The first question isn't why but why not? Vitamins, probiotics, omega-3 capsules, antioxidant pills: they can't hurt, right? Around the corner of each advancing birthday lurks a possible affliction arthritis, cancer, Alzheimer's and a giant industry has emerged to try to prevent them all. Americans now spend an estimated $28 billion a year on dietary supplements more than twice what we spent in 1995 and more than $5 billion more than what we pay each year for gym memberships. But do supplements actually work?
Dietary supplements occupy the broad, poorly regulated space between two more-defined kinds of consumables:...