Evidence explainerSelenium (selenomethionine vs selenite)
How Many Brazil Nuts a Day Is Too Many? The Selenium Math in One Handful
A single Brazil nut can carry more selenium than an adult needs in a day. Here is the math, the early warning signs of too much, and why a selenium pill on top may be one source too many.
Key takeaways
- One Brazil nut contains about 68 to 91 mcg of selenium, more than the adult RDA of 55 mcg [1].
- A 1-ounce serving (6 to 8 nuts) averages 544 mcg, above the adult upper limit of 400 mcg/day [1].
- Selenium intakes in North America are already well above the RDA, even in low-selenium regions [1].
- Early signs of too much are garlic breath and a metallic taste; chronic excess causes hair loss and brittle nails [1].
- In a long-term trial, 200 mcg/day of selenium was linked to more new type 2 diabetes than placebo (HR 1.55) [2].
Safety at a glance
From our Selenium (selenomethionine vs selenite) monograph. Published interactions and cautions only, each with its source.
Interactions
- Cisplatin. Can lower selenium levels in hair and serum; whether this matters clinically is unknown. A Cochrane review found insufficient evidence that selenium eases chemotherapy side effects.[1]
Who should be careful
- High intakes. Chronic excess causes selenosis: hair loss and brittle nails, garlic breath, a metallic taste, skin rash, nausea, diarrhea, fatigue and nerve problems. Brazil nuts (68–91 mcg each) can supply toxic amounts if eaten regularly, and misformulated products have caused severe poisoning.[1]
- Blood sugar and prostate findings. One long-term trial found more type 2 diabetes with 200 mcg/day, and ODS reports that SELECT investigators advised men over 55 against selenium above recommended intakes after finding more high-grade prostate cancer in men with high baseline selenium.[4][1]
- Thyroid autoimmunity in pregnancy. ODS reports that the American Thyroid Association recommends against selenium supplements for pregnant women with thyroid autoimmunity.[1]
- Before you start. Talk to your pharmacist or clinician before adding this supplement, especially if you take prescription medicines, are pregnant or breastfeeding, or have a chronic health condition.[1]
The evidence, graded
Pulled from our monograph evidence tables. Grades follow our evidence-grading policy.
- Grade CImprovedDeveloping high blood sugar over 7.7 yearsType 2 diabetes developed in 58 people on selenium vs 39 on placebo (HR 1.55; 95% CI 1.03 to 2.33), with the highest risk in those with the highest baseline selenium (HR 2.70).
- No effectNo effectSELECT trial primary end point (men 50+)Selenium did not lower prostate cancer risk (HR 1.04; 99% CI 0.87-1.24) or any other prespecified cancer end point. Type 2 diabetes was non-significantly more common with selenium (RR 1.07; 99% CI 0.94-1.22).
The math in one handful
Brazil nuts are the richest food source of selenium by a wide margin. ODS puts a single nut at 68 to 91 mcg [1]. The adult RDA is 55 mcg a day, so one nut can cover it [1].
A 1-ounce serving, about 6 to 8 nuts, averages 544 mcg in USDA data, nearly ten times the daily value and above the 400 mcg/day adult upper limit [1]. Values from other analyses vary widely, because selenium in plant foods depends on the soil they grew in [1].
In Europe the ceiling is lower still: in 2023 the European Food Safety Authority set an upper limit of 255 mcg/day for all adults [1].
Most people already get enough
Selenium intakes in North America, even in low-selenium regions, are well above the RDA, likely because most people eat food grown across a wide area [1]. Seafood, meat, poultry, organ meats, grains and dairy all contribute [1].
Common sources from the ODS food table:
What too much looks like
Chronic excess is called selenosis. The earliest signs are a garlic odor on the breath and a metallic taste in the mouth; the most common clinical signs are hair loss and brittle or lost nails [1]. Skin rash, nausea, diarrhea, fatigue, irritability and nerve problems can follow [1]. ODS warns that Brazil nuts could cause selenium toxicity if eaten regularly [1].
Supplements carry their own risk. In 2008, 201 people had severe reactions to a liquid supplement that contained 200 times the selenium on its label [1].
Why a pill on top may be one source too many
Selenium-only supplements typically contain 100 to 400 mcg [1]. The long-term trials of 200 mcg/day found no protective benefit and raised a blood-sugar question. In the Nutritional Prevention of Cancer trial, 58 people on selenium developed type 2 diabetes compared with 39 on placebo over an average of 7.7 years (HR 1.55), and the risk was highest in those with the highest starting selenium levels [2].
In SELECT, 35,533 men took 200 mcg/day of selenomethionine or placebo; there was no effect on prostate or other cancers, and type 2 diabetes was slightly but not significantly more common with selenium [3]. The full picture is on the selenium monograph, and the thyroid question is covered in selenium and thyroid antibodies.
Quick answers
Is one Brazil nut a day too much?
One nut supplies about 68 to 91 mcg, above the 55 mcg RDA but well under the 400 mcg adult upper limit [1]. The concern ODS raises is regular intake of larger amounts, and the selenium content of nuts varies widely [1].
Can I eat Brazil nuts if I take a multivitamin with selenium?
Add the two together. Products that combine selenium with vitamin E or other ingredients generally contain 50 to 200 mcg, and the adult upper limit counts food and supplements combined at 400 mcg/day [1].
What are the first signs of too much selenium?
A garlic odor on the breath and a metallic taste in the mouth are early indicators; hair loss and brittle nails are the most common signs of chronic excess [1].
Keep reading
Full monographs
Related articles
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Browse the full Supplement & Nutrient Encyclopedia or all supplement articles.
References
- [1]Selenium: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements, 2025.
- [2]Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Ann Intern Med, 2007.
- [3]Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA, 2009.