For four weeks, 20 healthy volunteers checked into a research center hospital and were served a variety of tempting meals: cinnamon french toast, stir-fry beef with broccoli and onions, turkey quesadillas and shrimp scampi. Researchers scrutinized everything that was eaten and came away with the first hard evidence to support a long-held suspicion: Heavily processed foods could be a leading factor in America?s obesity epidemic.
The unusual clinical trial compared the volunteers? calorie consumption and weight gain when they ate a diet based on unprocessed ingredients and when they ate meals dominated by ultra-processed foods. Both daily menus had matching amounts of calories, fat, sugar, carbohydrates and salts, and diners said they were equally tasty and satisfying.
Yet the volunteers chose to consume an average of 508 additional calories per day on the ultra-processed diet. After two weeks, they weighed an average of 2 pounds more than their counterparts who had dined on unprocessed foods.
The findings, published Thursday in the journal Cell Metabolism, will force scientists to rethink the complicated relationship between dietary habits and health.
?I thought it was all about the nutrients,? said study leader Kevin Hall, a section chief at the National Institute of Diabetes and Digestive and Kidney Diseases, which is part of the National Institutes of Health.
?There?s something other than the sugar and fat on the food label that causes people to overeat and gain weight,? Hall said. ?We don?t fully know the mechanism yet, but processed foods aren?t just innocent bystanders.?
The American diet has changed drastically over the past century. Home-grown produce and local poultry have given way to canned vegetables and deep-fried chicken tenders. Doctors have long suspected that changes in food preparation were among the key contributors to the obesity epidemic, but they?ve struggled to find ways to reverse the trend.
Almost 40% of adults in America are now obese, more than double the percentage in 1980, according to the Centers for Disease Control and Prevention. The obesity rate among children has almost tripled in the same time period.
Hall and his colleagues decided it was time to get serious with a randomized controlled trial, considered the gold standard for medical research. They recruited 20 people who weren?t picky eaters and were willing to spend a month living at the NIH?s Metabolic Clinical Research Unit in Bethesda, Md.
The volunteers were given three meals per day and were allowed to refill their plates as much as they wanted. They also had access to unlimited snacks. They were randomly assigned to consume either an ultra-processed diet or an unprocessed one for the first two weeks of the experiment. Then they switched menus for the remaining two weeks.
If volunteers ate everything put on their plates all day long, those on the unprocessed diet would have consumed the same number of calories and nutrients as those on the ultra-processed diet. In reality, their consumption was different because the researchers served up gargantuan amounts of food ? an average of 5,400 calories each day ? and participants left different amounts of food on their plates.
Dietitians at the National Institutes of Health designed custom menus to test the health effects of ultra-processed and unprocessed diets on study participants. NIDDK/JENNIFER RYMARUK/TNS
Participants said both diets were filling and delicious. That may sound trivial, but it?s important for a nutrition study because it helps eliminate the influence of factors like food preference that could influence the experiment?s results.
