(Tyr, L-tyrosine)

A nonessential amino acid required to build proteins and other important compounds. Tyrosine is readily synthesized by the body from the essential amino acid phenylalanine. Tyrosine in the diet decreases the requirement of phenylalanine because less needs to be shunted to tyrosine synthesis. In addition to serving as a raw material for proteins, tyrosine is converted by the nervous system into a family of neurotransmitters, chemicals that help transmit nerve impulses between cells. These “catecholamines” include dopamine, norepinephrine, and epinephrine. They play a role in sensitivity to pain and in feeling alert. In addition, the adrenal glands convert tyrosine to epinephrine (adrenaline) and norepinephrine, which are released as hormones to gear up the body to respond to stress. The thyroid gland combines iodine with tyrosine to form the thyroid hormone thyroxine, which regulates basal metabolism. Tyrosine also forms melanin, the pigment of skin. Normal transformations of tyrosine require niacin, copper, and vitamin C.

In humans, tyrosine appears to increase performance under stress. Under stress nerve cells release norepinephrine more rapidly. Tyrosine supplements could theoretically increase norepinephrine formation and indirectly help maintain vigilance. Tyrosine may help reduce depression and fatigue associated with premenstrual syndrome (PMS). tryptophan and tyrosine have been used to minimize cravings and depression during withdrawal from addictive substances. In lab animals, administration of tyrosine can increase brain uptake and increase brain production of neurotransmitters. One hypothesis of schizophrenia proposes that dopamine is increased in certain individuals. Some antipsychotic medications are designed to block the conversion of tyrosine to dopamine. Tyrosine seems to be most effective for nerve cells that fire frequently, therefore supplements can affect an aspect of physiology differently depending on the conditions. For example, certain brain cells fire rapidly in hypertension (high blood pressure); thus, giving tyrosine to hypertensive rats lowers the blood pressure. With low blood pressure, sympathetic nerves outside the brain are being triggered in order to raise blood pressure. Administration of tyrosine in this case helps raise blood pressure. Liver cirrhosis can lead to a brain condition called encephalopathy (hepatic encephalopathy). Unusually high blood levels of phenylalanine and tyrosine could increase brain formation of tyramine, and this could disrupt normal nerve function. In any event, cirrhosis alters tyrosine formation from phenylalanine as well as altering tyrosine degradation.

Tyrosine supplements can either raise or lower blood pressure depending on the individual, and migraine headaches can worsen, depending on the conditions. Tyrosine and phenylalanine supplements should not be taken at the same time as anti-depressant drugs containing monoamine oxidase inhibitors. Taken together they can raise blood pressure to fatal levels. (See also amino acid metabolism.)

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