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Leucine is in the group of branched-chain amino acids (BCAAs), along with valine and isoleucine. Leucine is the second most common amino acid found in protein besides glycine. Leucine, in conjunction with valine and isoleucine, boosts the healing of muscle, skin, and bones, aids in recovery from surgery, and lowers blood glucose levels. Leucine is necessary for the optimal growth of infants and for nitrogen balance in adults. Measurement of leucine is included in the Amino Acids, Plasma and Amino Acids, Urine tests along with 23 other amino acids.

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Leucine (Leu/L) is one of the amino acids found in proteins. Leucine is classified as a hydrophobic amino acid. Leucine, isoleucine, and valine are branched-chain amino acids (BCAAs). Leucine is an essential amino acid that cannot be synthesized by the body but must be obtained from food. Leucine is the most abundant branched-chain amino acid (BCAA) found in animal and plant proteins. It plays an important role in protein synthesis and metabolic function and is sometimes used as a dietary supplement or flavor enhancer.

Leucine levels decrease significantly after exercise. Leucine is transported around the body in the blood and incorporated into new proteins or oxidized in tissues. It helps regulate blood sugar levels, stimulates muscle protein synthesis, and promotes the growth of muscle and bone tissues. The amount of leucine in the blood increases shortly after consumption of protein-rich food, then returns to fasting levels.

After leucine is ingested, it is utilized in many organs including the liver, skeletal muscle, adipose tissue, and gut. Leucine is mostly converted to acetyl coenzyme-A (CoA) in a series of reactions taking place in the cytosol and in the mitochondria. To a lesser extent, leucine is also converted into β-hydroxy β-methyl butyric acid (HMB), a precursor of ketone bodies and cholesterol.

Studies have shown that gut bacteria affect the bioavailability of amino acids, including leucine. In humans, elevated BCAA concentrations have been linked to obesity, insulin resistance, diabetes, and other metabolic diseases.

Leucine activates the mammalian target of the rapamycin (mTOR) signaling pathway, which plays a role in many aspects of metabolism and physiology, including autophagy, muscle protein synthesis, and insulin signaling.

Recent studies suggest a daily intake of around 40 mg/kg body weight per day would be optimal for adults, with higher doses recommended for individuals who participate in intensive physical activity. Although leucine is widely available in foods and deficiency is rare, supplementation may be recommended to increase muscle mass or prevent protein degradation in muscle-wasting conditions. Because of leucine’s role in building and repairing muscle tissue, it is of particular interest to athletes and is sometimes used to promote recovery.

Leucine supplementation is sometimes encouraged in older people to mitigate the effects of sarcopenia, which is the age-related loss of lean muscle mass. Research shows that leucine supplementation may improve muscle strength and function in older people, and while evidence on the optimal dose is unclear, international guidelines have recommended 3 g of leucine with 25–30 g of protein at three main meals per day. Major dietary sources of leucine include high-protein animal products, dairy products, eggs, pulses, and whole grains. Because the three BCAAs (leucine, valine, and isoleucine) are typically consumed together, they tend to be studied as a group, though there are important differences in their synthesis and functions.

Leucine has also been investigated as a potential therapy for atherosclerosis, Parkinson’s disease, cirrhosis, and the physical effects of eating disorders.

Studies have linked impaired BCAA metabolism to the development of neurological conditions such as Alzheimer’s disease. BCAAs are identified among the metabolites that might be associated with Alzheimer’s disease or Alzheimer’s disease risk. Studies have shown that mixed BCAA therapy helped to improve cognitive recovery in patients with severe traumatic brain injury such as stroke.

Studies also show a link between amino acid metabolism and autoimmune disorders such as multiple sclerosis. 

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