Potato chips and French fries -- and generally any potato products -- contribute to the biggest weight gain over time, according to a new Harvard study, which researchers say is the first to look at long-term weight gain pegged to specific foods.
The study, conducted over 20 years and published in The New England Journal of Medicine, found that among more than 100,000 men and women whose weight was evaluated at four-year intervals, the average weight gain over each period was 3.35 pounds. This corresponded with an average weight gain just shy of 17 pounds over 20 years.
The researchers also tracked how much weight specific foods led people to gain over each four-year period. Potato chips were the worst culprit, associated with a weight gain of 1.69 pounds, followed by potatoes in general at 1.28 pounds. (French fries were worse than boiled or mashed potatoes.) This, explained Dr. Dariush Mozzafarian, a professor of epidemiology at the Harvard School of Public Health and the study's lead author, could be because starches and refined carbohydrates produce bursts in blood glucose and insulin, increasing hunger and thus upping the total amount of food people eat at their next meal.
There were plenty of non-potato culprits, too: Sugary beverages accounted for a one pound weight gain, while alcohol was linked with an average gain of 0.41 pounds over four years. Unprocessed meats accounted for a 0.95-pound uptick in weight, while processed meats were right behind at 0.93 pounds.
"Our findings indicate that small dietary and other lifestyle changes can together make a big difference, for bad or good," Mozzafarian wrote in an email, adding, "For diet ... eat fewer starches and refined foods like potatoes, white bread, low-fiber breakfast cereals, processed meats, sweets and soda."
Instead, the study suggests, opt for healthier options if you want to lose weight.
People who added a daily serving of vegetables lost an average of 0.22 pounds over four years, the researchers found. People who added whole grains lost 0.37 pounds, and those who ate fruits shed almost half a pound. Nuts and yogurt also resulted in weight loss -- all under one pound, but those losses can add up over time.
The authors point out that while these foods contain calories and fat as well, eating them usually causes people to avoid unhealthier, more calorie-dense options -- "displacing" them, in a way -- which ultimately leads to weight loss.
Also, because they have higher fiber content, they may be more satisfying.
"Satiety is a big thing," said Jeannie Moloo, R.D., a national spokesperson for the American Dietetic Association. "Consuming these foods [i.e. chips, sweet beverages and meat] could be less satiating and less filling, triggering hunger signals." Moloo praised the study, saying it was important to have finally documented the advice that dietitians and medical practitioners have been giving for years.
Another example of this advice? The fact that being sedentary isn't good for the waistline.
The study showed that changes in physical activity were related to long-term changes in weight, singling out TV watching as an example. Watching an hour of TV per day led to a 0.31 pound weight gain over each four year period -- a finding the authors chocked up to people's tendency to snack while they watch.
Sleep also plays a role: Participants who slept between six and eight hours a night were less likely to gain weight than those who got fewer than six hours or more than eight.
"Be active," Mozzafarian said, "turn off the TV, and get enough sleep."