What is Leucine? High and low values | Lab results explained

Leucine, together with isoleucine and valine, are essential amino acids that are referred to as branched-chain amino acids (BCAAs).

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Leucine is nutritionally essential and is required for formation of body proteins, enzymes and some hormones. Leucine itself has a hormone-like activity which is stimulation of pancreatic release of insulin. The branched-chain structure of leucine makes it very important for the formation of flexible collagen tissues, particularly elastin in ligaments. Leucine is relatively abundant in all protein foods.

Function of BCAAs:

– synthesis of enzymes

– transport proteins

– structural components of cells

Unlike other amino acids, BCAAs do not serve as precursors for bile acids or neurotransmitters, but are involved in:

– control mechanisms for neurotransmitters,

– muscle development and repair,

– and blood-sugar regulation.

Leucine stimulates insulin production and promotes protein synthesis.

Although leucine is abundant in muscle tissue it cannot substitute for glucose in the fasting state, but is converted into other compounds that serve as an alternate source of energy for the body.

Low BCAAs are an indication of:

– chronic depletion from low protein intake

– poor digestion

– or increased utilization from chronic over-exercising.

Lower levels:

Leucine, an essential amino acid, was low in this specimen. Leucine is a branched-chain amino acid that is a common constituent of proteins, peptides and hormones. It also promotes wound healing, insulin release from the pancreas, and is a component of elastin (ligaments).

– Low leucine can result from protein malnutrition, zinc deficiency (Zn dependent peptidase), or other gastrointestinal dysfunctions such as hypochlorhydria (aka achlorhydria). Food intolerances and malabsorption may be coincident with subnormal leucine.

– Low leucine can point to potential catabolism of skeletal muscle. Check 3-Methylhistidine to confirm this.

Subnormal leucine is unusual but can coincide with:

– deficient activity of digestive peptidases

– zinc deficiency

– pancreatic dysfunction

– or poor quality diet.

– Leucine can also be subnormal in hepatic encephalopathy (=decline in brain function that occurs as a result of severe liver disease), and absorption and uptake can also be deficient following intestinal bypass procedures.

Higher levels:

High fasting levels may be caused by a deficiency of vitamin B6, elevated insulin levels, or excessive supplementation.

Leucine, an essential amino acid, is abnormally high in this specimen. Leucine, isoleucine and valine are branched-chain structural amino acids whose catabolism requires multiple nutrient co-factors including:

– P-5-P (vitamin B₆)

– thiamin (vitamin B₁)

– riboflavin (vitamin B₂)

– lipoic acid

– Coenzyme-A

– biotin (vitamin B₇)

– magnesium

Branched chain amino acids are often elevated as a result of deficiency of B-6 and/or B-1, or excessive supplementation with protein (eg. whey protein) or the specific amino acids for body building/exercise recovery purposes.

A more serious, but rare cause of branched-chain hyperaminoacidurea is known as Maple Syrup Urine Disease (MSUD) that results from an inherited enzymatic defect. Symptoms of MSUD may be episodic with protein intake.

In infants and children MSUD symptoms include:

– vomiting

– irritability

– lethargy

– food refusal

– muscle hypertonicity

– respiratory

Convulsions and seizures may occur in acute cases. Individuals presenting with moderate branched-chain hyperaminoacidemia should be retested after reduction in protein intake and supplementation with vitamins B-6 and B-1. Those with MSUD should pursue professional advice regarding the avoidance of foods that are high in branched-chain amino acids.


The information on is NOT intended to replace a one-on-one relationship with a qualified health care professional and is not intended as medical advice.

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