Selenium: Hama et al., 2026
A cross-sectional study of 201 adults in Iraq found mean selenium intake of 291 µg/day—roughly double the U.S. RDA—with cereals contributing 69% of total intake. Despite high dietary selenium and iodine exposure, thyroid function remained normal in most participants. This is an observational snapshot, not a controlled trial, so it shows association in one population but cannot establish cause or optimal intake.
Key takeaways
- Mean selenium intake was 291 µg/day, with cereals accounting for 69% of total dietary selenium.
- Biomarkers showed adequate-to-high selenium status, and most participants remained euthyroid (normal thyroid function).
- Dietary iodine intake was not independently associated with TSH after adjusting for demographic and nutritional factors.
- This is a cross-sectional study—it captures a moment in time but cannot show whether high selenium intake causes thyroid changes or whether the body adapts over time.
- The population studied relied heavily on staple grains; these findings may not generalize to Western diets with more varied selenium sources.
The study
Hama and Hawrami published this work in Nutrition Research and Practice in 2026 (PubMed). They recruited 201 healthy adults aged 18-60 in Halabja City, Kurdistan Region, Iraq. Researchers used a semi-quantitative food frequency questionnaire combined with a total diet study approach—composite food samples were analyzed by inductively coupled plasma-mass spectrometry for iodine and selenium content. Urinary iodine (expressed as iodine-to-creatinine ratio), serum selenium, and thyroid-stimulating hormone (TSH) were measured. Multivariable regression models examined associations between iodine intake and TSH, adjusting for demographic and nutritional covariates. This was a cross-sectional design: researchers took measurements at a single time point, capturing a snapshot rather than tracking changes over time.
How to read this study
What this paper does well: The total diet study methodology is a strength. Instead of relying solely on food tables—which may not reflect local soil selenium levels or food processing—the authors analyzed composite food samples from the actual diet. That gives a more accurate picture of real exposure. They also measured both dietary intake and biomarkers (urinary iodine, serum selenium), which cross-validates the estimates. The sample size of 201 is adequate for a nutritional surveillance study, and the multivariable adjustment for demographic and nutritional covariates is appropriate—they didn't just run a crude correlation.
What this paper is missing: This is a cross-sectional design, so it captures one moment. We don't know if these selenium intakes have been sustained for years or months, and we don't know thyroid function before or after. The study cannot establish whether high selenium intake over time leads to subclinical thyroid dysfunction—it only shows that at the moment of measurement, most participants were euthyroid. The abstract mentions "possible physiological adaptation" and notes that "subclinical effects cannot be excluded"—that's honest, but it also means we can't rule out long-term risk. There's no comparison group with lower selenium intake.
How I'd weight this paper: I treat this as useful surveillance data—it flags that staple-heavy diets can drive selenium intake well above RDA levels without apparent acute thyroid dysfunction. It does not tell us whether 291 µg/day is safe long-term, nor does it establish an optimal range. For someone considering selenium supplementation on top of a Western diet, this reinforces the need to know baseline intake—cereals alone can contribute substantial selenium. It's not a reason to fear selenium, but it is a reason to track total exposure rather than supplement blindly.
What they found
Mean selenium intake was 291.14 µg/day—roughly double the U.S. RDA of 55 µg/day for adults, and above the 200 µg/day that some practitioners use as an upper target for supplementation. Cereals and cereal-based foods contributed 68.82% of total selenium intake. Mean iodine intake was 206.99 µg/day, with cereals contributing 65.97% of iodine. Biomarker findings indicated adequate-to-high iodine and selenium status at the population level—meaning blood and urine levels were in or above normal reference ranges. Most participants remained within the euthyroid range (normal TSH). In multivariable regression models adjusted for demographic and nutritional covariates, dietary iodine intake was not independently associated with TSH. The authors note that this suggests possible physiological adaptation to high intake, but they also caution that subclinical effects cannot be excluded with a single cross-sectional snapshot.
What it means for the average man
Selenium is essential for thyroid hormone metabolism—the thyroid gland has the highest selenium concentration per gram of tissue in the body. But this study shows that food sources, particularly grains, can provide selenium well above RDA levels without supplementation. If you eat bread, pasta, or rice as staples and you're already taking a selenium supplement, you may be stacking intake into the 300-400 µg/day range or higher—above the tolerable upper limit of 400 µg/day set by the NIH. The takeaway: know your baseline. If your diet is grain-heavy or you live in a region with high-selenium soil (the U.S. Great Plains, for example), you may not need supplemental selenium. If you do supplement, track total intake and consider periodic selenium testing to avoid chronic excess, which can cause selenosis (hair loss, brittle nails, garlic breath, neurological symptoms).
The caveats
This study was conducted in a population with a staple-heavy diet in Iraq. Selenium content in grains varies widely by soil—wheat grown in high-selenium regions like the Great Plains contains far more selenium than wheat from low-selenium regions like parts of China or Scandinavia. These findings may not apply to Western diets with more varied protein sources (meat, fish, Brazil nuts) or to populations with different agricultural practices. The cross-sectional design means we don't know if participants have been consuming these selenium levels for years or months, and we don't know what their thyroid function was before or will be in the future. The lack of an independent association between iodine and TSH after adjustment suggests that other factors (age, sex, nutritional status) matter more in this sample, but it does not rule out subclinical effects that a longer follow-up or more sensitive thyroid markers might detect. No funding or conflict of interest statement was included in the abstract.
Frequently asked questions
Can a cross-sectional study prove that high selenium intake is safe?
No. A cross-sectional study captures one moment in time—it can show that at the moment of measurement, participants with high selenium intake had normal thyroid function, but it cannot show whether that intake is safe over years or whether subclinical changes might emerge later. To prove safety long-term, you'd need a prospective cohort study following people over time, or ideally a randomized controlled trial comparing different selenium intakes.
How much selenium should I take if I'm supplementing?
The U.S. RDA is 55 µg/day for adults. Many men's health protocols use 200 µg/day. The tolerable upper limit is 400 µg/day. If your diet includes meat, fish, eggs, or grains from high-selenium regions, you may already be getting 100-200 µg/day from food. Adding a 200 µg supplement could push you to 300-400 µg/day—near the upper limit. If you're not tracking food intake, consider a lower-dose supplement (50-100 µg/day) or test your selenium status before supplementing.
Do cereals in the U.S. contain as much selenium as cereals in Iraq?
It depends on the soil. Wheat grown in the Great Plains (North Dakota, South Dakota, Montana) is high in selenium; wheat from the Pacific Northwest or imported wheat is typically lower. Food labels in the U.S. do not usually list selenium content, so unless you're eating a standardized diet or have access to food composition databases, you won't know your exact intake. The takeaway: if you eat a lot of bread, pasta, or rice and you supplement selenium, you may be stacking more than you think.
What are the signs of selenium toxicity?
Chronic selenium toxicity (selenosis) symptoms include hair loss, brittle nails, garlic breath, fatigue, irritability, and neurological symptoms like numbness or tingling. Acute toxicity from very high doses can cause nausea, vomiting, and diarrhea. The tolerable upper limit is 400 µg/day for adults; intakes above that over months increase risk. If you're supplementing above 200 µg/day, consider periodic selenium testing.
Sources
- Hama D, Hawrami K. Dietary iodine and selenium exposure and thyroid function in adults: a total diet study with biomarker assessment. Nutr Res Pract. 2026. PubMed
- National Institutes of Health Office of Dietary Supplements. Selenium: Fact Sheet for Health Professionals. Updated 2024.
These statements have not been evaluated by the Food and Drug Administration. This article is for educational purposes only and is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.
