Selenium Benefits: What Trials Found, Food Sources and Toxicity

Selenium benefits are clear at the level of basic nutrition: your body needs this trace mineral to build proteins that support thyroid hormone metabolism, DNA synthesis and defense against oxidative damage. As a supplement for people who already eat enough, the picture is different. Large trials found no protection against cancer, the margin between enough and too much is narrow, and a mislabeled product once poisoned 201 people. Here is what selenium does, where you get it, what the research shows and what to watch for on labels.

Peptide Labs sells a product that lists selenium among its ingredients, but this article does not recommend it or any other product. It is general information, not medical advice.

What is selenium?

Selenium is an essential mineral found in soil, water and food. The NIH Office of Dietary Supplements (ODS) explains that it is built into 25 selenoproteins, such as the glutathione peroxidases, the thioredoxin reductases and selenoprotein P. These proteins play roles in thyroid hormone metabolism, DNA synthesis, reproduction and protection from oxidative damage and infection. The thyroid holds more selenium for its size than any other organ, and selenoproteins help convert the thyroid hormone T4 into its active form, T3.

Plants take up selenium from soil and turn it mostly into selenomethionine, which is the main form in food. The body absorbs dietary selenium readily and controls its levels mainly by excreting the extra in urine.

Selenium benefits: what the research shows

Because selenoproteins act as antioxidants and affect the immune system, researchers hoped extra selenium would lower the risk of cancer and heart disease. The large trials did not bear that out for most people. The table summarizes what ODS and the major studies report.

Cancer trials

The idea gained traction from the Nutritional Prevention of Cancer Trial, in which 1,312 people with a history of skin cancer took selenium yeast or placebo. Selenium did not reduce skin cancer, but a secondary analysis found 49% fewer prostate cancers among the men. The Selenium and Vitamin E Cancer Prevention Trial (SELECT) then tested the idea directly in 35,533 men in the United States, Canada and Puerto Rico. Published in JAMA in 2009, it found that selenium, alone or with vitamin E, did not lower prostate cancer risk, and it reported a statistically nonsignificant increase in type 2 diabetes in the selenium-only group. A later SELECT analysis found a higher risk of high-grade prostate cancer in men who started with high selenium status, and the researchers advised men over 55 to avoid selenium supplements above recommended intakes.

A 2018 Cochrane review of 10 randomized trials with 27,232 participants, 94% of them men, concluded that selenium supplements had little to no effect on cancer incidence or cancer deaths.

Heart disease and thyroid research

ODS describes the observational research on selenium and heart disease as conflicting and says trial evidence does not support using selenium supplements to lower cardiovascular risk, especially in people who get enough from their diet. For the thyroid, low selenium status has been linked to thyroid problems in women but not men. Trials in people with autoimmune thyroiditis found that selenium lowered some thyroid antibody levels, while others found no effect on thyroid function or quality of life. The American Thyroid Association recommends against selenium supplements for pregnant women with thyroid autoimmunity. If you have a thyroid condition, decisions about selenium belong with your endocrinologist, and iodine matters too; our sea moss guide covers how much iodine can vary in seaweed products.

Food sources of selenium

ODS lists seafood, meat, poultry and organ meats as rich sources, with breads, grains, meat, poultry, seafood and eggs supplying most of the selenium in US diets. Brazil nuts stand apart. A one-ounce serving of six to eight nuts provides close to ten times the Daily Value, and ODS notes that a single nut can contain more than the full Daily Value. The selenium in plant foods also varies with the soil they grew in, so published values are averages.

• Brazil nuts: by far the richest source; regular large servings could cause selenium toxicity, according to ODS.
• Fish and shellfish: yellowfin tuna, sardines and shrimp are all high.
• Meat, poultry and eggs: steady everyday sources because animals keep fairly constant selenium levels in their tissues.
• Grains and beans: pasta, bread, oatmeal, brown rice and baked beans add smaller amounts.

Selenium deficiency: who is at risk?

ODS calls selenium deficiency very rare in the United States and Canada, because food travels widely and even low-selenium regions eat produce grown elsewhere. On its own it rarely causes illness, but it has been tied to Keshan disease, a heart muscle disease first identified in 1935 in low-selenium areas of China, and to Kashin-Beck disease, a joint disorder in parts of China, Tibet, Siberia and North Korea. Groups at higher risk include:

• People whose diet comes mostly from crops grown in selenium-poor soil.
• People on long-term kidney dialysis, which removes some selenium from the blood.
• People living with HIV, who often have low selenium levels.
• People eating vegetarian or vegan diets, who may have lower intakes, and people who smoke, who tend to have lower selenium status.

Selenium supplement forms

A selenium supplement may contain selenomethionine, selenium-enriched yeast, sodium selenite or sodium selenate, and ODS says the body absorbs up to about 90% of each. Selenium appears in multivitamins, in combinations with vitamin E and as a stand-alone product, and ODS notes that single-ingredient products tend to contain more. About 17% to 19% of US adults take a supplement with selenium, which makes it easy to get it from more than one product at once.

Selenium toxicity and safety

Selenium toxicity, called selenosis, comes from chronically high intakes. ODS lists a garlic odor on the breath and a metallic taste as early signs, and hair loss and brittle or lost nails as the most common ones, along with rash, nausea, diarrhea, fatigue, irritability and nerve problems. The adult upper limit set by the Food and Nutrition Board is only about seven times the recommended intake, a narrow gap, and in 2023 the European Food Safety Authority set an even lower adult limit. Hair shedding has many causes, but excess selenium is one of them; our guide to hair growth vitamins explains why more is not better.

The clearest warning came in 2008. CDC investigators reported in Archives of Internal Medicine that a liquid dietary supplement contained 200 times the selenium stated on its label. They identified 201 cases in 10 states. Diarrhea, fatigue, hair loss, joint pain, nail changes and nausea were the most common symptoms, and about half still had fingernail discoloration or loss 90 days later. The authors wrote that pharmaceutical-style manufacturing standards might have prevented the outbreak, a good reason to favor products with independent testing; see our guide to third-party tested supplements.

• Cancer care: the chemotherapy drug cisplatin can lower selenium levels, and a Cochrane review found insufficient evidence that selenium eases chemotherapy side effects. Ask your oncology team before taking it.
• Pregnancy: stay within label directions and ask your obstetric clinician, especially with a thyroid condition.
• Stacking: add up selenium across multivitamins, mineral blends and Brazil nuts.

Reading labels that list selenium

In our catalog, selenium appears inside a multi-ingredient formula rather than as a stand-alone product. Sandhu’s Liver Cleanse & Detox capsules list selenium as L-selenomethionine at 91% of the Daily Value, zinc at 136% and vitamin E at 100%, plus milk thistle, dandelion root, andrographis, berberine, beet root, grape seed, turmeric and ginger. The label advises pregnant or nursing women, anyone under 18 and people with a medical condition to consult a physician first. Note that it supplies zinc without copper; our copper supplement guide explains why long-term high zinc matters, and our zinc guide covers zinc itself.

This label is not evidence of any liver or detox effect. Our articles on liver cleanse supplements and berberine side effects cover the other ingredients and their cautions.

Frequently Asked Questions

Questions about a label? Contact us or read our Buyer’s Guide. Follow the label and ask a healthcare provider before starting any supplement, especially if you take medicines or are pregnant. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

Last reviewed October 2026.

Related guides: Iodine Supplement Guide: Deficiency, Kelp and Thyroid Safety, Zinc Supplement Guide: Benefits, Side Effects and Immune Facts.