NEJM | All Calories Are Not Created Equal | Food Type Matters More

Mmmmmmm ... needs bacon and melted cheese.

In the game of life and long-term weight maintenance, calories count, but the types of foods might matter more, according to a study by Harvard researchers published last week in the New England Journal of Medicine.

Diets that include potatoes, white bread, sugar-sweetened beverages and meats — well, all that defines modern America — were associated with the greatest weight gain over the 20-year study period. Surprisingly, eating French fries led to more weight gain than eating sugary desserts. And eating whole grains was associated with weight loss, diametrically opposite to the significant weight gain associated with refined grains despite equal caloric content. These results prompted the Harvard researchers to claim that the mantra to "eat less, exercise more" might be overly simplistic.

The Harvard study was one of the first to examine factors linked to long-term weight gain. Most other research has focused on dieting after the subjects have gained extra weight. American adults gain at least a pound per year, on average, so the impact on health after a few decades can be significant. The researchers tapped into three large, ongoing studies — the Nurses' Health Study, the Nurses' Health Study II, and the Health Professionals Follow-up Study — following over 120,000 adults who were free of obesity and chronic diseases at the beginning of the study. "Of course, in the end, 'calories in' versus 'calories out' is what causes weight gain," said lead author Dariush Mozaffarian of the Harvard School of Public Health. "The key question is how to achieve that balance, since everyone is trying to do it and nearly everyone is failing."


America's love of fast food and junk food apparently isn't the way to achieve balance. For example, a daily serving of French fries was associated with 3.35 pounds of weight gain every four years; potato chips were associated with 1.69 extra pounds. Potatoes in general were among the biggest dietary offenders, followed by sweetened soda pop (one-pound gain every four years), and processed meat and unprocessed red meat (about a 0.95-pound gain).

Conversely, eating more of several specific foods — vegetables, fruits, nuts and whole grains — was associated with less weight gain, chafing the conventional wisdom that all calories are equal, Mozaffarian said. Nuts are calorie-dense, but their consumption was associated with weight loss. Whole and low-fat milk were equally associated with weight loss, despite the calorie difference. Yet a bag of potato chips, with only about 150 calories per serving, has fewer calories than many items on the Harvard researchers' list and was associated disproportionally with so much weight gain.

No laws of thermodynamics are being broken here. "Differences in weight gain seen for specific foods and beverages could relate to varying portion sizes, patterns of eating, effects on satiety, or displacement of other foods or beverages," the researchers wrote in their paper. That is, eating potatoes and white bread might be less satiating compared with less-processed, higher-fiber foods with the same number of calories, increasing subsequent hunger signals in the brain and thus the total caloric intake, the researchers said. Higher-fiber foods and their slower digestion, on the other hand, could augment satiety, the researchers said. Their increased consumption would displace more highly processed foods in the diet, "providing plausible biologic mechanisms whereby persons who eat more fruits, nuts, vegetables and whole grains would gain less weight over time," as stated in the paper.

Changes in Diet and Lifestyle and Long-Term Weight Gain in Women and Men. Mozaffarian, D., Hao, T., et.al. N Engl J Med 2011; 364:2392-2404, June 23, 2011.

Christopher Wanjek at LiveScience