Submitted to: Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/21/2003
Publication Date: 11/6/2003
Citation: Hawkes, W.C., Keim, N.L. 2003. Dietary selenium intake modulates thyroid hormone and energy metabolism in men. Journal of Nutrition. 133:3443-3448, 2003.
Interpretive Summary: Selenium is a mineral that has been officially recognized as essential to human health only since the 1980's. Selenium can also be toxic when consumed at only 14 times the recommended intake. Recent research suggests that selenium supplementation to about 5 times the RDA can reduce the risk of lung, colon and prostate cancers by 50% or more. No studies have been done to assess the possible adverse effects of these high selenium intakes in healthy humans. We confined 11 healthy men for 120 days to strictly control their eating and activities while we changed the amount of naturally occurring selenium they ate without changing anything else about their diets. We found that high selenium (about 5 timesthe RDA) decreased the amount of active thyroid hormone in their blood and led to a steady slow weight gain. On the other hand, we observed that low selenium increased their active thyroid hormone levels and led to a steady, slow weight loss. These responses were opposite to what's been found in animals, showing the need to do such studies in humans. The weight increases caused by high selenium could be an undesired side effect of taking selenium supplements to prevent cancer.
Technical Abstract: Eleven healthy men were fed a controlled diet of foods naturally high or low in selenium for 120 days. Selenium intake was 47 :g/d for 21 days, then either 13 or 297 :g/d for 99 days, leading to significantly different blood selemium and glutathione peroxidase concentrations. 3, 3N, 5-Triodothyronine (T3) decreased in the high seleium group, increased in the low selenium group, and was significantly different between groups from day 45 onward. The changes in T3 induced by dietary selenium were opposite to those previously observed in rats, but were consistent with the other metabolic changes in this study. By day 64 the high selenium group started to gain weight, while the low selenium group began to lose weight, and the changes were significantly different between groups from day 92 onward. Decrease in serum T3 followed by compensatory increases in serum thyrotropin suggested that a relatively hypothyroid state was induced by the high selenium diet, while increases of serum T3 and serum triglycerides followed by losses of body fat suggested that a relatively hyperthyroid state was induced by the low selenium diet. More research is needed to assess the health implications and clinical significance of these apparent effects of dietary selenium.