selenium
Meaning of selenium: The gap between too little and too much is narrow, and where food was grown largely decides which side people land on.
Definition of selenium
Selenium is a trace mineral the body needs in very small amounts and cannot make for itself. It is built into a group of proteins that includes an enzyme protecting cells from oxidative damage and the enzymes converting thyroid hormone into its active form. Food supplies it, chiefly seafood, meat, grains and Brazil nuts, in amounts that vary with the soil.
What does the body use it for?
Selenium does not act loose in the body. It is incorporated into about two dozen proteins, and everything it does happens through them. One family neutralizes peroxides formed as an unavoidable side effect of using oxygen, acting as part of the cell's antioxidant defense and working alongside vitamin E rather than in place of it. A second family sits in the thyroid and in tissue throughout the body, and its job is to trim one iodine atom from the main thyroid hormone to switch it into the form that actually works on cells. This is why a shortage of the mineral can disturb thyroid function even when the gland itself is healthy and iodine intake is adequate. Further selenium containing proteins take part in immune function, in maintaining sperm, and in folding new proteins correctly.
What happens at each extreme?
| Intake | What follows |
|---|---|
| Well below need | Muscle aching and weakness; in severely depleted regions, a heart muscle disease and a joint and bone disorder are described |
| Adequate | Selenium containing enzyme activity plateaus, and more intake adds nothing measurable |
| Well above need | Brittle hair and nails progressing to hair loss, a garlic odor on the breath, nausea, rash and irritability |
Soil is the reason intake varies so widely between places. Plants take up whatever selenium the ground holds, animals eat the plants, and both extremes have been documented in populations eating locally grown food. A single Brazil nut can carry a substantial share of a day's requirement, which makes them an efficient source and an easy one to overshoot.
Does it prevent cancer?
The idea came from observation that regions with selenium poor soil reported more of certain cancers, and from early work suggesting supplements might lower risk. Large randomized trials designed to test the question directly did not confirm a protective effect, and one raised the separate concern that supplements might slightly increase the risk of diabetes. What remains is unremarkable: the shortfall states are real and correctable, and beyond the point where the selenium dependent enzymes are saturated there is no evidence that additional intake helps, while the margin before toxicity is smaller than for most nutrients.
Also known as
Used in a sentence
The patient's selenium level was checked as part of the nutritional evaluation.
Good to know
Selenium toxicity from excessive intake, though uncommon, is a real possibility alongside deficiency, since both extremes of intake can cause health problems.
Word origin
Where “selenium” comes from
Named after Selene, the Greek goddess of the moon, paired with tellurium, named for the earth, since the two elements were discovered in close association.