Histidine

What Is Histidine?

Histidine is the amino acid with an imidazole ring in its side chain, a small ring of carbon and nitrogen that can pick up or release a hydrogen ion at the body's normal pH. That one property explains most of what histidine does.

What Histidine Does in the Body

Holds iron in hemoglobin. In hemoglobin and myoglobin, a histidine in the protein chain binds the iron atom at the center of each heme group. Another histidine sits beside the spot where oxygen attaches. Histidine is also part of the active site of many enzymes, where its ring passes hydrogen ions back and forth.

Becomes histamine. One enzyme step removes histidine's carboxyl group, released as carbon dioxide, and turns it into histamine. Mast cells release it in allergic reactions, cells in the stomach lining use it to trigger acid, and neurons in the brain use it to keep you awake, which is why older antihistamines cause drowsiness.

Builds carnosine. Muscle joins histidine to beta-alanine to make carnosine, which buffers the acid that builds up in hard exercise. The same imidazole ring does the buffering.

When Fish Turns Histidine Into Histamine

Tuna, mackerel, sardines and anchovies carry unusually high amounts of free histidine in their muscle. If the fish warms up after the catch, bacteria convert that histidine into histamine. Eating it causes scombroid poisoning: flushing, headache, a peppery taste, a rash and an upset stomach, usually within an hour.

It is often mistaken for a fish allergy, but the cause is histamine in the fish, not the immune system. Cooking, canning and freezing do not remove histamine once it has formed, which is why the fish has to stay cold from boat to plate.

Foods High in Histidine

Every food protein contains histidine, so meeting the need is mostly a question of eating enough protein. In the Caltide food library, baked cod has 0.67 g per 100 g, roasted turkey 0.78 g and peanuts 0.60 g. Cooked lentils have 0.25 g, so a 70 kg adult would cover the 700 mg from about 280 g of lentils or about 105 g of cod.

Compare more foods in the food library, or read about tryptophan, the essential amino acid with the smallest daily need.

Common Histidine Mistakes

Treating histidine as optional for adults. Adults store some histidine in hemoglobin and carnosine and can draw on it for weeks, which made short studies miss the need. Longer studies showed it, and it has counted as essential for adults since 1985.

Confusing histidine with histamine in food. A food rich in histidine is not a high histamine food. Histamine builds up through aging, fermenting or spoiling, as in aged cheese, cured meat and fish kept too warm.