Small lifestyle changes like eating fewer potatoes, drinking less pop and not staying up too late could help prevent long-term weight gain, researchers say.

A series of three studies in Thursday's issue of the New England Journal of Medicine looked at how changes in diet and other lifestyle factors lead to gradual weight gain over the decades of about one pound per year.

Potato chips were the worst culprit, researchers at Harvard University found.

"They're very tasty and they have a very good texture. 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.

Both diet and exercise are important to prevent weight gain, but making wise food choices clearly plays a bigger role, said Dr. Frank Hu, a senior of the paper and a professor of nutrition and epidemiology at the Harvard School of Public Health.

The researchers looked at lifestyle factors and weight gain every four years over 12 to 20 years for three separate groups of 50,422 women, 47,898 women and 22,557 men who were all free of obesity or chronic diseases when the study began.

Study participants gained an average of 3.35 pounds during each four-year period, which added up to  a weight gain of 16.8 pounds over the 20-year period.

Foods associated with the greatest weight gain over the 20-year study period included:

  • Potato chips (for each one increased daily serving, +1.69 pounds more weight gain every four years).
  • Other potatoes (1.28 lb).
  • Sugar-sweetened beverages (1.00 lb).
  • Unprocessed red meats (0.95 lb).
  • Processed meats (0.93 lb).

Several foods were tied to less weight gain when their consumption increased, such as vegetables (loss of 0.22 pounds), fruits (-0.49 pounds), nuts (-0.57 pounds), whole grains (-0.37 pounds) and yogurt (-0.82 pounds).

Changes in physical activity and TV viewing also influenced changes in weight. Watching an hour of TV a day was tied to a 0.31-pound increase.

Also, those who slept six to eight hours a night gained less weight than those who slept less than six hours, or more than eight hours.

The study was funded by the U.S. National Institutes of Health and the Searle Scholars Program. Several researchers reported receiving fees from drug and nutrition companies.

With files from The Associated Press