Blame the potato chip. It's the biggest demon behind that nearly pound-a-year weight creep that plagues many of us, a major diet study found. Bigger than soda or ice cream. And the reason is partly that old advertising cliché: You can't eat just one. "People generally don't take one or two chips. They have a whole bag," said obesity expert Dr. F. Xavier Pi-Sunyer of the St. Luke's-Roosevelt Hospital Center in New York. In the new study, reported in Thursday's New England Journal of Medicine, scientists from Harvard analyzed changes in diet and lifestyle habits of 120,877 people from three long-running medical studies. On average, participants gained nearly 17 pounds over the 20-year period. Potato chips were the biggest dietary offender. Each extra daily serving of 1 ounce led to a 1.69-pound uptick over four years. For potatoes other than chips, the gain was 1.28 pounds; sugar-sweetened beverages, 1 pound; and sweets and desserts, .41 of a pound. Weight loss was associated with the intake of daily servings of yogurt, .82 of a pound; nuts, .57 of a pound, and fruits, .49 of a pound.

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