What Is Selenomethionine?

Selenomethionine (SeMet) is one of the main natural forms of selenium. This nutrient is found in many foods, including nuts, whole grains, meat, eggs, and dairy.

As a form of selenium, SeMet is important for normal thyroid gland function. It plays a role in reproduction, DNA production, and protecting the body from infection. It also has been studied for heart and other health benefits.

Selenomethionine combines with proteins in the body to form antioxidants called selenoproteins. These compounds help protect against free radicals, the unstable molecules that can cause damage to cells in your body.

This article looks at the possible benefits of selenomethionine and some of the research on selenium's health impacts. It will also help you know what to look for if you decide to try a SeMet product.

Uses of Selenomethionine

Much of the research on SeMet has centered around thyroid disease, cancer, and heart disease prevention. It also has been studied for its role in mental health and how it may prevent cognitive decline.

Thyroid Disease

Selenium levels in the human body are highest in the thyroid gland. The mineral plays a key role in making thyroid hormone and in its metabolism. Because of this, selenomethionine has been studied for its effects on diseases of the thyroid.

One study looked at 192 people who had hypothyroidism, or mildly low and symptom-free thyroid levels because of Hashimoto’s disease. For the study, participants were given either 83 mcg of oral SeMet or a placebo (sham treatment) every day for four months.

At the end of the study, 31.3% of people who took SeMet had restored thyroid function. That number was just 3.1% in the placebo group.

Another study looked at how children and teens with autoimmune thyroiditis would respond to SeMet. Seventy-one people were given 200 mcg of selenomethionine or a placebo every day for six months.

At the end of the study, the SeMet group showed a higher reduction in immune proteins (antibodies) that attack thyroglobulin (Tg), a protein made by the thyroid, than the placebo group.

Another selenomethionine study looked at whether the supplements helped protect against thyroid autoimmunity during and after pregnancy. In it, 45 women who had thyroiditis in pregnancy were given either SeMet or a placebo. They were evaluated at around 10 weeks gestation, at 36 weeks gestation, and about six months after delivery. 

There were no real differences between the groups at the first check-in. However, the researchers saw a notable rise in selenium blood levels in the SeMet group at the second evaluation. There was also a decrease in autoantibodies, an antibody type often made by the body after an infection, following the baby's delivery in the selenomethionine group.

Based on these three studies, it seems that SeMet may help with some thyroid conditions in certain groups of people, including adults, pregnant women, children, and adolescents.

Cancer

Selenomethionine does not seem to help with cancer prevention.

The Adenomatous Colorectal Polyp (ACP) study was designed to see whether SeMet or vitamin E lowered the rate of colon and rectal cancers, as well as adenomas—tumors, or abnormal growths, that can lead to these cancers.

The study, based on a subset of people in a wider research program, included 2,286 men with one or more of these cancer-related adenomas who were treated for seven to 12 years.

The colorectal study subset was divided into four groups: those who got only selenium, those who got only vitamin E, those who got both, and those who got neither and had only placebos.

In the men who got selenomethionine, 34.2% were diagnosed with adenomas at least one year later, compared to 35.7% in the group given a placebo. This was not a significant difference and led to a conclusion that taking SeMet had no effect on colon cancer prevention for these people.

A 2018 review of previous studies also found that taking selenium, including in the form of selenomethionine, did not have an impact on any type of cancer risk.

Heart Disease

A 2015 review found that when selenium intake was higher, heart disease risk was lower. Yet the researchers were unable to tell if it was selenium alone that actually prevented heart disease or if there were other factors at work.

They also reported that SeMet is one of the best organic ways to boost selenium levels in the body. However, that form of selenium is not the most efficient in terms of how well it is processed and used by the body. The data did not support the use of selenomethionine for preventing heart disease, especially in healthy people who get enough selenium from their diet.

More clinical trials are needed to better understand the SeMet-heart risk connection.

Cognitive Function

Selenium levels in the body often decline with age. Because of this, low selenium may be linked to age-related cognitive declines, possibly because of the subsequent reduction in its impact as an antioxidant. However, study results are mixed.

Alzheimer's disease is often diagnosed in older people and leads to progressive cognitive decline. Yet there is no clear evidence to support a role for selenium—in SeMet form or otherwise—for treating it.

Selenium may have a proven cognitive decline prevention benefit one day, but more studies are needed.

What Are The Side Effects of Selenomethionine?

