While processed food has long been suspected as a contributing factor in obesity, it has been difficult to make a direct connection because of a number of variables. However, researchers using a controlled study with 20 healthy participants were able to determine eating a diet of processed food led to increased calorie consumption and weight gain of a pound per week.
The results of the study published in the journal Cell Metabolism showed even though the two diets contained similar amounts of carbohydrates, fat, sugar, salt and calories, those on the processed diet consumed more food and gained weight. In fact, the processed food group consumed an average of 508 additional calories per day.
"I was surprised by the findings from this study, because I thought that if we matched the two diets for components like sugars, fat, carbohydrates, protein, and sodium, there wouldn't be anything magical about the ultra-processed food that would cause people to eat more," says lead author Kevin Hall from the National Institutes of Health. "But we found that, in fact, people ate many more calories on the ultra-processed diet, and this caused them to gain weight and body fat."
The work was done at the NIH’s Metabolic Clinical Research Unit where the 20 participants were studied for a month. They were assigned to either the processed or unprocessed food group for two weeks and then switched. That eliminated some of the variables that have led to unanswered questions from previous studies.
Subjects were given three meals per day and had access to bottled water and either processed or unprocessed snacks. They were instructed to eat as much as they wanted and what they ate was tracked and measured.
An example breakfast for the processed group included cereal, whole milk with added fiber and a packaged blueberry muffin with margarine. The unprocessed breakfast contained a parfait made with plain Greek yogurt, strawberries, bananas, walnuts, salt, and olive oil and apple slices with fresh-squeezed lemon. Both groups said their food tasted good and was satisfying so food preference was eliminated as a factor.
During the two weeks on the diet of processed food the average weight gain was two pounds, while the average weight loss during the two weeks on the unprocessed diet was two pounds. Similarly, the subjects on the processed diet gained body fat and the ones on the unprocessed diet lost body fat.
Researchers have several hypotheses for the additional food consumption among the processed food consumers. One is the speed at which they ate, which was faster than the unprocessed group.
"There may be something about the textural or sensory properties of the food that made them eat more quickly," Hall says. "If you're eating very quickly, perhaps you're not giving your gastrointestinal tract enough time to signal to your brain that you're full. When this happens, you might easily overeat."