Daily selenium intake and the toxicity upper limit

Short answer: Most adults need 55 micrograms (mcg) of selenium per day, according to the NIH Office of Dietary Supplements (ODS). The tolerable upper intake level for adults is 400 mcg per day from food and supplements combined, and staying below that ceiling is strongly advised unless a clinician says otherwise.

What is selenium and why does the body need it?

Selenium is an essential trace mineral, meaning the body cannot make it and must obtain it from food or, when necessary, supplements. According to the NIH ODS, selenium serves as a structural component of 25 selenoproteins. Three of the most studied groups are the glutathione peroxidases, the thioredoxin reductases, and selenoprotein P. Together these proteins support antioxidant defense, helping protect cells from oxidative damage, and they play a critical role in thyroid hormone metabolism, including the conversion of the prohormone T4 into the active hormone T3. The NIH ODS also notes that selenium supports DNA synthesis and normal reproduction. The evidence behind these physiological roles is graded as strong.

How much selenium do adults need each day?

The Recommended Dietary Allowance (RDA) for selenium in adults is 55 mcg per day, as established in the Dietary Reference Intakes published by the National Academies and reported by the NIH ODS. Requirements increase modestly during pregnancy (60 mcg) and lactation (70 mcg).

For perspective, the NIH ODS reports that the average daily selenium intake from foods and beverages for people age 2 and older in the United States is about 108 mcg, which is roughly double the adult RDA. This means most Americans already meet their selenium needs through diet alone, without any supplementation.

Common food sources

Selenium content in plant foods varies by soil concentration, but reliable dietary sources include:

• Brazil nuts (one of the most concentrated food sources available)
• Seafood such as tuna, halibut, and shrimp
• Beef, pork, and chicken
• Eggs and dairy products
• Enriched breads and cereals

The 400 mcg upper limit: what it means and why it matters

The Tolerable Upper Intake Level (UL) for selenium in adults is 400 mcg per day from food and supplements combined, according to the NIH ODS. This figure was set on the basis of hair and nail brittleness and loss, the earliest observable signs of excess selenium intake. A UL is not a recommended target; it is the highest daily amount considered unlikely to cause harm for most healthy adults.

Chronic intake above the UL can lead to selenosis, a condition whose symptoms may include brittle or falling hair, brittle nails, a garlic-like odor on the breath, nausea, diarrhea, skin rashes, fatigue, and in severe cases neurological effects. Acute selenium poisoning from very high doses is rare but serious.

Because average dietary intake in the United States is already around 108 mcg per day, adding a high-dose selenium supplement can push total daily intake closer to or beyond the 400 mcg ceiling more quickly than many people expect. Anyone considering supplementation should calculate their combined intake from all sources.

What does the research say about selenium supplements?

It is important to note that selenium supplements are not intended to diagnose, treat, cure, or prevent any disease, and these statements have not been evaluated by the FDA. The following is a summary of what current evidence shows, graded honestly to reflect the strength of that evidence.

• Antioxidant defense and selenoprotein function: Evidence is strong that selenium is an essential component of 25 selenoproteins and supports antioxidant defense against oxidative damage.
• Thyroid hormone metabolism: Evidence is strong that selenium is required for normal thyroid hormone metabolism, including the conversion of T4 to active T3.
• DNA synthesis and reproduction: Evidence is strong that selenium supports DNA synthesis and normal reproduction.
• Hashimoto's thyroiditis: Evidence is limited that selenium supplementation improves outcomes in chronic autoimmune (Hashimoto's) thyroiditis. People with this condition should discuss selenium status with their endocrinologist before supplementing.
• Cancer risk: Evidence is insufficient to conclude that selenium supplementation reduces the overall risk of cancer or specific cancers.
• Cardiovascular disease: Evidence is insufficient to conclude that selenium supplementation reduces cardiovascular disease risk.
• Cognitive decline or Alzheimer's disease: Evidence is insufficient to conclude that selenium supplementation reduces cognitive decline or Alzheimer's disease risk.
• HIV outcomes: Evidence is insufficient to conclude that selenium supplementation benefits people living with HIV.
• Male fertility or sperm quality: Evidence is insufficient to conclude that selenium supplementation improves male fertility or sperm quality.

Grading sources: NIH ODS Selenium Health Professional Fact Sheet and the National Academies Dietary Reference Intakes.

Drug interactions and medication considerations

Selenium supplements may interact with medications, and certain medications can affect body selenium levels. Anyone taking medications regularly should discuss their selenium status with a health care provider or pharmacist before starting a supplement.

One specific interaction flagged by the NIH ODS involves cisplatin, a platinum-based chemotherapy agent. Cisplatin can reduce selenium levels in hair and serum, though whether these reductions are clinically significant is not yet known. A Cochrane Review found insufficient evidence that selenium supplementation alleviates chemotherapy side effects. Patients undergoing chemotherapy should not adjust selenium intake without guidance from their oncology team.

Practical takeaways

• The adult RDA for selenium is 55 mcg per day, per the NIH ODS.
• Average dietary intake in the United States is already about 108 mcg per day, so most people do not need a supplement.
• The tolerable upper intake level is 400 mcg per day from all sources combined. Exceeding this level over time raises the risk of selenosis.
• Strong evidence supports selenium's physiological roles in selenoprotein function, antioxidant defense, thyroid hormone metabolism, DNA synthesis, and reproduction.
• Evidence for supplementation benefits beyond correcting a deficiency is limited to insufficient for most health outcomes studied to date.
• Always consult a clinician or pharmacist for personal dosing guidance, and talk to a doctor before combining selenium supplements with any medication.

For the full data behind this guide, see the NIH ODS Selenium Health Professional Fact Sheet at ods.od.nih.gov and the National Academies Dietary Reference Intakes at ncbi.nlm.nih.gov/books/NBK225483.