Research published in The Lancet Global Health estimates around 37% of people don't get enough selenium to meet the recommended intake, which is higher than most people would expect for a mineral this obscure.
Your body uses selenium to support thyroid function, protect cells as part of its antioxidant system, and help your immune response. When intake stays low over time, that's associated with a weakened immune response and effects on thyroid function.
Adults need around 55 mcg a day. Brazil nuts are by far the richest source, to the point that a couple can cover a day's worth, alongside fish, eggs, and whole grains.
Selenium has one of the narrower gaps between what you need and what's too much, so the ceiling matters: it's best not to exceed 400 mcg a day. Consistently high intake is associated with hair and nail brittleness and digestive upset, and because Brazil nuts are so concentrated, eating them in large quantities daily is the most common way people approach that limit.