Methionine

Overview

Amino acids are the building blocks of proteins and are the end products of protein digestion.

The environment consists of 80 amino acids; however, the body only utilizes 20 of them. Essential amino acids are those that are not synthesized in the body so they must be taken in through the diet.

Methionine is the only amino acid that forms sulfide bonds in proteins. It is important to a variety of biochemical pathways; most cells can use homocysteine as a substitute for methionine. However, there are methionine-dependent cells that cannot use homocysteine in place of methionine. (1) , (2)

Methionine levels affect the amount of sulfur-containing compounds, such as glutathione, which defend against toxic compounds, in the liver. When higher levels of toxic compounds are present, more methionine is needed. (3)

Through its supply of sulfur, methionine improves the tone and pliability of skin, conditions hair, and strengthens nails. Methionine also protects cells from oxidation. Methionine is essential for the absorption, transportation and bioavailability of selenium and zinc. Methionine chelates heavy metals, such as lead, cadmium, and mercury.

Methionine lowers blood levels of histamine by supporting its breakdown. Methionine is important in cartilage structures, especially proteoglycans and glycosaminoglycans. It is incorporated into tissue proteins and enzymes and is a component of pain-reducing peptides, such as enkephalin and endorphins.

The range of human need for methionine is estimated at between 800 and 3,000 milligrams per day.

Dosage Info

Dosage Range

500mg – 20 grams daily.

Most Common Dosage

1,500mg daily.

Dosage Forms

Tablets, capsules, powder, oral, and intravenous administration.

Adult RDI

None established

Adult ODA

None established

RDA

Active Forms

DL-methionine and L-methionine.

Absorption

The absorption of amino acids occurs along the entire length of the small intestine.

Toxicities & Precautions

General

Excess amino acids burden the liver and kidneys; ammonia builds up in the liver when amino acids are in high concentrations. This may result in poor digestion and impairment of liver function.

Too much methionine can be toxic. The dose in adults that may cause toxicity is estimated to be approximately 350mg/kg or about 25 grams daily for an average person.

Symptoms of toxicity reportedly may include ataxia, hyperactivity, hemosiderosis, reduced growth, loss of appetite, and suppressed hematocrit. N-acetylmethionine is less toxic at these doses.

High doses increase urinary excretion of calcium and may induce hallucinations (at very high doses); however, doses of 1-3 grams daily are well tolerated with rare, idiosyncratic adverse responses.

Homocysteine is formed by the demethylation of the essential amino acid methionine. Homocysteine accumulates as a result of B6 deficiency.

Individuals with elevated levels of homocysteine and cholesterol are at increased risk of cardiovascular disease. (4)

Side Effects

Supplemental amino acids can produce temporary flatulence or diarrhea.

Functions in the Body

Neurotransmitter Synthesis

Dopamine, norepinephrine, epinephrine, and particularly serotonin.

Lipotropic

Inhibits fat accumulation in liver.

Protein Synthesis

Component of proteins, collagen, and elastin.

Antioxidant

Provides efficient oxidant scavenger.

Choline Synthesis

Supports function of the brain.

Methyl Donor

Contributes to synthesis of numerous compounds.

Sulfur Donor

Provides sulfur for detoxification.

Chelator

Binds with heavy metals, facilitating excretion.

Folate Levels

A methionine deficiency induces functional folate deficiency.

Clinical Applications

Histadelic Depression

Depressed patients who have elevated histamine levels (histadelic) have been treated successfully with methionine. (5)

Schizophrenia

Schizophrenic patients who have elevated levels of histamine have been treated successfully with methionine. (6)

Liver Detoxification

Due to its function as a sulfur donor, methionine aids in liver detoxification functions and as a precursor to glutathione, (7) an antioxidant in the liver.

Symptoms and Causes of Deficiency

Strict vegetarians (vegans) are at higher risk for deficiency, however deficiencies are still possible with non-vegetarians. Suspected reasons for deficiency include poor diet, stress, infection, impaired absorption, and age. Single amino-acid deficiencies are unknown except in people on crash diets consisting of only a few foods. Amino-acid deficiencies appear more commonly as a result of total protein deficiency.

Moderate methionine deficiency symptoms include retarded growth and low levels of essential proteins in blood. Severe deficiencies may manifest as apathy, depigmentation of hair, edema, lethargy, liver damage, loss of muscle and fat, skin lesions, and weakness.

Dietary Sources

Food sources include sunflower seeds, beef, eggs, cottage cheese, chicken, fish, pork, liver, sardines, yogurt, pumpkin seeds, sesame seeds, and lentils.

References

  1. View Abstract: Kokkinakis DM, von Wronski MA, et al. Regulation of O6-methylguanine-DNA Methyltransferase by Methionine in Human Tumor Cells. Brit J Cancer. 1997;75(6):779-88.
  2. View Abstract: Kokkinakis DM, Schold SC Jr, et al. Effect of Long-term Depletion of Plasma Methionine on the Growth and Survival of Human Brain Tumor Xenografts in Athymic Mice. Nutrition Cancer. 1997;29(3):195-204.
  3. View Abstract: Levine RL, Mosoni L, et al. Methionine Residues as Endogenous Antioxidants in Proteins. Proc Natl Acad Sci. USA. Dec1996;93(26):15036-40.
  4. Hennig B, Toborek M. Is Methionine an Athergenic Amino Acid? J Opt Nutr. 1994;3(2):80-83.
  5. Braverman E, et al. The Healing Nutrients Within. New Canaan, CT: Keats Publishing Inc; 1997:189-190.
  6. Braverman E, et al. The Healing Nutrients Within. New Canaan, CT: Keats Publishing Inc ;1997:191-92.
  7. View Abstract: Donaldson WE, Leming TK. Effect of Dietary Methionine and Lysine on the Toxicity of Ingested Lead Acetate in the Chick. J Nutr. 1984;114:2155-59.