For the study, Mozaffarian and his colleagues analyzed data collected from a total of 120,877 healthy American men and women. The volunteers detailed their eating, exercise and other habits for the Nurses Health Study, the Nurses Health Study II and the Health Professionals Follow-up Study— large, highly respected Harvard studies examining a host of health issues. The researchers followed the participants for four-year intervals to see how changes in what they ate, drank and did affected their weight.
Within each period, the subjects gained an average of 3.35 pounds. Every additional daily serving of potatoes pushed up the scale by more than a pound every four years. As expected, the type of potato, however, was important. Every order of french fries put on 3.35 pounds; a snack of potato chips added 1.69. But even each helping of boiled, baked or mashed potatoes contributed a little more than a half-pound.
Although the study did not evaluate why potatoes would be particularly fattening, other research shows that starches and refined carbohydrates such as potatoes cause blood sugar and insulin to surge, which makes people feel less satisfied and eat more as a result, Mozaffarian said.
Many people might also be surprised that every extra serving of refined grains, such as white bread, added 0.39 pounds — almost as much as indulging in some sweets.or desserts.
Researchers will surely scramble to try to explain why yogurt appears so helpful. It may be because of subtle shifts of microbes in the digestive tract, or perhaps because people who eat more yogurt also tend to do other healthy things, the researchers said.
Lifestyle factors were clearly important. Those who exercised more gained nearly two pounds less than those who increased their physical activity the least. People who slept less than six hours a night — or more than eight hours — were more likely to gain weight, possibly by unbalancing hunger hormones such as ghrelin. Every extra hour per day of television watching added about a third of a pound, perhaps by encouraging snacking.
But some researchers expressed caution.The precise “serving size” varied among foods, and relied on participants’ memory and honesty, for example.
“To attempt to isolate the effect of specific foods on weight changes is fraught with problems,” said Lawrence J. Cheskin, who heads the Johns Hopkins Weight Management Center. “One is that people may conclude that if they simply stop eating X, they will reduce the chance of weight gain. This is unlikely, and a false conclusion.”Similarly, it is likely more a result of people who eat fruit being more health-conscious than fruit per se causing less weight gain.”
Nevertheless, the consistency across all three data sets made the researchers confident that the findings are generally accurate for sketching an outline of which food choices encourage overeating and which are associated with maintaining a healthier weight.
With no magic bullet weight-loss pills in sight, and study after study showing that dieting only helps a little, other researchers said the findings offer valuable clues to the only other option for fighting the obesity epidemic: preventing weight gain.
“What we now need are effective strategies and possibly public health policies to help people adopt lifestyle behaviors that will prevent them from becoming obese,” said Samuel Klein of the Washington University School of Medicine in St. Louis. “An ounce of prevention is worth a pound of fat when it comes to obesity.”