High intake of selenium from the diet or supplements has been associated with increased risk of type 2 diabetes. Get the details in the Concerns and Cautions section of our Selenium Supplements Review.
Taking too much selenium was found to increase the risk of death in a newly published study. How much is too much? Get the details in the Concerns and Cautions section of the Selenium Supplements Review.
A long-term study among men in the U.S. found that giving high doses of vitamin E and/or selenium did not significantly affect their risk of developing dementia. The study was part of a larger study which found that these supplements increased the risk of prostate cancer, type 2 diabetes, hair loss, and dermatitis. For details, see the "Alzheimer's disease/dementia" section of the Selenium Supplements Review >>
Some Spirulina supplements and many A. flos-aquae supplements were found to be contaminated with microcystin toxin — a liver toxin, according to a recently published study. For details, see the "Concerns and Cautions" section of the Greens and Whole Food Supplements Review >>
A recent study found that women with the highest intakes of selenium from supplements had a much lower risk of ovarian cancer than those not getting selenium from supplements. Get the details in the "What It Does" section of the Selenium Supplements Review (which includes our tests of products) >>
A new study suggests that men who have prostate cancer should not take supplements providing high-dose selenium. Earlier research has similarly advised against high-dose selenium for people with already adequate selenium intake. For details and more about selenium, see the Selenium Supplements Review >>
A study in older men in the U.S. found that taking large daily doses of vitamin E and/or selenium did not significantly reduce the risk of developing cataracts. But could the dose have been too high to be helpful? Other research has shown that "less is more" when it comes to antioxidant supplementation for cataracts. For more information, see the updates to the Vitamin E Supplements Review and the Selenium Supplements Review >>
A new, long-term study of Americans found the risk of dying over the course of the study (about 14 years) was lowest when antioxidant levels in the blood were above the lowest levels (the bottom 20% of the population). However, for people in the top 20% of blood levels for vitamins A and E, the risk of death increased compared to people with moderate levels. For selenium and beta-carotene, there was no significant difference in the death rate between moderate and high levels, although for vitamin C some additional benefit was seen at high, but not the highest, levels.
The results suggest that antioxidant supplements may be useful for those who are nutritionally deficient, but, as noted by the researchers, "beyond a certain threshold, higher levels do not lead to additional benefit, and may potentially be toxic." More details (including specific serum levels) are found in the linked updates to the following reviews, which include our test results and quality ratings of products: