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Threonine is an amino acid important in the formation of proteins, collagen, elastin (connective tissue protein), and tooth enamel. It is also important for neurotransmitter production and nervous system health. Threonine helps maintain the proper balance of proteins in the body and liver function, metabolism, and assimilation. Measurement of threonine is included in the Amino Acids, Plasma and Amino Acids, Urine tests along with 23 other amino acids.

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Threonine (Thr/T) is an essential amino acid for humans. It is mainly found in eggs, milk, cottage cheese, gelatin, fish, poultry, meat products, lentils, black beans, malt, and sesame. Threonine deficiency is more common in strict vegetarians. Threonine is an important amino acid and participates in the formation of many proteins, such as tooth enamel, collagen, and elastin. Threonine helps keep connective tissues and muscles strong and elastic throughout the body including the heart, where it is found in significant amounts. It also helps build strong bones and tooth enamel and can speed wound healing or recovery from injuries. Threonine is an important amino acid for the nervous system and plays an important role in porphyrin metabolism and fat metabolism by preventing the accumulation of fat in the liver (along with aspartic acid and methionine).

Severe threonine deficiency can cause neurological dysfunctions. Threonine also acts as an immunostimulant that promotes the development of the thymus gland while it can also promote the defense functions of cells. Threonine has been used in the treatment of congenital spasticity and multiple sclerosis at a dose of 1 gram daily. The threonine content of most infant formulas on the market today is about 20% higher than the threonine concentration in human milk. Because of this high threonine content, its plasma concentrations are up to twice as high in formula-fed preterm infants than in human-milk-fed infants. Catabolism of threonine appears to be primarily (70-80%) by the enzyme threonine dehydrogenase which oxidizes threonine to 2-amino-3-oxo butyrate ultimately forming glycine and acetyl-CoA, while the enzyme threonine dehydrogenase which catalyzes threonine to 2-oxo butyrate and ammonia, is less active. Increased plasma threonine concentrations lead to the accumulation of threonine and glycine in the brain. This accumulation affects the balance of neurotransmitters that can have consequences for brain development during early infancy. For this reason, excessive intake of threonine during infant feeding should be avoided.

Symptoms of threonine deficiency include emotional disturbances, confusion, indigestion, and fatty infiltration of the liver. Threonine supplementation is very helpful in intestinal disorders and indigestion and has also been used to control stress and mild depression. Threonine is available in supplements such as protein powder and amino acid tablets. The typical dose is between 100 and 500 mg per day. Exceeding recommended doses of threonine can disrupt liver function and cause the formation of increased concentrations of urea and toxic ammonia.

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