Leucine

What is leucine?

Leucine is an essential amino acid; it is one of the 3 branched-chain amino acids (BCAAs) ^[1]. In foods, leucine is incorporated into proteins.

Leucine abbreviation (symbol): Leu

Leucine Functions in the Human Body

Leucine is ^[1]:

• Important for the synthesis of hemoglobin, other proteins and hormones, regulation of blood sugar levels, growth and repair of bones and muscles and wound healing
• Ketogenic amino acid — it can be converted into ketones ^[15]
• Not a glucogenic amino acid — it cannot be converted into glucose ^[15]

Foods High in Leucine

• ANIMAL FOODS: beef, pork, chicken, turkey, fish, cheese ^[2]
• PLANT FOODS: cereals, legumes, nuts, seeds, baker’s yeast and spirulina ^[2]

Foods low in leucine: fruits and vegetables ^[2]

L-Leucine Supplements

Oral L-leucine supplements without prescription (over-the-counter) are available.

Leucine Supplements Benefits

Leucine and Muscle Growth

Leucine supplements are POSSIBLY EFFECTIVE in enhancing protein synthesis and promoting muscle growth ^[8]:

• In 3 studies ^[9,11,12] and 2 systematic reviews of studies ^[13,14] consuming protein with added leucine after exercise, increased protein synthesis in young and older men.
• In one 2009 study, consuming whey with added free leucine by young males did not result in greater muscle anabolic effect than consuming whey alone ^[7].

Leucine supplements are PROBABLY INEFFECTIVE in:

• Increasing short-term physical performance ^[3] or strength ^[14]
• Increasing long-term physical performance or preventing muscle loss in various illnesses ^[4], including muscle loss during immobilization ^[10]
• Reducing blood glucose levels in diabetes type 2 ^[5]

Leucine Safety: Side Effects and Toxicity

The safety of leucine supplements is described under branched-chain amino acids.

Who should limit leucine intake?

Individuals with the following conditions should limit leucine intake from foods and supplements:

• Isovaleric acidemia ^[6]
• Maple syrup urine disease ^[6]

Isovaleric Acidaemia (Aciduria)

Isovaleric acidemia (aciduria) is a hereditary disorder of the leucine metabolism in which isovaleric acid builds up in the blood, urine and brain ^[16].

Symptoms, which develop in the first days after birth, include shaking, hypothermia, poor feeding, vomiting and, if not treated, coma and death; in a chronic form, recurrent attacks with vomiting, extreme tiredness and the smell of sweaty feet can occur ^[16].

Treatment includes a life-long low-lysine (low-protein) diet and carnitine and glycine supplements ^[17].