Multifunctional Role of Methionine Essential Amino Acid

Amino acids are absolutely critical for proper neurological, immunological, and gastrointestinal functions. They are used to construct enzymes, antibodies, neurotransmitters, immunoglobulins, and hormones. They are also an important part of the detoxification process. They join with toxic substances and escort them from the body. Methionine, cysteine, taurine, and homocysteine are the four sulfur-containing amino acids. While methionine and cysteine are the only two incorporated into proteins, taurine and homocysteine also play vital physiological roles. Methionine and cysteine are the principal source of sulfur in our diets. The body can make cysteine from methionine, but not vice versa, so that methionine is a dietary essential.1 The multifunctional role of methionine includes its participation in the synthesis of carnitine, creatine, glutathione, nucleic acids, polyamines, catecholamines (neurotransmitters), as well as serving as a substrate for protein synthesis. Methionine helps in the breakdown of fats and is a powerful antioxidant.

Dietary Sources of Methionine

Following is the list of foods high in methionine:

  • Brazil nuts
  • Cheese (brie, cheddar, parmesan)
  • Crayfish
  • Eggs (dried and whole)
  • Dried milk
  • Meats (beef, chicken, and pork)
  • Fish (salmon, halibut)
  • Soybeans
  • Sunflower seeds

Methionine Can Increase the Growth of Certain Types of Cancer

Since rapidly dividing cells have an increased demand for certain amino acids, doctors have worried about the wisdom of giving these amino acids in large amounts. Several studies have shown that methionine and arguinine can increase the growth of certain types of tumors and may increase the risk of metastasis as well.2 The strongest evidence concerning the connection between tumor growth and amino acids is found with L-methionine. Most of the evidence stems from the use of diets depleted of methionine. For example, giving patients with advanced gastric cancer intravenous nutritional supplementation using mixtures containing no methionine significantly slowed the growth of their tumors and made the tumors more susceptible to chemotherapy. Similar reactions have been seen in prostate cancer when methionine was restricted in the diet.2

Crayfish is an important source of methionine

Methionine Can Prevent Cancer

While methionine can increase the growth of existing cancers, in the normal individual it appears to play a vital role in the prevention of cancer development. Methionine's major role in preventing cancer development is based on the fact that it is the primary methyl donor in cells. Biochemically, L-methionine is converted to S-adenosylmethionine (SAMe), which then donates methyl groups for DNA regulation and repair. When there is a deficiency of methyl groups, cancer risk is greatly increased. This is why the supplement SAMe should not be taken by people with a strong family history of cancer.2

Role of Homocysteine in the Development of Heart Disease

One of the main roles of methionine in the body is to provide methyl groups for metabolic processes to occur. When methionine loses a methyl group it becomes homocysteine. In order for homocysteine to convert back to methionine, it must receive a methyl group either from folic acid, or vitamin B6. Vitamin B12 is needed as a co-factor for this reaction to occur.

Homocysteine can be measured in the bloodstream; a high reading usually indicates that we do not get enough vitamins B6, B12, or folic acid in our diet. These vitamins are found in high amounts in fresh fruit and vegetables, as well as animal products (meats, eggs, and fish). Having high blood levels of homocysteine is thought to be a major risk factor for heart disease and several other diseases.4

Effects of Dietary Methionine Deficiency in Dogs and Cats

Removal of methionine from diets of puppies and adult dogs results in immediate decrease in food intake and weight loss. Puppies may exhibit dermatitis. When methionine is returned to the diet, lesions rapidly disappear. When dogs are given high-carbohydrate, high-fat food, that contain protein near their requirements but low in methionine and deficient in taurine, they consistently develop gallstones corrected with methionine supplementation, but not taurine.3

Removal of methionine from the diets of kittens and adult cats results in weight loss. Animals develop lethargy and abnormal secretions from the eyes.3


  1. Amino acids and proteins for the athlete: the anabolic edge. Mauro G. Di Pasquale
  2. Natural Strategies for Cancer Patients. Russell L. Blaylock
  3. Nutrient requirements of dogs and cats. National Research Council (U.S.). Ad Hoc Committee on Dog and Cat Nutrition
  4. Cholesterol: The Real Truth. Sandra Cabot, Margaret Jasinka



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