|Three things make this study special #1 it's human study, #2 it was conducted in a metabolic ward and #3 it didn't use potentially toxic Se supplements|
In view of the fact that the mechanism that triggers this side effect for selenium is a bit more straight forward than it is for iodine, I would like to start this new series with the initially mentioned essential trace element.
Now, from previous SuppVersity posts on things like zinc or chromium (both may worsen your insulin sensitivity in high doses), you will be well aware that despite being "essential", i.e. it being absolutely necessary to have them in your diet, consuming too much of these "essentials" can and usually will backfire.
In the case of selenium the consequences became most obvious in a 2003 study from the U.S. Department of Agriculture (USDA). In said study (Hawkes. 2003b), which was conducted by researchers from the Western Human Nutrition Research Center and Department of Nutrition in Davis, CA, the scientists didn't even have to use copious amounts of supplemental selenium to slow down their subjects' thyroid and have them gain weight.
|Table 1: Sample meal plan for one week. I've taken this from a previous paper by the same researchers (Hawkes. 2003), in which they used the protocol for the first time - albeit for a different purpose..|
|Brazil nuts & selenium: "How much is too much?" A weighty question answered solely based on toxicity, not thyroid hormone metabolism, here.|
As Hawkes and Keim point out, "[t]his difference may be due to the fact that serum T 3 in humans comes mainly by deiodination of T 4 in liver, whereas serum T3 in rats is released mainly preformed from the thyroid" - that's a significant difference, because it implies that the selenium containing deiodinase enzymes that convert T4 to T3 and deactivate T3 are far more critical in man vs. mice / rats. In addition, it may be relevant that most rat studies induced a state of profound selenium deficiency, with glutathione peroxidase activities suppressed by as much as 90%, whereas the mild selenium deficiency in the study aat hand lead to a decrease in plasma glutathione peroxidase activity of only 12% in the low selenium group.
|Figure 1: Changes in relevant markers of thyroid and lipid metabolism (left | p < 0.05 for inter-group differences), as well as changes in lean and fat mass (right | only the reduction in body fat in the low selenium group was statistically sign.)|
Against that background, it's not really surprising that the high selenium group began to gain while the low selenium group began to lose weight by day 64 (the weight gain is not reflected in in the data in Figure 1, because taken on it's own, the 700g the subjects gained were not significantly; that's in contrast to the inter-group difference with p = 0,001). It did thus take no more than 6 weeks on the high vs. low selenium diet to trigger measurable effects not just on the subjects' metabolism, but also on their body weight; and by day 92, the weight gain / loss in the high vs. low selenium groups was pronounced and uniform enough to be statistically significant.
- Berry, Marla J., and P. Reed Larsen. "The Role of Selenium in Thyroid Hormone Action*." Endocrine reviews 13.2 (1992): 207-219.
- Hawkes, Wayne Chris, Fulya Zeynep Alkan, and Lynn Oehler. "Absorption, distribution and excretion of selenium from beef and rice in healthy North American men." The Journal of nutrition 133.11 (2003a): 3434-3442.
- Hawkes, Wayne Chris, and Nancy L. Keim. "Dietary selenium intake modulates thyroid hormone and energy metabolism in men." The Journal of nutrition 133.11 (2003b): 3443-3448.
- Hawkes, Wayne Chris, et al. "High-selenium yeast supplementation in free-living North American men: no effect on thyroid hormone metabolism or body composition." Journal of trace elements in medicine and biology 22.2 (2008): 131-142.
- Kenfield, Stacey A., et al. "Selenium Supplementation and Prostate Cancer Mortality." Journal of the National Cancer Institute 107.1 (2015): dju360.
- Köhrle, J. "Thyroid hormone deiodination in target tissues--a regulatory role for the trace element selenium?." Experimental and clinical endocrinology 102.2 (1993): 63-89.