Methionine: functions, food sources and side effects

Amino acids help build the proteins that make up the tissues and organs of your body.

In addition to this critical function, some amino acids have other special functions.

Methionine is an amino acid that produces several important molecules in your body. These molecules are essential for the proper functioning of your cells.

Due to the important molecules that it produces, some recommend increasing the intake of methionine. However, others recommend limiting it due to possible negative side effects.

This article will discuss the importance of methionine and if you should worry about the amount of this in your diet. The sources and possible side effects are also discussed.

What is methionine?

Methionine functions, food and side effects

Methionine is an amino acid found in many proteins, including the proteins in food and those found in the tissues and organs of your body.

In addition to being a building block for proteins, it has other unique characteristics.

One of them is its ability to become important molecules that contain sulfur (1).

Sulfur-containing molecules have a variety of functions, including protecting their tissues, modifying their DNA and maintaining proper functioning of their cells (2, 3).

These important molecules must be made of amino acids that contain sulfur. Of the amino acids used to produce proteins in the body, only methionine and cysteine ​​contain sulfur.

Although your body can produce the amino acid cysteine ​​alone, methionine must come from your diet (4).

In addition, methionine plays a critical role in initiating the process of producing new proteins within cells, something that occurs continuously as older proteins break down (1).

For example, this amino acid begins the process of producing new proteins in your muscles after an exercise session that damages them (1, 5).

Summary Methionine is a unique amino acid. It contains sulfur and can produce other molecules that contain sulfur in the body. It is also involved in the start of the production of proteins in your cells.

It can produce molecules critical for normal cell function

One of the main functions of methionine in the body is that it can be used to produce other important molecules.

It is involved in the production of cysteine, the other amino acid that contains sulfur used to build proteins in the body (1, 6).

Cysteine ​​can, in turn, create a variety of molecules, including proteins, glutathione and taurine (1).

Glutathione is sometimes called the "master antioxidant" because of its critical role in the defenses of your body (2, 7).

It also plays a role in the metabolism of nutrients in the body and in the production of DNA and proteins (2).

Taurine has many functions that help maintain the health and proper functioning of your cells (8).

One of the most important molecules in which methionine can be converted is S-adenosylmethionine or "SAM" (1).

SAM participates in many different chemical reactions by transferring part of itself to other molecules, including DNA and proteins (3, 9).

SAM is also used in the production of creatine, an important molecule for cellular energy (10, 11).

In general, methionine is directly or indirectly involved in many important processes in the body due to the molecules in which it can be converted.

Summary Methionine can be converted into several sulfur-containing molecules with important functions, such as glutathione, taurine, SAM and creatine. These molecules are critical for the normal functions of the cells in your body.

Play a role in DNA methylation

Your DNA contains the information that makes you who you are.

Although much of this information may remain the same throughout your life, environmental factors can change some aspects of your DNA.

This is one of the most interesting roles of methionine: it can be converted into a molecule called SAM. SAM can change its DNA by adding a methyl group (a carbon atom and its hydrogen atoms attached) (3, 9).

The amount of methionine in your diet can affect the amount of this process that occurs, but there are many unanswered questions about this.

It is possible that increasing methionine in the diet increases or decreases the amount of changes in your DNA as a result of SAM (12).

In addition, if these changes occur, they may be beneficial in some cases, but harmful in others (12).

For example, some research has shown that high-nutrient diets that add methyl groups to their DNA can reduce the risk of colorectal cancer (13).

However, other research has shown that a higher intake of methionine could worsen conditions such as schizophrenia, perhaps due to the addition of more methyl groups to DNA (12, 14).

Summary One of the molecules produced by methionine, SAM, can alter its DNA. It is not completely clear how the methionine content of your diet affects this process, and it is possible that this process may be beneficial in some cases and harmful in others.

Diets low in methionine prolong the life of animals

Although methionine has important functions in the body, some research shows the benefits of diets that are low in this amino acid.

Some cancer cells depend on the methionine in the diet to grow. In these cases, limiting the intake of your diet could be beneficial in helping cancer cells die of hunger (15).

Since plant proteins are usually lower in methionine than in animal proteins, some researchers believe that plant-based diets could be a tool to combat some types of cancer (15, 16).

In addition, several animal studies show that methionine reduction can increase lifespan and improve health (17, 18, 19).

One study found that shelf life was more than 40% longer in mice fed a low methionine diet (17).

This longevity may be due to a better resistance to stress and metabolism, in addition to maintaining the reproduction capacity of the body's cells (20, 21).

Some researchers concluded that the low methionine content acts to actually decrease the rate of aging in mice (19).

It is not yet clear whether these benefits extend to humans or not, but some test tube studies have shown the benefits of low methionine content in human cells (20, 22).

However, human research is needed before conclusions can be drawn.

Summary In animals, reducing the methionine content of the diet can decrease the rate of aging and increase the lifespan. Some studies have shown the benefits of reducing methionine in human cells, but research is needed in living humans.

