What it is
Selenium is an essential mineral.
What it does
Selenium is incorporated into proteins (creatively called selenoproteins) that are involved in thyroid hormone metabolism, reproduction, protection of oxidative stress, and DNA metabolism.
The recommended intake for selenium is 70mcg per day for men and women who aren’t lactating. Lactation can increase intake requirements for selenium.
The safe upper limit for selenium intake is 400mcg per day.
Likelihood of tracking completeness: Very low
Selenium is not a nutrient that food manufacturers are required to disclose on nutrition labels. The vast majority of food and beverage manufacturers do not voluntarily list selenium content on nutrition labels, so most branded products in the MacroFactor database lack information on selenium. So, if you’d like to accurately track your selenium intake, you’ll need to make a point of mostly tracking “common foods,” which come from research-grade databases that have full nutrient reporting.
For more on when you can track using branded foods versus common foods when you’re trying to accurately monitor your intake of particular nutrients, you should check out this article.
Likelihood of insufficient intake: Varies regionally
Selenium primarily enters the food cycle when plants absorb selenium from the soil. So, selenium levels in the food supply vary based on regional differences in soil selenium levels. Intake of selenium is typically adequate in selenium-rich regions, and inadequate in selenium-poor regions, though there’s not a perfect correlation due to food imports and exports, and because selenium levels in farmland may not perfectly reflect typical regional selenium levels due to soil depletion or the use of selenium-supplemented fertilizers.
Overall, insufficient selenium intake seems to be pretty uncommon in most of the US, and relatively common in most of Europe, but regional differences also exist within both the US and Europe. Similar dynamics have also been observed in China, suggesting that this is a pretty generalizable phenomenon.
For more on nutrients with a greater likelihood of insufficient or excessive intake, you should check out this article.
Effects of of deficiency or excess
Selenium deficiency is linked to Keshan disease (a particular type of cardiomyopathy observed in selenium-deficient regions), Kashin-Beck disease (a particular type of osteoarthritis observed in selenium-deficient regions), and a general increase in illness and infection.
The dose-response relationship between selenium intake, selenium concentrations, and various diseases varies by disease type. For instance, higher selenium intakes and concentrations are associated with increased rates of type II diabetes, but decreased rates of certain cancers (though the precise relationship may differ between different types of cancers). In aggregate, most researchers think the relationship between selenium intake and health follows a U-shaped curve, though the relationship may be more linear for specific conditions, with a wide array of diseases and conditions associated with both high and low selenium intakes and concentrations.
Foods rich in selenium include shellfish, mushrooms, fish (particularly predatory fish, since selenium accumulates up the food chain), eggs, pork, wheat products, beef, and soy-based products. Brazil nuts are also exceptionally rich in selenium, but most other nuts and seeds are not.
If you’d like to learn more about micronutrients generally, there’s a five-part series on the MacroFactor website you might enjoy.