Histidine

Histidine comes in two forms (L and D) just like almost all the other structural amino acids (ones that get made into protein) but the L- prefix is not usually bothered with for this amino acid.
Histidine is a semi-essential amino acid; it is manufactured in sufficient quantities in adults, but children may at some time have a shortage of histidine. It is one of the basic (reference to pH factor) amino acids due to its aromatic nitrogen-heterocyclic imidazole side chain. This amino acid is metabolized into the neurotransmitter histamine and the set of genes that produce the enzymes responsible for histidine synthesis.
Histamine is also a compound released by immune system cells during an allergic reaction.
Histidine is needed for growth and for the repair of tissue, as well as the maintenance of the myelin sheaths that act as protector for nerve cells.
Histidine is further required for the manufacture of both red and white blood cells, and helps to protect the body from damage caused by radiation and in removing heavy metals from the body.
In the stomach, histidine is also helpful in producing gastric juices, and people with a shortage of gastric juices or suffering from indigestion, may also benefit from this nutrient.
It is reported that an increase in the intake of histidine helps with the lengthening of orgasms and also more intense sexual enjoyment.
Some people take 1,000 mg of histidine two to three times per day in capsule or tablet form but it is best to work out the dosage requirements as 8-10 mg per day per kilogram of body weight.
There are no reported side effects with histidine, but too high levels of histidine may lead to stress and mental disorders such as anxiety and people with schizophrenia have been found to have high levels of histidine.
People suffering from schizophrenia or bipolar (manic) depression should not take a histidine supplement without the approval of their medical professional.
Histidine is best taken with vitamin B3 (niacin) and B6 (pyridoxine).
Although not conclusively proven - it is thought that histidine may be beneficial to people suffering from arthritis and nerve deafness.
Histidinemia is an inborn error of the metabolism of histidine due to a deficiency of the enzyme histidase, where high levels of histidine are found in the blood and urine, and may manifest in speech disorders and mental retardation. It is quite rare, but if you have this problem you should not take additional histidine without well-qualified medical advice.