Key Takeaways (expand)
- Methionine is one of the nine essential amino acids that we must consume from our diets, and one of the 20 amino acids that can help build proteins.
- Methionine is also one of only a handful of sulfur-containing amino acids—the others being cysteine, taurine, and homocysteine.
- Due to its sulfur content, methionine makes a significant contribution to the body’s available pool of sulfur (which is then needed for numerous biological functions).
- Methionine serves as a precursor to a number of other important molecules, including the methyl donor S-adenosyl methionine (SAMe), phosphatidylcholine, the antioxidant glutathione, and the amino acids homocysteine, cysteine, carnitine, taurine, and creatine.
- Through its precursor role, methionine contributes to tissue building and repair, immune system function, the breakdown of neurotransmitters, hormone regulation, detoxification, metabolism, digestion, and antioxidant defense.
- Some studies, although mostly observational, suggest a protective effect of dietary methionine on liver cancer, pancreatic cancer, breast cancer, and colorectal cancer.
- Excess methionine has been linked to increased risk of cardiovascular disease, non-alcoholic fatty liver disease, and certain cancers, especially in animal models—although the evidence is less compelling in humans.
- Some preliminary evidence suggests high intakes of methionine could worsen schizophrenia symptoms and induce detrimental changes in the gut microbiome.
- Good sources of methionine include eggs, chicken, beef, pork, fish, Brazil nuts, sesame seeds, hempseeds, chia seeds, and soybeans.
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Methionine is one of the nine essential amino acids, and one of only two proteinogenic sulfur-containing amino acids (the other being cysteine). It was first isolated in 1921 by the American biochemist John Howard Mueller.
Along with being used for protein biosynthesis, methionine serves as a precursor for other amino acids, glutathione (AKA the “master antioxidant!”), the methyl donor SAMe, and other sulfur-containing compounds. This gives it an important role in numerous metabolic processes, immune function, and digestion!
Good sources of methionine include eggs, chicken, beef, pork, fish, Brazil nuts, sesame seeds, hempseeds, chia seeds, and soybeans.
The Biological Roles of Methionine
Broadly, amino acids are molecules with the molecular formula of R-CH(NH2)-COOH-NH2, where -NH2 is the basic amino group, COOH is an acidic carboxyl group, and R represents a molecular unit called a side chain. That side chain is unique for each amino acid, and its chemical properties create different classes of amino acids: nonpolar and neutral, polar and neutral, polar and acidic, and polar and basic.
Although hundreds of amino acids exist, only 20 of them are used for what amino acids are arguably most known for: forming the building blocks of proteins. Proteins are not only an essential macronutrient in the human diet; they’re molecules that perform most of the various functions of life. In addition to being major structural components of cells and tissues, proteins have incredibly diverse roles that range from driving chemical reactions (e.g., enzymes) to signaling (e.g., some types of hormones) to transporting and storing nutrients. Proteins are synthesized within cells through a two-phase process of transcription and translation, during which amino acids get linked together to form long chains (spanning anywhere from 20 to over 2,000 amino acids in length!).
So, while all proteins are made of amino acids, not all amino acids are used for making proteins! We use the term proteinogenic amino acids to refer specifically to the amino acids that get encoded into our DNA and incorporated into proteins. Meanwhile, non-proteinogenic amino acids do neither of these things (although they still have some very important biological roles!).
Amino acids can be further classified based on whether we can create them in our bodies, or need them from our diet. Essential amino acids are amino acids that can only be obtained from foods; these include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Non-essential amino acids are amino acids our bodies can synthesize metabolically from other molecules; these include alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine. And, some amino acids are conditionally essential, meaning our bodies can normally make them, but some circumstances (like illness or stress) can limit their synthesis and create a dietary requirement. These include arginine, cysteine, glutamine, tyrosine, glycine, ornithine, proline, and serine.
As an essential amino acid, methionine has to come from the foods we eat; we can’t produce it on our own. And, as a proteinogenic amino acid, methionine is used by the body for protein synthesis!
Methionine is also one of only a handful of sulfur-containing amino acids—the others being cysteine, taurine, and homocysteine (though the latter two aren’t incorporated into proteins). Due to its sulfur content, methionine makes a significant contribution to the body’s available pool of sulfur, which is needed for electron transport, cell structure, metabolism, synthesizing and repairing DNA, supporting skin and tendon health, and more. And because sulfur is more electronegative than oxygen, its presence in methionine influences many of its molecular properties, including how hydrophobic methionine is!
Within the body (and especially within the liver), methionine is broken down and regenerated in a series of reactions called the methionine cycle. This cycle results in the production of a number of important molecules with a range of biological functions. First, methionine is converted to the universal methyl donor S-adenosyl methionine (SAMe) through the activity of an enzyme called methionine adenosyltransferase; SAMe then gets converted to S-adenosyl homocysteine (SAH), and then to the amino acid homocysteine—which can then be recycled to form methionine again! What’s more, these methionine metabolites can go on to form other critical molecules: for example, homocysteine can be converted into cystathione and then to the amino acid cysteine, which can then be used to synthesize the critical antioxidant glutathione. Methionine also serves as a precursor to phospholipids (such as phosphatidylcholine) and additional non-essential amino acids (including carnitine, taurine, and creatine).
