A Medicine Chest or a Grocery Shelf?
Article on health benefits of 'functional foods,' being marketed by drug and consumer-goods companies; says grocery aisles are stacked with rows of margarines, breakfast cereals, salad dressings and snacks that promise to fight heart disease, osteoporosis and other ailments; in some cases, products are billed as alternatives to medicine; says critics contend that some manufacturers exaggerate products' health benefits, especially when brand may contain ingredients that do as much harm as good; Michael F Jacobson, executive director of Center for Science in the Public Interest, says there is a lot of quackery; photos (M)