Methionine: Benefits, Side Effects, and Safety

Methionine is an essential amino acid, meaning our bodies cannot synthesize it, and we must obtain it from food.

It has many crucial functions, including producing protein for tissue growth and repair, regulating metabolic processes, and antioxidant activities. Therefore, it is important to consume enough methionine, especially for young people whose bodies are still developing.

However, there is also some evidence that older individuals may benefit from limiting the amount of methionine in their diets. Research points to a reduced risk of weight gain, diabetes, heart disease, and more. There is also a suggestion that methionine restriction could increase life expectancy.

So, is it a good idea to take a methionine supplement, or is it something we should be avoiding? Read on to find out more.

What Is Methionine?

Methionine is one of nine essential amino acids humans must find from dietary sources. It is primarily found in animal products such as meat, fish, eggs, and milk. It also exists in smaller quantities in plant protein, including pulses, nuts, seeds, and wheat.


It is a sulfur-containing amino acid and has numerous functions within the body. It is involved in protein synthesis and, therefore, the growth and repair of various tissue types. It also has roles in metabolism, immunity, and digestive function.

Furthermore, the molecule contributes to the body’s antioxidant system and thus offers some protection against chronic disease. Methionine also mitigates liver toxicity, and doctors sometimes use it to treat acetaminophen poisoning.

How Does Methionine Work?

When a person consumes methionine, it is transported to the liver, where it is converted to other bioactive compounds. This occurs via several distinct processes.

One of these processes is known as trans-sulfuration. It converts methionine to a chemical called homocysteine and then to cysteine. Cysteine is another amino acid with essential physiological functions. It is a precursor to glutathione, the body’s primary antioxidant compound.

Cysteine is also required for the production of taurine and coenzyme A.

While cysteine has many benefits for health, the intermediary compound homocysteine can be harmful, increasing the risk of cardiovascular disease. Therefore, the trans-sulfuration reaction must occur efficiently. This requires an adequate intake of vitamins B6, B9 (folate), and B12.

Methionine also undergoes another process known as transmethylation. It involves the donation of methyl (CH₃) groups to DNA via the compound S-adenosylmethionine (SAM). This is crucial because it significantly influences gene expression and cellular health.

Additionally, methionine is necessary for synthesizing other compounds, including creatine, phosphatidylcholine, and polyamines. They have vital roles in energy production and maintaining cellular function.

Methionine Benefits

Many of methionine’s benefits stem from the reactions described above. Its antioxidant effects are especially significant. They mean it protects cells from the damage caused by environmental pollutants. It also aids the excretion of harmful heavy metals from the body and slows cellular aging.

According to a publication by the University of Rochester Medical Center, there are also several unsubstantiated claims regarding the benefits of methionine, including:

  • May reduce fatigue
  • May reduce the risk of premature balding
  • May help treat osteoporosis
  • May protect the body from ionizing radiation

However, there is currently insufficient scientific evidence to support these claims.

Methionine Restriction

Despite its benefits, many experts now believe that overconsumption of methionine can be harmful. In fact, a growing body of evidence suggests that limiting one’s methionine consumption could improve health and increase life expectancy.

According to a 2021 review, studies on methionine restriction in yeast, nematodes, fruit flies, mice, and rats have yielded positive results. Early human studies have also highlighted several potential benefits.

It seems that methionine restriction helps to improve “metabolic flexibility” and insulin sensitivity. This can lead to numerous benefits, including a reduced risk of obesity, diabetes, and heart disease. It has also been linked with reduced levels of inflammation and DNA damage.

According to a 2017 paper, some of the benefits of methionine restriction include:

  • Increased glutathione levels
  • Reduced oxidative stress
  • Increased trans-sulfuration
  • Decreased lipid (fat) production and storage

That said, individuals should consult their healthcare provider before implementing any major dietary changes. Furthermore, methionine is essential for healthy development and should not be restricted during pregnancy, childhood, and adolescence. The practice is likely most suited to older adults and should be supported by adequate nutritional intake and physical activity.

Methionine Dosage

When discussing dietary methionine, it is often listed alongside cysteine, the other sulfur-containing amino acid. The average daily requirement for methionine plus cysteine is 13mg per kilogram of body weight for adults.


Single high doses of methionine have not been shown to be harmful. However, regularly consuming excessive amounts could cause problems. This is because increasing methionine consumption could increase homocysteine levels and the risk of heart disease.

Doses of five times the daily requirement have been shown to increase homocysteine levels significantly.

Methionine Side Effects

High doses of methionine have been associated with the following side effects:

  • Nausea
  • Vomiting
  • Dizziness
  • Constipation
  • Bad breath
  • Drowsiness
  • Low blood pressure
  • Irritability

The amino acid could exacerbate the symptoms of schizophrenia and bipolar disorder, so people with these conditions should not take methionine supplements. People with a rare genetic disorder called homocystinuria should also avoid excessive methionine consumption as it could cause kidney stones.

Supplements are possibly unsafe for pregnant and lactating individuals.

Should I Take a Methionine Supplement?

Most people get enough methionine from food and do not require dietary supplements. Moreover, due to the risk of increased homocysteine levels, people should only use methionine supplements if advised to do so by a professional. Such individuals should also consume plenty of B vitamins to help the body convert homocysteine into cysteine.

It is also essential to choose supplements wisely. Single amino acid supplements have been linked with altered nitrogen balance in the body, which can adversely impact metabolism. They could also overwork the kidneys and impair growth in children. Therefore, combination products might be a safer option.

As always, consumers should look for brands that are transparent about their ingredients and provide lab reports for their products. Customer reviews on third-party sites are another excellent way to determine whether a company is reliable. We have listed some of the most reputable products in our Best Amino Acids Review.

The Bottom Line on Methionine

The role of methionine in human health is undeniable. However, consuming too much of this amino acid could have detrimental effects. Therefore, taking a methionine supplement without medical advice could do more harm than good.

Individuals considering supplementation should consult a knowledgeable physician or dietician for further information. It is also crucial to choose supplements carefully due to the unregulated nature of the market.

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