Evidence Level
Moderate
Selenium research highlights the importance of baseline status. The Nutritional Prevention of Cancer (NPC) trial (Clark et al., 1996, n=1,312) initially showed a 52% reduction in prostate cancer with 200mcg selenium yeast daily, but this was a secondary endpoint. The subsequent SELECT trial (Lippman et al., 2009, n=35,533) found no cancer prevention benefit with selenomethionine in a selenium-replete population, and even suggested possible diabetes risk at 200mcg/day. For thyroid autoimmunity, a systematic review by Wichman et al. (2016, 16 studies) confirmed selenium supplementation reduces TPO antibody levels in Hashimoto's thyroiditis, though clinical significance remains debated.