Vitamins and dietary supplements are big -- more than 110 million Americans forked over a collective $28 billion in 2010 on little bottles of would-be health magic. Research is unclear, however, on whether shoring up your diet with extra vitamins, minerals, and other supplements helps or hurts -- in the short run or in reaching for the century mark.For more than a decade, for example, researchers followed more than 35,000 men enrolled in SELECT, a clinical trial designed to see whether taking selenium and vitamin E might help prevent prostate cancer.