Long-term Selenium Intake and Cognitive Decline
In a recent epidemiological study of rural Chinese seniors, researchers found that lower selenium levels in nail samples were associated with lower cognitive scores.
Specifically, researchers observed that people in the group with the 20% lowest selenium levels scored 10 years older on a standardized cognitive test than people with the 20% highest selenium levels.
Selenium is an essential trace mineral — meaning you only need very small amounts of it (micrograms - mcgs, or millionths of a gram) and have to get it from your diet or through supplements. Selenium may also be toxic in higher amounts (e.g., more than 400 mcg/day), so this is a case where a lot more is not better.
The mineral plays very important roles in the body, such as inclusion in key enzymes:
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Glutathione peroxidases, which help one of the body’s primary antioxidants (glutathione) to neutralize free radical molecules.
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Thyroid hormone deiodinases, which help convert inactive thyroid hormone T4 into the biologically active form T3, among other functions.
Selenium may help to reduce the risk of certain cancers (e.g., prostate, colon), improve immune function, and lower the risk of cardiovascular disease (via reduced oxidative damage to the arterial walls). Also, selenium appears to sequester, although not chelate, mercury.
Brazil nuts are the food highest in selenium (potentially > 100 mcg per nut, depending on the soil). Organ meats and seafood are also high in selenium. Selenium levels in grains and plant foods can vary greatly depending on the soil in which they are grown. Here’s an (old) map that shows soil selenium levels in North America.
As the authors of the study above note, it’s important to realize that the effects of selenium deficiency likely occur over a long period. Selenium supplementation is not a quick fix. Including a reasonable amount of selenium (e.g, 200 mcg/day) in a well-utilized form (e.g., selenomethionine) as part of a preventive program started well before cognitive decline symptoms appear makes the most sense.