Food sources of methionine

While virtually all protein-containing foods have some methionine, the amount varies widely. Eggs, fish and some meats contain high amounts of this amino acid (23).

It is estimated that about 8% of the amino acids in egg whites are amino acids that contain sulfur (methionine and cysteine) (24).

This value is around 5% in chicken and meat and 4% in dairy products. Vegetable proteins usually have even smaller amounts of these amino acids.

Some research has also examined the total amount of sulfur-containing amino acids (methionine and cysteine) in different types of diets (24).

The highest content (6.8 grams per day) was reported in high protein diets, while vegetarians (3.0 grams per day) and vegans (2.3 grams per day) had lower intakes.

Despite the low intake among vegetarians, other research has shown that they actually have higher concentrations of methionine in the blood than those who eat meat and fish (25).

This finding led the researchers to conclude that the dietary content and blood concentrations of methionine are not always directly related.

However, these studies found that vegans have a low dietary intake and low concentrations of methionine in the blood (24, 25).

Summary Animal proteins often have a higher methionine content than plant proteins. Those who follow a plant-based diet have a lower dietary intake of sulfur-containing amino acids, although they may have higher or lower levels of methionine in the blood.

Intake, toxicity and side effects

Researchers have established a recommended daily intake of sulfur-containing amino acids (methionine and cysteine), but studies have also examined the side effects of higher doses.

Recommended intake

The recommended daily intake of methionine plus cysteine ​​is 8.6 mg / lb (19 mg / kg) per day for adults, which is around 1.3 grams for a person who weighs 150 pounds (68 kilograms) (4) .

However, some researchers have recommended consuming twice this amount according to the limitations of the studies used to establish the recommended intake (24).

The elderly often have a low methionine intake, and studies have shown that they may need higher intakes of 2 to 3 grams per day (24, 26).

Despite the fact that certain groups can benefit by increasing their methionine consumption, many diets exceed 2 grams per day of methionine plus cysteine.

It is estimated that a variety of diets, including vegan, vegetarian, traditional and high protein diets, contain between 2.3 and 6.8 grams per day of these amino acids (24).

Effects on homocysteine

Perhaps the biggest concern associated with high methionine consumption is due to one of the molecules that this amino acid can produce.

Methionine can be converted to homocysteine, an amino acid associated with several aspects of heart disease (27, 28).

The high intake of methionine can lead to an increase in homocysteine, although some people are more susceptible to this process than others (29).

Interestingly, research indicates that the potential hazards of high methionine intake may be due to homocysteine ​​rather than to methionine itself (30).

However, there are other factors that can alter homocysteine ​​levels.

For example, although they have a lower intake of methionine in the diet, vegans and vegetarians may have a higher homocysteine ​​than omnivores due to low intake of vitamin B12 (31).

Other research has shown that a diet high in protein and high in methionine did not increase homocysteine ​​after six months, compared to a diet low in protein and low in methionine (32).

In addition, altering intake by up to 100% does not appear to affect homocysteine ​​in healthy adults without vitamin deficiencies (33).

Side effects

To evaluate the body's responses to methionine, researchers will administer a large dose of this amino acid and observe the effects.

This dose is much higher than the recommended intake, often around 45 mg / lb (100 mg / kg), or 6.8 grams for a person who weighs 150 lbs (68 kg) (29).

This type of test has been performed more than 6,000 times, mainly with minor side effects. These minor side effects include dizziness, drowsiness, and changes in blood pressure (29).

One major adverse event occurred during one of these tests, which resulted in the death of a person with high blood pressure, but otherwise good health (34).

However, it seems likely that an accidental overdose of approximately 70 times the recommended intake caused the complications (34).

In general, it seems that methionine is not particularly toxic in healthy humans, except in extremely high doses that would be practically impossible to obtain through the diet.

Although methionine participates in the production of homocysteine, there is no evidence that ingestion within a typical range is dangerous to heart health (29).

Summary People who follow many types of diets will often exceed the minimum recommended intake of methionine. Side effects in response to large doses are often minor, but can become dangerous at extremely high doses.

The bottom line

Methionine is a unique amino acid that contains sulfur that can be used to build proteins and produce many molecules in the body.

These include the antioxidant glutathione and the SAM molecule, which is used to modify DNA and other molecules.

Methionine is found in a variety of foods that contain protein and is often higher in animal proteins than in plant proteins. Although it has been shown that diets low in methionine prolong the lifespan of animals, it is not yet clear if this is important for humans.

People who consume many different types of diets usually comply with the recommended intake of methionine, although some elderly people may benefit by increasing their intake.

Side effects in response to large doses are usually minor, but could become dangerous at extremely high doses beyond what could be obtained with a normal diet.

According to research available in healthy humans, you may not need to specifically limit or increase your intake of methionine in your diet.

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