Tyrosine

What is tyrosine?

Tyrosine is a conditionally essential amino acid, which can be produced in your body from another amino acid phenylalanine, but in certain circumstances, such as young age or heavy illness, you need to obtain additional amounts from food to be healthy ^[1]. In foods, tyrosine appears as part of proteins.

The name origin: from the Greek tyri = cheese

Tyrosine abbreviation (symbol): Tyr

Tyrosine Functions in the Human Body

Tyrosine is ^[1]:

• A building block of proteins
• A glucogenic amino acid — it can be converted into glucose ^[7]
• A ketogenic amino acid — it can be converted into ketones ^[7]
• A precursor of:
  • The thyroid hormone (thyroxine)
  • The neurotransmitter dopamine in the brain
  • The neurotransmitter norepinephrine (noradrenaline)
  • The hormone epinephrine (adrenaline)
  • Melanin–a pigment in skin, hair and iris

Tyrosine Rich Foods

• ANIMAL FOODS: most animal foods ^[2]
• PLANT FOODS: peanuts, seeds, baker’s yeast, spirulina ^[2]

Foods low in tyrosine: cereals, fruits ^[2].

Tyrosine Supplements

Oral tyrosine supplements without prescription (over-the-counter) are available:

• L-tyrosine
• N-acetyl-l-tyrosine is, according to certain producer’s claim, absorbed faster and excreted at a slower rate than l-tyrosine.

Tyrosine Health Benefits

Tyrosine supplements are POSSIBLY EFFECTIVE ^[3] in improving alertness, memory ^[11] and mental performance.

Tyrosine supplements are POSSIBLY INEFFECTIVE ^[3] in the prevention or treatment of depression or attention deficit disorder (ADD) in adults and attention deficit hyperactivity disorder (ADHD) in children or in increasing physical performance ^[10].

There is INSUFFICIENT EVIDENCE ^[3] about the effectiveness of tyrosine supplements in the prevention or treatment of alcoholism, Alzheimer’s disease, chronic fatigue syndrome (CFS), cocaine dependence, erectile dysfunction, excessive sleepiness (narcolepsy), heart disease, high blood pressure, impotence, Parkinson’s disease, phenylketonuria ^[8,9], premenstrual syndrome (PMS), schizophrenia, stress or wrinkled skin, or in promoting of weight loss.

Tyrosine Safety: Side Effects, Toxicity

Tyrosine in doses up to 150 mg/kg for three months is POSSIBLY SAFE for most adults ^[3].

Side effects may include nausea, heartburn, headache, fatigue, joint pain ^[3].

Who should not take tyrosine supplements?

• Pregnant and breastfeeding women
• Individuals with hyperthyroidism (Grave’s disease)
• Individuals who take thyroid hormones or levodopa
• Reference: ^[3]

Tyrosine-Drugs Interactions

Tyrosine supplements may decrease the effects of levodopa (a drug for Parkinson’s disease) and increase the effects of thyroid hormones, such as levothyroxine ^[3].

Tyrosinemia

Tyrosinemia means elevated blood tyrosine level. Three types of hereditary (autosomal recessive) tyrosinemia are known: tyrosinemia I, II and III.

Tyrosinemia I

In tyrosinemia I or hereditary infantile tyrosinemia tyrosine is accumulated in the blood and internal organs due to a deficiency of the enzyme fumarylacetoacetate hydrolase ^[5].

Symptoms develop in the first months of life and include vomiting, bloody stools, nosebleeds, jaundice, cabbage-like odor and failure to thrive ^[5]. Complications can include liver cirrhosis and cancer, kidney disorders, peripheral neuropathy, rickets and seizures.

Diagnosis. Blood tests show elevated levels of tyrosine, methionine and phenylalanine.

Without treatment, the disorder is deadly until the tenth year of life.

Tyrosinemia II

Tyrosinemia II is caused by a deficiency of the enzyme tyrosine aminotransferase.

Symptoms often begin in early childhood and include excessive tearing, abnormal sensitivity to light (photophobia), eye redness, painful skin lesions on the palms and soles, and mental retardation ^[5]. 

Diagnosis. A blood test shows increased tyrosine levels and normal levels of other amino acids.

Tyrosinemia III

Tyrosinemia III is caused by a very rare hereditary deficiency of the enzyme 4-hydroxyphenylpyruvate dioxygenase. Symptoms include seizures, periodic loss of balance and coordination, mental retardation ^[5]. 

Diagnosis. A blood test shows increased tyrosine levels.

Diet and Treatment in Tyrosinemia I, II and III

A life-long low-tyrosine and low-phenylalanine diet is prescribed ^[5]. In tyrosinemia I, a drug NTBC can help.

Transient Tyrosinemia

Transient tyrosinemia is a benign disorder with elevated blood tyrosine levels in normal or preterm infants, supposedly caused by immaturity of enzymes that metabolize tyrosine, and vitamin C deficiency ^[6].

Symptoms may include difficult swallowing, lethargy, jaundice, but, often, no symptoms develop. Blood tests show elevated tyrosine and phenylalanine levels. Low-protein diet and vitamin C supplements may be prescribed. Transient tyrosinemia usually resolves spontaneously within the first month of age.

Alkaptonuria

Alkaptonuria is a rare inherited disorder in which amino acids tyrosine and phenylalanine cannot be properly metabolized.