Something about the industrial processing of food makes us more likely to overeat, according to a new study. Volunteers ate more and gained more weight on a heavily processed diet than an unprocessed one, even when the two diets had the same available calories and nutrients.
The study is “a landmark first,” and a “shot over the bow” in a debate over the health of processed food, says Steven Heymsfield, an obesity researcher at Louisiana State University’s Pennington Biomedical Research Center in Baton Rouge who was not involved with the work. But some experts question whether the study controlled for important differences between the diets.
The definition of “processed food” is controversial. Nearly all the food at grocery stores is subject to some processing: It’s pasteurized, vacuum sealed, cooked, frozen, fortified, and mixed with preservatives and flavor enhancers. Some of these processes can change its nutritional qualities. And some studies have found associations between processed diets and increased risk of obesity, cancer, and even earlier death, but none has shown a causal link.
Still, some health officials and national governments have seized on processing as a culprit in the global epidemic of obesity and related diseases. The official dietary guidelines of Brazil, for example, recommend that people “limit consumption of processed foods.”
Kevin Hall, a physiologist at the National Institute of Diabetes and Digestive and Kidney Diseases in Bethesda, Maryland, suspected that processed foods were linked to poor health simply because they were likely to contain lots of fat, sugar, and salt. So in the new experiment, he and his team tried to rule out those factors. They recruited 20 healthy people and gave each about $6000 to surrender some freedoms, dietary and otherwise. Participants spent 28 straight days in a National Institutes of Health facility—with no excursions. They wore loose-fitting scrubs to make it harder for them to guess whether their weight was changing. Each was restricted to an “ultraprocessed” diet or a “minimally processed” diet for 2 weeks, and then switched to the other diet for 2 more weeks.
The study used a food classification system called NOVA developed by a team of researchers in Brazil. It describes “ultraprocessed” foods as ready-to-eat formulations with five or more ingredients, often including flavor-enhancing additives, dyes, or stabilizers. To be considered “minimally processed,” foods can be frozen, dried, cooked, or vacuum packed, but they can’t include added sugar, salt, or oil. Meals in the ultraprocessed arm of the study included packaged breakfast cereals, sweetened yogurt, canned ravioli, and hot dogs. Those in the unprocessed diet included oatmeal, steamed vegetables, salads, and grilled chicken. Dietitians carefully matched the processed and unprocessed diets for calories, sugar, sodium, fat, and fiber.
The captive participants did enjoy one big freedom: They chose how much to consume. Once they ate their fill, Hall’s team calculated their intake by painstakingly weighing the leftovers, down to every dollop of ketchup that didn’t make it onto a hot dog. The researchers found that by the second week of each diet, people were eating, on average, about 500 more calories per day when the fare was ultraprocessed. That extra consumption led to a weight gain of about a kilogram during the 2 weeks on the ultraprocessed diet, versus a loss of about a kilogram on the unprocessed diet, they report today in Cell Metabolism.