Through its precursor role, methionine ultimately assists in the functions of the various molecules it helps form. By helping produce glutathione, for example, methionine supports tissue building and repair, antioxidant function, and immune system function. The conversion of methionine into SAMe gives methionine a role in modifying DNA, breaking down neurotransmitters, maintaining cell membranes, regulating hormones, and activating various chemicals. The production of cysteine from methionine contributes to cysteine’s powerful antioxidant properties and detoxification abilities. And by helping produce taurine, methionine supports energy production, metabolism, digestion, nerve growth and nervous system functioning, the processing of bile acids, and fluid balance!
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Methionine in Health and Disease
Most research on methionine has focused on the potential longevity and anti-cancer benefits that come from restricting its intake, especially in animal models. But, some evidence (albeit mostly observational) suggests methionine could play a protective role in certain cancers.
Methionine and Liver Cancer
Compared to other cancer types, methionine may have a uniquely protective role in liver cancer—possibly due to the liver being the site of both methionine synthesis and degradation. In a 2020 experiment using liver cancer cells, for example, methionine supplementation reduced the cancer cells’ growth rate and triggered some cancer-protective changes in metabolic pathways. Another in vitro experiment from 2017found that supplementation with SAMe, formed from methionine, was able to block liver cancer cell growth and inhibit liver cancer cell invasion. A mouse study from 2017 likewise found that short-term supplementation with SAMe significantly decreased the number of tumor nodules and proliferating liver cells, while also increasing the expression of tumor suppressor proteins. (Interestingly, these effects reversed with longer-term SAMe supplementation—suggesting that different mechanisms come into play after an initial induction of cell cycle arrest!)
All that being said, much more research is need in humans in order to understand whether methionine (or molecules formed from methionine) are protective against liver cancer in living humans.
Methionine and Pancreatic Cancer
A 2007 prospective study, following nearly 82,000 Swedish adults for an average of seven years, found that methionine intake was significantly inversely associated with risk of pancreatic cancer. Men in the highest versus lowest quartile of methionine intake had a 68% lower risk, while women had a 41% lower risk (for all participants combined, the risk was 56% lower). Although this study can’t prove cause and effect, findings from in vitro and animal studies support the idea that nutrients involved in methyl group metabolism (including methionine) could help reduce pancreatic carcinogenesis, due to their protective influence on toxic damage and cellular differentiation in the pancreas. More research is needed in humans!
Methionine and Breast Cancer
A protective effect of methionine intake on breast cancer has been found in some observational studies. A 2013 meta-analysis found that among post-menopausal (but not pre-menopausal) women, risk of breast cancer dropped 4% for every 1 g per day increase of methionine intake. A 2013 observational study found that methionine intake appeared protective of estrogen receptor positive breast cancer, with women in the highest quartile of intake having a 17% lower risk (29% lower risk for Hispanic women, specifically). And, some mechanistic studies have shown that methionine is cytotoxic to human breast cancer cells, leading to lower proliferation and growth—largely by reducing the expression of a key cell cycle regulatory protein called p53. Controlled studies are needed in humans to further explore these findings!
Methionine and Colorectal Cancer
A 2013 meta-analysis of eight prospective studies, encompassing over 431,000 participants, found a significant inverse association between methionine intake and colorectal cancer. Specifically, people with the highest versus lowest intake of methionine had an 11% lower risk of developing colorectal cancer, a 23% lower risk of developing colon cancer, and a 12% lower risk of developing rectal cancer. A stratified analysis of the data found that men had a particularly significant drop in colorectal cancer risk with higher methionine intake—25%, to be exact!
Likewise, a cohort study from 2008, tracking nearly 121,000 people, found that men in the highest versus lowest quintile of methionine intake had a 43% lower risk of proximal colon cancer, while women in the highest versus lowest quintile of intake had a 55% lower risk of rectal cancer. And, a 2014 study of people with Lynch syndrome (a type of inherited cancer syndrome) found that for people carrying the MTHFR 677TT allele (which impacts methionine metabolism), a low methionine intake was associated with a significantly higher risk of developing colorectal tumors.
Although these observational studies can’t confirm causation, some animal studies have also supported a protective effect of methionine and its metabolites on colorectal cancer. For example, a mouse study from 2012 found that supplementation with SAMe inhibited several pathways involved in colon carcinogenesis, inducing apoptosis and reducing colon cancer tumor load by 40%. Additional animal research has likewise shown that methionine can help reduce inflammation-induced colon cancer.
On the whole, some plausible mechanisms exist for a protective role of methionine on colorectal cancer risk, but more research is needed!
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Health Effects of Methionine Deficiency
Long-term methionine deficiency can lead to stunted growth, reproductive issues, fertility loss, hair loss, liver dysfunction, bone-related disorders, and elevated homocysteine levels (hyperhomocysteinemia).
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Problems From Too Much Methionine
Although consuming methionine is essential for human health, high intakes have been implicated in some health issues and chronic diseases.
