RDA for selenium is 55 micrograms/day for men and women.
The Upper Intake Level (UL) is 400 mcg/d. Intakes above the UL may cause adverse effects.
Selenium Content of Selected Foods, USDA National Nutrient Database.
Selenium content of plant foods depends on the selenium content of the soil in which the plants were grown. Selenium content of animal foods depends on the selenium content of the plants the animals were fed, and on the form provided. (See "Forms" below.)
The US Geological Survey provides a nice map of selenium concentrations across the US. If you're interested in a local area, you can click the map to visit their site where they display selenium concentration by county.
Some areas of the country contain such high levels of selenium they're toxic to wildlife. Selenium accumulates in soil through both natural and man-made processes. (The poisoning deaths of wild birds at a California Wildlife Refuge in the early 1980s were attributed to selenium in drainage from irrigation water.)
µg is the same as mcg (micrograms). "µ" is the Greek symbol for "micro". I like to spell things out because I'm fanatical.
Absorption rates vary, as they do for many nutrients. Usually there's a feedback mechanism, which is true in this case. That is, if you already get plenty of selenium in your diet (food or supplement) you will absorb less of any new incoming selenium.
Animal sources may be better absorbed that plant sources since plant phytates can interfere with absorption. Although seafood is an excellent source of selenium, mercury and other heavy metals in some fish species decrease absorption.
- Selenium is found in 2 forms: organic and inorganic.
- Organic selenium (selenomethionine, selenocysteine) is better absorbed than inorganic selenium (sodium selenate or sodium selenite).
- Selenium in foods is mostly organic. Supplements often contain inorganic selenium, although that's changing. Be sure to check the label.
- Inorganic forms may act as pro-oxidants, the opposite of antioxidants. They may deplete glutathione peroxidase (GP, an antioxidant enzyme) instead of replenishing it.
- Vitamin C is thought to enhance absorption of organic selenium but interfere with absorption of inorganic selenium.
Possible Risks Associated with Marginally High Intake
Weight gain and increases in body fat.
Insulin resistence and diabetes.