Though several studies have linked processed food intake to problems such as obesity and intestinal inflammation in mice, nothing concrete had been shown in the case of humans.
Previously researchers have reported associations between processed foods and health outcomes, such as an increased risk of developing obesity, cancer, autoimmune conditions, and even death in humans, but there was nothing conclusive linking the two.
Now scientists working at the National Institutes of Health in the US say they have proof that suggests eating ultra-processed foods leads to weight gain in humans in as little as two weeks.
Ultra-processed foods, including soft drinks, packaged snacks, meat nuggets, frozen meals, and foods high in additives and low in unprocessed ingredients, are increasingly making up a significant chunk of energy intake around the world.
In their study, the researchers recruited male and female volunteers who stayed at the test center for 28 days. Half of the participants ate ultra-processed food for the first two weeks while the others received unprocessed foods. After the two-week period, the groups switched, allowing each participant to eat both the ultra-processed food and the unprocessed food for two weeks.
The volunteers ate three meals per day, and the researchers asked them to eat as much or as little as they wanted. They also had access to snacks and bottled water all day. After the 28-day period, the researchers found that when the volunteers were on the ultra-processed diet, they ate an average of 508 calories more each day than when they were on the unprocessed diet. As a result, they put on an average of 0.9 kilograms during this time, mostly in the form of body fat.
Participants in the unprocessed food group lost an average of 0.9 kg after the two-week study period. This group also saw increases in the gut hormone peptide YY, which suppresses hunger, and decreases the hunger hormone ghrelin.
The researchers also found that though the study participants rated the pleasantness and familiarity of the diets as equal, they ate significantly faster in the ultra-processed group. In fact, they consumed an extra 17 calories, or 7.4 grams of food per minute, than their counterparts in the unprocessed food group.
The study suggests that textural or sensory elements of ultra-processed foods could be making the volunteers eat more quickly, not giving their gastrointestinal tract enough time to signal the brain that they were satiated, and thereby overeating.
The study also observed that the ultra-processed food group actually consumed more carbohydrates and fat than the unprocessed food group, which had slightly more protein.
Meals in the ultra-processed group had a higher energy density than in the unprocessed group, which also likely contributed to the observed excess energy intake by that group.
The researchers acknowledge that they did not take into consideration how cost, convenience, and skill influence consumers to choose ultra-processed over unprocessed foods. Ultra-processed foods contribute to more than half the calories consumed by many people as they are cheap and convenient options.
Therefore policies that discourage consumption of ultra-processed foods should take into account the time, skill, expense, and effort required to prepare meals from minimally processed foods — resources that are often not easily available to those in lower socioeconomic classes.