Selenium

Daily value: 70 mcg. Average consumption in US diet: 83-129 mcg

Selenium is another very popular antioxidant widely marketed by the nutritional supplement industry. However, current research indicates that dietary levels are adequate to maximize plasma glutathione peroxidase (9), while the dosage at which long-term toxicity occurs has yet to be clearly established (10-11). Like beta-carotene, the requirement for selenium is spared by the much safer antioxidant vitamins C and E (12). For these reasons, no selenium is added to My Personal Health supplements.

added 2022: "Selenium mediates exercise-induced adult neurogenesis and reverses learning deficits induced by hippocampal injury and aging"
Highlights
Selenium mediates the exercise-induced increase in adult hippocampal
neurogenesis
Selenium increases hippocampal precursor proliferation and adult
neurogenesis
Selenium reverses cognitive decline in aging and in hippocampal
injury

9. Levander, O. A., and Whanger, P. D. "Deliberations and evaluations of the approaches, endpoints, and paradigms for selenium and iodine dietary recommendations", J. Nutr. 126: 2427S-34S (1996)

Table 1 shows the mean dietary intakes of U.S. men and women (90 and 74 mcg/day respectively) are in excess of the RDAs (70mcg/day). "[50 mcg/day] was selected [by the Food and Nutrition Board] as the lower limit of the Estimated Safe and Adequate Daily Dietary Intake of this mineral. The upper limit...was set at 200 mcg/d" "It was found that [41 mcg/day] was sufficient to maximize plasma glutathione peroxidase activity [the important antioxidant function of selenium]"

10. Williams, S. R., Nutrition and Diet Therapy, Mosby, St. Louis, 1993, pg. 712

"Selenium is also toxic in high chronic doses, but the level at which this toxicity occurs is uncertain."

11. Hathcock, J. N. "Safety limits for nutrients", J. Nutr. 126: 2386S-89S(1996)

"The RfD method uses a no observed adverse effect level or a lowest observed effect level [to determine the maximum safe dosage of nutrients established by the Environmental Protection Agency and utilized by the Food and Nutrition Board to set safe limits for nutrients.]" Table 1 shows the RfD [safe level of ingestion] of selenium is at most 350 mcg per day. "If the composite SF [safety factor] is adjusted to 10, the resulting RfD is 100 mcg/day, a limit that may be deemed appropriate by many nutritionists."

12. Watson, R. R. and Leonard, T. K., "Selenium and Vitamins A, E, and C: Nutrients with Cancer Prevention Properties", J. Am. Dietetic Assoc. 86:505-510 (1986)

"It has been estimated that 35% of all cancer incidence is related to diet." "Vitamin E spares selenium."[therefore, if you take enough vitamin E, you don't need selenium] "toxicity occurs at low dietary levels in animals, indicating a small range of safe intake." "Selenium compounds possess a high degree of toxicity, and an excess may have severe consequences...Only [with more human studies] should general use of selenium supplements be considered."