For one, methionine is needed for the growth and metabolism of cancer cells, giving excess dietary methionine a potential role in cancer growth. In contrast to healthy cells, cancer cells have metabolic defects that make them uniquely dependent on the presence of methionine—a phenomenon known as the “Hoffman effect.” A variety of animal and in vitro experiments have shown that restricting dietary methionine helps inhibit the growth of various cancer types, including melanoma, prostate cancer, and sarcoma. And in animal models, methionine restriction has been shown to enhance the effectiveness of chemotherapy and radiation therapy.
However, while many studies suggest methionine restriction could reduce cancer growth (at least in animal models), relatively fewer studies have linked excess methionine to increased cancer growth. One case-control study from 2013 did find that among men with at least one copy of the MTHFR A1298C allele (which impairs the activity of an enzyme involved in maintaining methionine and homocysteine homeostasis), those with a high dietary methionine intake were significantly more likely to develop prostate cancer—a 670% increased risk! However, the association wasn’t present for non-carriers of this allele. Clearly, this is an area where much more research is still needed, especially where diet and genetics might intersect.
Excess methionine has also been linked to a potential increase in cardiovascular disease risk. This may be due in part to its conversion to the non-essential amino acid homocysteine, which itself has been implicated in cardiovascular injury. For example, a number of case-control and epidemiological studies have linked elevated homocysteine levels with higher risk of heart disease, atherosclerosis, arterial damage, and stroke. Mechanistically, elevated homocysteine has been shown to harm the endothelial layer, promote inflammation, and promote oxidative stress—all potentially contributing cardiovascular disease. That being said, studies of methionine supplementation in humans have shown that it typically takes very high intakes (about five times the amount usually obtained from diet), and/or the presence of nutrient deficiencies, in order for methionine to cause appreciable elevations in homocysteine.
Animal experiments have shown that independent of changes in homocysteine levels, methionine supplementation itself can induce changes associated with cardiovascular disease development. However, the results of randomized controlled trials haven’t shown a compelling effect of methionine on cardiovascular risk factors in humans. So, in all, the detrimental potential of methionine on the cardiovascular system is still under debate!
Some research suggests that high dietary methionine intake could contribute to non-alcoholic fatty liver disease—specifically by decreasing the production of hydrogen sulfide, which plays a protective role in fatty liver pathology. For example, a 2022 mouse study found that compared to a normal diet, a high-methionine diet reduced the expression of enzymes needed for the production of hydrogen sulfide, triggering increased inflammation and fat buildup in the liver. A methionine-restricted diet was able to then reverse these effects! And, a 2022 cross-sectional study found significant associations between methionine metabolites in the blood and the prevalence of non-alcoholic fatty liver disease. People in the highest versus lowest quartiles of serum S-adenosylhomocysteine levels and homocysteine levels, for example, were at a 65% and 63% higher risk (respectively) of having fatty liver. Greater levels of these methionine metabolites were also correlated with greater severity of the disease.
There’s also evidence that excess methionine could contribute to or worsen some psychiatric disorders, such as schizophrenia. Early studies from the 1960s experimented with high-dose methionine supplements for schizophrenic patients, and found that methionine actually worsened schizophrenia symptoms. And, animal experiments have used methionine supplementation to induce schizophrenia-like symptoms. Researchers hypothesize this could be due to methionine-induced alterations in DNA methylation at specific gene regions related to schizophrenia, particularly those affecting the neurotransmitter GABA.
Although the research is preliminary, methionine has shown potential to alter the gut microbiome (and its metabolites) in ways that negatively affect cognitive health. A mouse study from 2022 found that feeding the animals a high-methionine diet, compared to a regular or low-methionine diet, led to a decreased abundance of short-chain fatty acid producing bacteria such as Roseburia, Blautia, Faecalibaculum, and Bifidobacterium, as well as serotonin-producing bacteria such as Turicibacter; meanwhile, there was a significant increase in pro-inflammatory bacteria such as Escherichia-Shigella. These shifts in microbiome composition were linked to oxidative stress and inflammation in the mouse brain, and also seemed to downregulate the expression of memory-related genes. Interestingly, a low-methionine diet was able to reverse all of these results!
A number of other rodent studies have supported a role of methionine restriction for improving or protecting gut health (and in some cases, subsequently cognitive health), especially in older animals or those fed high-fat diets. However, most of these experiments compared normal methionine intake to deliberately restricted methionine intake, making it unclear whether excess methionine would worsen gut health relative to normal dietary intakes.
Some evidence suggests that reducing methionine levels in the diet could have important metabolic benefits. Across a variety of animal studies and cell culture studies, methionine restriction has been shown to decrease inflammation, adiposity (body fat), and oxidative stress, while improving insulin sensitivity and extending lifespan. However, this research hasn’t been compellingly replicated in humans, and doesn’t necessarily mean that excess methionine would exacerbate these issues!
How Much Methionine Do We Need?
For adults aged 19 and older, the recommended dietary allowance (RDA) for methionine is 14 mg daily per kg of body weight (or 6.4 mg per lb). During times of physical trauma or illness, however, methionine needs may be higher!
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