The histidine, like arginine is nutrionally semiessential, as adult human can be maintained in nitrogen balance for short periods in the absence of histidine.

The decarboxylation of histidine gives histamine. This is catalysed by aromatic amino acid decarboxylase, as well as enzyme histidine deccarboxylase present in most cells. The decarboxylase is inhibited by methyl amino acids, which this is used as antihypertensive agents. The histamine formation is concerned with initiating and sustaining rapid tissue growth.

Histamine has important actions on the circulation, on smooth muscle, on secretions, and in producing itch or possibly pain. In circulation it produces arteriolar as well as precapillary sphincter dilatation. An injection of histamine causes fall of blood pressure and a rise of heart rate. Histamine increases the tone of most types of smooth muscle, e.g. intestine and bronchioles. Histamine stimulates salivary, pancreatic, intestinal, and pepsin secretions. When pricked into the skin, produce itching or pain due to stimulation of nerve endings. Histamine release in urticaria is associated with itching.

The synthesis of histidine leads to formation of urocanic acid, glutamic acid, formic acid, and ammonia. An inherited defect in the enzyme histidase causes plasma levels of histidine to increase, and pyruvate and other histidine metabolites appear in the urine. A majority of patients are clinically normal, although abnormalities as mental retardation and speech defects may be seen. The enzyme deficiency is manifested in skin and liver biopsy specimens.

Histidine compounds in the body include ergothioneine in red blood cells and liver. It is not synthesized in the body and comes purely from dietary source. Others are carnosine and anserine found in muscle, and give methylhistidine in urine.

The genetic disorders for metabolism of histidine are:

Imidazole amino aciduria; wherein large amounts of histidine and its above named compounds are excreted through urine together with a condtion of cerebromacular degeneration Histidinemia; it is due to hypoactivity of the enzyme histidase present in the liver causing retarded speech. It is essential for growth and the repair of human tissues. Its deficiency can cause joints pains, rheumatoid arthritis. It is beneficial in cardiovascular disorders as it dilates the blood vessels. It promotes sexual arousal, cures allergic conditions, and treats anaemia and auditory nerve disorders. It stimulates Hcl secretion in the stomach, and thus who have ulcers should avoid its intake.

Histidine is found in parboiled rice, barely, red gram dal, cow pea, moth beans, rajmah, spinach, drumstick leaves, radish, yam, green plantain, French beans, gingelly seeds, sunflower seeds, banana, grapes, beef, and cheese. The RDA is 10 mg per kg. of body weight.