The Committee for Skeptical Inquiry recently published a useful piece called 'Science and Pseudoscience in Adult Nutrition Research and Practice'. It's by Reynold Spector who has held professorships of medicine at Iowa, Stanford, and Harvard-MIT, and is presently a professor at the Robert Wood Johnson Medical School. The piece discusses what we know about human nutrition (a lot), what we know about the value of supplements (in most cases we know it's little or nothing), and issues relating to who gains and who loses from the massive trade in diet related products, and the often unhelpful reporting and advertorial on these topics.
It's well worth a read, and a link, and being brandished in the face of a popular and pernicious kind of idiocy. Here's the piece: Science and Pseudoscience in Adult Nutrition Research and Practice.