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.
“They showed that the effect [of processing] goes beyond nutrients,” says Carlos Monteiro, an epidemiologist at the University of São Paulo in São Paulo, Brazil, who helped develop the NOVA classification system and supports government interventions to limit processed food consumption. Simply reformulating packaged foods to contain less sugar, salt, or fat—as many large companies are now attempting—won’t eliminate their risks, he says.
If participants continued eating those extra 500 calories, they would “gain a lot of weight—a lot —over time,” says Heymsfield, though he notes that their gusto for the ultraprocessed diet might have waned if the study had gone on a few weeks longer. He suspects people overate processed food because it was more appealing. “The ultraprocessed foods look like foods I might overeat also, given the chance,” he says.
Yet on surveys, the participants rated the processed meals as no more pleasant than the unprocessed ones. If they weren’t enjoying the food more, why were they eating more of it?
One possibility is that industrial processing produces softer foods that are easier to chew and swallow—and thus easier to scarf down. The participants ate faster on the ultraprocessed diet, and studies have found that people tend to eat more when they eat faster. Blood tests also revealed that, while on the unprocessed diet, people had higher levels of an appetite-suppressing hormone called PYY and lower levels of the appetite-stimulating hormone ghrelin, though it’s not clear how these changes relate to food processing.
And despite the researchers’ efforts to perfectly match the nutrition of the diets, there were some differences that may have influenced how much people ate. The ultraprocessed meals contained slightly less protein, and some research has found that people tend to eat until they reach a certain protein target. If that protein is more diluted, those studies hint, people will consume more calories to hit the same target.
Ultraprocessed foods also tend to be more energy-dense—they have many more calories per gram, notes Barbara Rolls, an obesity researcher who studies eating behavior at Pennsylvania State University in State College. (Although Hall’s team concluded the two diets were roughly equal in energy density, the measurements included low-energy-density beverages added to the ultraprocessed diet to boost fiber via dissolved supplements.) Rolls’s team has found that more energy-dense foods lead people to eat more calories because they tend to eat a consistent weight or volume of food day to day.
Hall and his colleagues are now planning a similar-size study with a few tweaks: They’ll bump up the protein in the ultraprocessed diet and swap fiber-enriched beverages for soups, which may encourage people to eat more slowly.
For now, some researchers aren’t convinced that processing itself is a menace. “A lot of … the ultraprocessed foods in this study are perhaps ones that we [shouldn’t] to be eating too often,” Rolls says. And most people don’t have the time or resources to prepare farm-to-table meals, she adds. “If we had to live without processed foods, I don’t think we would be able to feed the population—nor would people like it.”