Selenium

What Is Selenium?

Selenium is unusual among minerals: the body does not just bind it to proteins, it builds it into them. It becomes part of selenocysteine, an amino acid that cells assemble into about 25 selenoproteins. Without enough selenium, those proteins are made more slowly or not at all.

What Selenium Does in the Body

Three groups of selenoproteins do most of the known work. Glutathione peroxidases break down hydrogen peroxide and other by-products before they damage cell membranes and DNA. Thioredoxin reductases keep other proteins in their working state. Iodothyronine deiodinases remove one iodine atom from thyroxine (T4) to make triiodothyronine (T3), the active thyroid hormone.

That last job ties selenium to iodine. The thyroid holds more selenium per gram than any other organ, and it needs both minerals: iodine to build the hormone, selenium to switch it on. Selenium also works next to vitamin E, since both protect the fats in cell membranes from oxidation.

Why Selenium in Food Depends on the Soil

Plants take up selenium from the ground without needing it themselves, so the same crop can hold many times more in one region than another. Animals that graze those crops pass the difference on. This is why figures for grains, nuts and even meat are averages, and why food tables for selenium are less precise than for most minerals.

The clearest proof came from China. In low-selenium areas, children developed Keshan disease, a weakness of the heart muscle, and a joint disorder called Kashin-Beck disease was common. Keshan disease became much rarer once people in those areas were given selenium. In the US, food travels from many regions, which evens out local soil.

A Day of Food With Selenium, Without Brazil Nuts

Most people with a mixed diet get enough selenium. This is one day built only from library foods in usual portions, with no fish and no supplement.

Chicken breast alone gives 40 µg of the total.

Brazil Nuts and Too Much Selenium

Brazil nuts are in a class of their own. The NIH table lists about 544 µg in 1 ounce, 6 to 8 nuts, which is ten times the daily need and above the US upper limit of 400 µg. Single nuts vary widely with where the tree grew, so eating a handful every day is the easiest way to get too much from food.

Who Is Most Likely to Be Low in Selenium

Shortfalls are rare in the US. The groups at higher risk are people on kidney dialysis, who lose selenium in the process, people living with HIV, and people whose food comes mostly from low-selenium soils. Vegans rely on grains and nuts, whose content depends on where they grew.

Common Selenium Mistakes

More antioxidant is not better. Once the selenoproteins are full, extra selenium adds no enzyme activity. In the SELECT trial, 200 µg a day for years did not prevent prostate cancer.

Counting only the supplement. The upper limit covers food and supplements together. A 200 µg capsule on top of an ordinary day and a few Brazil nuts can pass 400 µg.