Iron, iodine, the fatty acids in oily fish: feed them to a deficient teenager and, sometimes, their scores on a memory test or a nonverbal reasoning task tick upward. Sometimes. The frustrating word runs through the whole literature, because just as often nothing moves at all, and the supplement that worked in a Mexican village fails in a Dutch classroom. Researchers at Swansea University have spent years pulling 73 New! Sign up for our email newsletter on Substack.