L-tyrosine is an amino acid that the body uses to synthesize protein. The body can produce tyrosine on its own, making it a non-essential amino acid. L-tyrosine is important when the body is under stress, fatigued, or cold. In adequate levels during stressful times, it appears to reduce stress-induced weight loss as well as levels of stress hormones. Under normal circumstances, tyrosine does not appear to have any effect on brain function, performance, or mood.
The body creates l-tyrosine from phenylalanine, which is abundant in many high protein foods, such as cottage cheese, peanuts, turkey, chicken, and soy. There is a condition called phenylketonuria, or PKU, in which the body lacks sufficient levels of the enzyme required to metabolize phenylalanine to l-tyrosine. The phenylalanine accumulates in the body and is excreted in urine.
PKU is a serious health condition that can lead to seizures, brain damage and progressive mental retardation. There is no cure for PKU, which is typically diagnosed during newborn health screenings. The condition is managed by lowering blood phenylalanine levels, by feeding a low-phenylalanine diet and protein supplements, supplementing with tyrosine, and monitoring cognitive development.
It is not necessary to supplement with l-tyrosine for the majority of the population. If someone chooses to supplement, the recommended dosage is between 500 and 1500 milligrams. High levels of l-tyrosine can reduce levels of dopamine in the body. Dopamine is a neurotransmitter necessary for normal central nervous system function.
Taking high doses of l-tyrosine can also decrease the body's ability to absorb other amino acids. Aside from individuals with PKU, other groups who may want to supplement with tyrosine include people suffering from depression, and those with kidney disease. People suffering from kidney disease often excrete large amounts of protein, and can develop deficiencies in l-tyrosine as well as other amino acids.
One group of people who should refrain from supplementing with tyrosine are those with thyroid conditions such as Graves disease or an overactive thyroid. Tyrosine is used by the body to manufacture the thyroid hormone thyroxine. Supplementing tyrosine may increase the amount of thyroxine in the body, worsening thyroid symptoms.
There are many health conditions that tyrosine may help, although medical evidence is mixed. Generally, supplementing tyrosine is considered safe, so individuals with attention deficit and attention deficit hyperactivity disorder, chronic fatigue syndrome, narcolepsy and individuals hoping to improve alertness after periods of sleep loss may find some relief with tyrosine supplements though it is important to discuss it first with a health care provider. Side effects from tyrosine are mild and relatively uncommon. They include joint pain, heartburn, nausea, headache, and fatigue.