Over the past several decades, fast-food and processed/packaged foods made with cheap ingredients like white flour and salt have come to dominate the American diet. While an established link between eating too much junk food and obesity has been made, there is still a need for more research to suss out the reasons why.
In a watershed study published in Cell Metabolism, researchers provided food for 20 participants split into two groups. For 2 weeks, one group ate an “unprocessed diet” (for example, a breakfast of plain yogurt and whole fruit), while the second group consumed an “ultra-processed diet,” which might include a breakfast of sugary boxed cereal and a packaged muffin. The investigators made sure that the calories, fat, carbohydrates and fiber were matched in each meal and snack but instructed subjects to eat as little or as much at each meal as they desired. After 2 weeks, the groups traded regimens.
Results showed that people ate significantly more when their meals were ultra-processed—around 500 additional calories per day—than they did when eating unprocessed meals. The added calories in the processed diet contributed to about a 2-pound weight gain, mainly from body fat mass. On the flip side, people lost about the same amount of weight when they were assigned to a healthier diet. Since participants agreed that both diets were flavorful enough to enjoy eating, the researchers surmised that the reason more calories were eaten on the less healthy, ultra-processed diet was not about meal satisfaction. Instead, they suspected people often ate easier-to-chew ultra-processed foods faster, leading to delayed satiety signals and, in turn, greater food consumption.