To most, the word “diet” equals weight loss. But diet soda may not be holding up its end of the bargain. Researchers at the University of Texas Health Science Center at San Antonio recently found that people who drank two or more diet sodas daily had a six-times-greater increase in waist circumference at the end of the 10-year study than those who didn’t drink diet soda at all. Those bigger waist sizes may be due to the “I saved here, I can splurge there” theory of dieting, says researcher Sharon Fowler, M.P.H. Or perhaps the artificial sweeteners in diet soda stoked diet-soda drinkers’ appetites, as other research suggests.