Selenomethionine (SeMet) is a mineral important for normal thyroid gland function. Although some research has explored SeMet’s ability to lower the risk of heart disease, decrease age-related cognitive decline, and treat ulcerative colitis, more research is needed to confirm these benefits.
Selenomethionine is one of the main natural forms of selenium. This nutrient is found in many foods, including nuts, whole grains, meat, eggs, and dairy. It 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.
In the United States, the Food and Drug Administration (FDA) does not regulate supplements the way it regulates prescription drugs. That means some supplement products may not contain what the label says. When choosing a supplement, look for independently tested products and consult a healthcare provider, registered dietitian nutritionist (RD or RDN), or pharmacist.
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Uses of Selenomethionine
Supplement use should be individualized and vetted by a healthcare professional, such as a registered dietitian nutritionist (RD or RDN), pharmacist, or healthcare provider. No supplement is intended to treat, cure, or prevent disease.
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 thyroid metabolism and making thyroid hormone. Because of this, selenomethionine has been studied for its effects on thyroid diseases.
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 every day for four months.
At the end of the study, 31.3% of people who took SeMet had restored thyroid function compared to just 3.1% in the placebo group.
Another study looked at how children and teens with autoimmune thyroiditis would respond to SeMet. Seventy-one participants were given 200 mcg of selenomethionine or a placebo daily for six months.
At the end of the study, the SeMet group showed a greater reduction in antibodies (immune proteins) that attack thyroglobulin (a protein made by the thyroid), than the placebo group.
Another study looked at whether the supplements helped protect against thyroid autoimmunity during and after pregnancy. Forty-five participants who had thyroiditis in pregnancy were given either SeMet or a placebo. They were evaluated at around 10 weeks gestation, 36 weeks gestation, and about six months after delivery.
Study results found:
- No differences between the groups at 10 weeks gestation
- A notable rise in selenium blood levels in the SeMet group at 36 weeks gestation.
- A decrease in autoantibodies (an antibody type often made by the body after infection) in the selenomethionine group following the baby’s delivery
Based on these three studies, it appears SeMet may help with some thyroid conditions in certain people, including adults, pregnant women, children, and adolescents.
What Is Keshan Disease?
Keshan disease is the only known disorder linked directly to low levels of selenium. It damages the heart muscle, leading to heart failure and other heart problems. The disease is often found in places with low soil levels of selenium, meaning the mineral levels in food grown there may be lower too. Keshan disease was discovered in 1935 in China, where it continues to cause fatal heart problems in children and younger women today.
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 cancer-related adenomas who were treated for seven to 12 years. Participants were divided into four groups: selenium only, vitamin E only, selenium + vitamin E, and placebo only.
Participants 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 the 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 contributing factors.
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 one day be shown to prevent cognitive decline, but currently more studies are needed.
Ulcerative Colitis
Ulcerative colitis (UC) is an inflammatory bowel disease that mostly impacts the large colon. It can create discomfort and nutrient deficiencies. A study published in 2023 suggested 200 mcg of selenomethionine daily for 10 weeks decreased UC symptoms and interleukin-17 (a marker of inflammation in the body) and increased quality of life in people with mild to moderate UC. More research is 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, including:
- 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.
What Is the Difference Between Selenomethionine and Selenium?
Selenomethionine is a natural form of selenium. It is the same form of selenium found naturally in the foods you eat. Seafood, nuts, and meat are all good dietary sources of selenomethionine.
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?
Always speak with a healthcare provider before taking a supplement to ensure that the supplement and dosage are appropriate for your individual needs.
The National Academies of Sciences sets the suggested daily amount needed for vitamins and minerals, including selenium. The Recommended Dietary Allowance (RDA) for selenium is based on age and life stage, including people who are pregnant or breastfeeding. The RDA includes your intake from all sources, such as food, drinks, and supplements.
| Age/Stage | RDA for Selenium |
|---|---|
| 1 to 3 years | 20 mcg/day |
| 4 to 8 years | 30 mcg/day |
| 9 to 13 years | 40 mcg/day |
| 14+ years | 55 mcg/day |
| Pregnancy | 60 mcg/day |
| Breastfeeding | 70 mcg/day |
What Happens if I Take Too Much Selenomethionine?
Taking too much selenium for long periods 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 warfarin or heparin. It may have a similar effect if you take it with aspirin.
In a rat study, selenium increased or extended the time barbiturates lasted. These findings may not apply to humans. However, let your healthcare provider know if you take barbiturates like Butisol (butabarbital) or Luminal (phenobarbital) and selenium supplements.
Cisplatin, a chemotherapy medication, reduces selenium levels. While some evidence suggests selenium might help Cisplatin toxicity, other studies suggest more research is needed to prove this effect.
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 but aren’t limited to the following:
| Food | Serving size- ounces (oz) | Micrograms (mcg) | % Daily Value (DV) |
| Brazil Nuts | 1 oz. (6 to 8 nuts) | 544 mcg | 989% DV |
| Yellowfin tuna | 3 oz. cooked | 92 mcg | 167% DV |
| Oysters | 3 oz. | 64.5 mcg | 117% DV |
| Halibut | 3 oz. cooked | 47 mcg | 85% DV |
| Sardines | 3 oz. | 45 mcg | 82% DV |
| Ham | 3 oz. roasted | 42 mcg | 76% DV |
| Shrimp | 3 oz. cooked | 40 mcg | 73% DV |
| Beef, bottom round | 3 oz. roasted | 33mcg | 60% DV |
| Macaroni, enriched | 1 cup cooked | 31.7 mcg | 58% DV |
| Turkey | 3 oz. roasted | 31 mcg | 56% DV |
| Salmon | 3 oz. cooked | 30.2 mcg | 55% DV |
| Clams | 3 oz. | 26 mcg | 47.5% DV |
| Chicken, light meat | 3 oz. roasted | 22 mcg | 40% DV |
| Cottage Cheese, 1% fat | 1 cup | 20 mcg | 36% DV |
| Pita bread | 6.5 inches | 16.3 mcg | 30% DV |
| Yogurt, plain, lowfat | 1 cup | 8 mcg | 15% DV |
| Milk, 1% fat | 1 cup | 8 mcg | 15% DV |
| Raisin bread | 1 piece | 5 mcg | 9.1% DV |
| Bran flakes cereal | 3/4 cup | 2.67 mcg | 4.9% DV |
Selenomethionine Supplements
SeMet is a common form of selenium. SeMet supplements are most often available in the form of tablets or capsules.
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, cognitive decline, or ulcerative colitis.
While supplements may have health benefits, it’s important to ask your healthcare provider before starting a supplement regimen, especially if you have pre-existing health conditions or take other medications.
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