of supplements are consumed, this can cause a decrease in the absorption of water at the intestinal level, causing gastrointestinal disturbances.

That is why it is recommended that supplements be taken accompanied by an abundant intake of water throughout the day.

Similar to many other amino acids, leucine is found in many foods that are rich in protein such as:

  • Meat foods such as fish, chicken, turkey and to a lesser extent, beef.
  • Dairy products like buttermilk, yogurt, and cheese.
    nuts such as almonds, soybeans, hazelnuts, walnuts, peanuts, pistachios among others.
  • Other foods such as eggs, legumes, brewer’s yeast, corn, potatoes, and seeds
  • Fruits also contain leucine, but to a lesser extent.

Leucine (also known as 2-amino-4-methylpentaenoic acid) is an essential amino acid from the branched-chain amino acid class (along with isoleucine and valine).

Of the three amino acids, leucine stands out for being the most powerful activator of proteins, its activation can positively influence the synthesis of muscle proteins) and also for being an exclusively ketogenic amino acid, that is, it produces ketone bodies after catabolism while the Valine is glucogenic and produces glucose, and isoleucine is both ketogenic and glucogenic.

Leucine classification

Leucine can be classified as:

  • L-Leucine is the natural version of the amino acid, it is found in the body’s proteins and is the main form used as a supplement.
  • D-Leucine is the mirror image of L-Leucine, which is created in the laboratory and is also used as a supplement.

Leucine functions

Leucine is important for growth as it is involved in the production of the human growth hormone: somatotropin, so it could help to slow down the aging process.

In addition, it has been recommended for use as humans age, and suffer from sarcopenia, a natural decrease in muscle that leads to an increased risk of injury and disability.

Leucine aids in the growth, maintenance, and regeneration of muscle tissue and bone tissue, it also prevents the breakdown of muscle proteins after severe trauma or stress, and may be beneficial for people with phenylketonuria.

Oral branched-chain amino acids are likely to improve symptoms and liver function in people with poor brain function caused by a liver disease known as hepatic encephalopathy.

Scientists believe that amino acids aid protein synthesis and prevent protein breakdown, which in turn helps prevent muscle fatigue and pain.

Strong evidence supports the use of branched chain amino acids (including leucine) to reduce fatigue and exertion after prolonged or intense exercise, thereby helping you recover from intense exercise.

Oral branched-chain amino acids probably reduce the symptoms of tardive dyskinesia.

Tardive dyskinesia is a movement disorder that causes involuntary movement. It is most commonly caused by long-term use of antipsychotic medications in people with schizophrenia or bipolar disorder.

In patients with burns, trauma, or sepsis (infection in the tissues), supplementation with branched-chain amino acids, which includes leucine and two other amino acids, reduces muscle wasting.

One study suggested that leucine supplementation can increase muscle strength.

Leucine is an important chemical element involved in protein synthesis and many other metabolic functions.

Leucine contributes to the regulation of blood sugar levels.
The breakdown of leucine produces two molecules that are used for energy: acetyl-CoA and acetoacetate.

Scientists think that fasting can lead to increased levels of leucine in the blood and increased activity of the enzymes that convert leucine to ketone bodies and are used for energy.

Patients with deficiencies in the molecule that breaks down leucine into acetyl-CoA and acetoacetate (3-hydroxy-3-methylglutaryl-CoA lyase) develop acidosis (low blood pH) and hypoglycemia (low blood glucose).

Leucine can play a key role in maintaining glucose levels and producing energy.

Presentations as a supplement

Leucine is a branched chain amino acid that is sold as a supplement.

The other two branched chain amino acids are valine and isoleucine, but leucine is the most popular of the three as a bodybuilding supplement.

Supplements generally lack solid clinical research. Regulations establish manufacturing standards for them, but do not guarantee that they are safe or effective.

Despite the popularity of leucine among gym enthusiasts, the science to back most of its uses is weak.

Amino acids are available as individual amino acids or in combinations.

They also come as part of multivitamins, proteins, and food supplements. Forms include tablets, fluids, and powders.

Chemical composition

  • C6H13NO2

Toxicity and interactions

Using a single amino acid supplement can lead to a negative nitrogen balance.

This can diminish the benefits of its use. It can make your kidneys work harder. In children, individual amino acid supplements can cause growth problems.

High doses of individual amino acids should not be taken for long periods of time.

Very high doses of leucine can cause low blood sugar (hypoglycemia). It can also cause pellagra. Symptoms of this can include skin lesions, hair loss, and gastrointestinal problems.

Pregnant or lactating women should not use leucine supplements.

People who have maple syrup urine disease, a rare inherited problem, should not take it. Nor should they take the other branched chain amino acids. These include isoleucine and valine.

The interactions of leucine with glucose are not clear, so it should be consumed with caution in people with diabetes.

Leucine possesses both blood sugar lowering properties (it can release insulin from the pancreas, it can directly stimulate glucose uptake in a cell without insulin) but also the reverse (by stimulating S6K, it can inhibit insulin-stimulated glucose uptake) .


Leucine tends to supplement in the range of 39 mg / kg / day and 45 mg / Kg / day for sedentary people. It tends to be taken on an empty stomach or with meals, accompanied by plenty of water.

Mechanism of action

Leucine or its metabolites appear to increase intracellular calcium, similar to muscle contractions, and the increase in calcium will activate proteins like mTOR which then induce muscle protein synthesis.

However, unlike muscle contraction, leucine probably does this in all cells rather than localizing to skeletal muscle.

Leucine Side Effects

Branched chain amino acids like leucine are possibly safe when used appropriately as oral supplements.

Pregnant and lactating women should avoid supplementation due to lack of safety data.

Leucine absorption in the brain can be influenced by diet. High-carbohydrate, low-protein diets increase branched-chain amino acids and decrease aromatic amino acids (serotonin precursors) in the brain, while a low-carbohydrate, high-protein diet can do the opposite.

The blood-brain barrier competes for the absorption in the brain of these amino acids, this competition can decrease the production of serotonin.

An overdose can lead to brain damage or liver disease as it can increase ammonia levels in the blood.


It should not be used in people with kidney problems.

Leucine supplementation lowers glucose levels, which can lead to unhealthy levels in people with:

  • People suffering from hypoglycemia (low blood sugar).
  • People who take medicine to lower blood sugar.

Therefore, its use is not recommended in diabetics.