Mineral
Foods high in selenium and why excess matters here
Selenium builds antioxidant enzymes. The gap between what you need and too much is unusually narrow for a mineral.
Highest sources
Ranked among the ingredients covered here. Reference amounts vary slightly between entries, so read this as a rough comparison rather than an exact order.
What it does
It forms part of glutathione peroxidase, protecting cells from oxidative damage. It also sits in the enzymes that convert thyroid hormone to its active form, and is known to offset some of the toxicity of heavy metals such as mercury.
Recommended intake
The usual adult reference is 55 µg a day. The upper limit is 400 µg — only about sevenfold above the requirement, a far narrower window than most minerals.
What affects absorption
Selenium from food is generally well absorbed. Content depends heavily on the selenium in the soil where the crop grew, so the same food can differ several-fold between growing regions.
What to watch out for
A single Brazil nut can exceed the daily requirement, so eating several every day makes it easy to pass the upper limit. Accumulated excess causes hair loss, brittle nails, garlic-smelling breath and neurological symptoms. Approach supplements carefully.
Sources
The databases and health authorities this article's nutrition figures and health statements draw on.