For starchy potatoes other than chips, the gain was 1.28 pounds. Within the spud group, french fries were worse for the waist than boiled, baked or mashed potatoes. That's because a serving of large fries contains between 500 to 600 calories compared with a serving of a large baked potato at 280 calories.

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Soda added a pound over four years. Eating more fruits and vegetables and other unprocessed foods led to less weight gain, probably because they are fiber-rich and make people feel fuller.

For each four-year period, these factors had these effects on weight:
- An alcoholic drink a day, 0.41-pound increase.
- Watching an hour of TV a day, 0.31-pound increase.
- Recently quitting smoking, 5-pound increase.

People who slept more or less than six to eight hours a night gained more weight.

The study was funded by the National Institutes of Health and a foundation. Several researchers reported receiving fees from drug and nutrition companies.

"Humans naturally like fat and sweet," said Dr. David Heber, director of the UCLA Center for Human Nutrition, who had no role in the study. "That's why we always tell people to eat their fruits and vegetables."

Pi-Sunyer, who also wasn't involved in the research, said the study gives useful advice.

"It's hard to lose weight once you gain it," he said. "Anything that will give people a clue about what might prevent weight gain if they follow through with it is helpful."

The federal government earlier this year issued new dietary guidelines advising people to eat smarter. This month, it ditched the food pyramid - the longtime symbol of healthy eating - in favor of a dinner plate divided into four sections containing fruits, vegetables, protein and grains.