Generally, side effects will only occur when there is too much selenium in the body. This is rare when taking supplements as directed, but it can happen if the doses you take are too high.

Some people may experience mild side effects when taking selenomethionine at lower doses. These may include:

• Stomach discomfort
• Headache
• Rash
  

Always talk with your healthcare provider before taking any dietary supplement, including SeMet. Be sure to follow the directions on the product label.

Precautions

Selenomethionine can be toxic at high doses, especially when taken long-term. Taking lower doses for long periods of time may increase your risk of developing diabetes.

Dosage: How Much Selenomethionine Should I Take?

The National Academies of Sciences sets the dietary reference intakes (DRIs) for vitamins and minerals, including selenium. The DRIs for selenium are based on age, as well as life stage, including people who are pregnant or breastfeeding. The DRIs include your intake from all sources, such as food, drinks, and supplements.

What Happens If I Take Too Much Selenomethionine?

Taking too much selenium for long periods of time can cause selenosis. This is the medical term for selenium poisoning. People with selenosis may experience symptoms such as:

• Garlic breath odor
• Metallic taste in the mouth
• Nausea
• Diarrhea
• Fatigue
• Irritability
• Hair and nail loss
• Skin lesions
  

Selenosis can also cause nerve damage. People with severe selenosis may experience:

• Tremors
• Respiratory distress
• Kidney failure
• Heart failure
• Death
  

Interactions

Selenium may increase the risk of bleeding or bruising if you take it with blood thinners such as Coumadin (warfarin) or heparin. It may have a similar effect if you take it with aspirin.

Selenium may increase or extend the length of time that barbiturates like Butisol (butabarbital) or Luminal (phenobarbital) affect you.

Talk to your healthcare provider if you are taking Zocor (simvastatin) with niacin to help raise your HDL ("good" cholesterol) levels. Selenium and other antioxidants may make this combination less effective. 

Selenium may also make certain chemotherapy drugs and immunosuppressive medication less effective.

How to Store Selenomethionine

Selenium should be kept in its original container at room temperature. Do not expose it to heat, moisture, sunlight, or freezing temperatures. 

Similar Supplements

Several other supplements have the same functions as selenomethionine and are possible alternatives:

• Myo-inositol: This compound helps regulate hormones including insulin and thyroid-stimulating hormone. It is sometimes taken in combination with selenium to help treat hypothyroidism. 
• Iodine: Your body needs iodine in order to produce thyroid hormones. However, it is possible to get too much iodine in your diet, which can also cause problems with your thyroid. Since iodine deficiency is uncommon in the United States, most healthcare providers don't recommend iodine supplementation.
• Omega-3 fatty acids: These supplements may help reduce the risk of colon cancer, though so far this evidence only comes from animal studies. Because a diet rich in omega-3 fatty acids can help reduce inflammation, these supplements may also be used to help treat thyroid conditions.

Sources of Selenomethionine and What To Look For

Most people can get enough selenium from the foods they eat. You may have trouble absorbing selenium, however, if you are on dialysis, if you have a GI condition such as Crohn's disease, or if you have HIV. In these cases, a selenium supplement may be helpful.

Food Sources of Selenomethionine

Foods high in selenium include:

• Brazil nuts
• Yellowfin tuna
• Oysters and clams
• Sardines
• Shrimp
• Salmon 
• Halibut
• Ham
• Enriched breads and cereal
• Pork, beef, Turkey, and chicken
• Milk, yogurt, and cottage cheese

Selenomethionine Supplements

SeMet is a common form of selenium. SeMet supplements are most often available in the form of tablets or capsules.

Remember, though, that selenomethionine and other supplements are not regulated by the FDA or any other government agency. The quality of the products and how effective they are varies greatly because of this.

Simply reading the label may not provide enough information to make a well-informed buying decision about SeMet. It’s best to ask your healthcare provider for help in choosing the right supplement for you. They can also advise you on the best dosage for you.

As a general rule, look for products certified by a third party, such as U.S. Pharmacopeia, NSF International, or Consumer Lab. This ensures quality and that the ingredients and amounts listed on the label are accurate.

Summary

Selenomethionine is present in many foods you eat, but it's also sold as a nutritional supplement. While SeMet may have health benefits, notably for thyroid function, researchers still have not shown clear benefits for cancer, heart disease, or cognitive decline.

The supplements are generally safe to take. Still, be sure to ask your healthcare provider about your own risks and benefits before you decide to add SeMet to your diet and nutrition plans.