Threonine is somewhat low in corn and some grains, though it is not the limiting (that is, the lowest relative to making a complete protein) amino acid in these foods. There are good levels of threonine in most flesh foods, dairy foods, and eggs and moderate levels in wheat germ, many nuts, beans, and seeds, as well as some vegetables. Threonine is an important constituent in many body proteins and is necessary for the formation of tooth enamel protein, elastin, and collagen. It is found in high amounts in newborns, and requirements seem to decrease with age yet increase with stress. Threonine also has a minor role (a greater one when choline is deficient) as a lipotropic in controlling fat buildup in the liver.
Threonine has a mild glucose-sparing effect and is a precursor of amino acids glycine and serine. Threonine is one of the immune-stimulating nutrients (cysteine, lysine, alanine, and aspartic acid are others), as it promotes thymus growth and activity. A deficiency of threonine in rats has been associated with a weakened cellular response and antibody formation. One gram of threonine twice daily may also be helpful in some cases of depression.