For a long time, many health experts have suspected that the increased consumption of processed foods over the past 50 years has been a key factor behind the obesity epidemic. But, remarkably, no one has conducted a randomized controlled trial (RCT) — considered the gold standard of research — to test that theory.
Until now, that is.
Late last week, scientists at the National Institutes of Health published the results of a small but tightly designed RCT that pitted an ultra-processed diet against an unprocessed one. The study found that even when ultra-processed and unprocessed meals are similar in terms of calories, protein, fat, carbohydrates, and other nutrients, the ultra-processed foods lead people to take in more calories and gain weight.
“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 Kevin Hall, the study’s lead author and a senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases, in a released statement. “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.”
How the study was done
For the study, Hall and his colleagues recruited 20 healthy volunteers (10 men and 10 women). All were in their late 20s or early 30s. None was obese, but some were overweight.
The volunteers agreed to live in an NIH lab for 28 days. When they arrived at the lab, they were randomly assigned to either an ultra-processed diet or an unprocessed diet for two weeks. Then they were switched to the other diet for another two weeks.
The volunteers were served three meals daily in the lab and were given access to snacks (also either ultra-processed or unprocessed) throughout the day. They were allowed to eat as much or as little as they wanted (although the meals were served for only an hour), and the amount of food and drink they consumed was carefully tracked and measured.
There is no universally accepted definition of ultra-processed foods, so the researchers used a diet classification system developed by scientists in Brazil, called NOVA. It places foods and beverages into four categories, ranging from “unprocessed/minimally processed” to “ultra-processed.”
Ultra-processed foods are defined in this system as “snacks, drinks, ready meals and many other products created mostly or entirely from substances extracted from foods or derived from food constituents with little if any intact food, which often contain flavours, colours and other additives that imitate or intensify the sensory qualities of foods or culinary preparations made from foods.”
Unprocessed/minimally processed foods are those that are unaltered (an apple, for example) or “altered by processes such as removal of inedible or unwanted parts, drying, crushing, grinding, fractioning, filtering, roasting, boiling, pasteurisation, refrigeration, freezing, placing in containers, vacuum packaging, or non-alcoholic fermentation. None of these processes adds substances such as salt, sugar, oils or fats to the original food.”
What the study found
During the two weeks that the volunteers ate the ultra-processed foods, they consumed, on average, 508 more calories a day then they did during the two weeks they ate the unprocessed foods. Those calories mounted up. For at the end of the two weeks on the ultra-processed diet, the volunteers had gained an average of two pounds compared with an average weight loss of two pounds during the two weeks they were on the unprocessed diet.
Hall and his colleagues offer several hypotheses for why the volunteers in their study took in more calories on the ultra-processed diet.
“There may be something about the textural or sensory properties of the food that made them eat more quickly,” says Hall. “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.”
Another possible explanation is that the unprocessed diet in the study contained slightly more protein than the ultra-processed one (about 15.6 percent of calories versus 14 percent).
“It could be that people ate more because they were trying to reach certain protein targets,” says Hall.
Yet another reason for why the people in the study took in more calories while on the ultra-processed diet may have to do with the diet’s effects on certain hormones. The study found that levels of the appetite-suppressing hormone PYY were higher — and levels of the so-called hunger hormone ghrelin were lower — among the volunteers during the two weeks of eating unprocessed foods.
Limitations and implications
The study comes with several caveats, starting with the fact that it involved only 20 participants, lasted only a month and was conducted in a highly controlled environment. The findings might have been different if the study had been larger, longer and more “realistic” (if it had involved people making food choices in real-life settings).
In addition, although the researchers did their best to match the nutrient compositions of the two diets, they weren’t identical. Most notably, to make sure the ultra-processed diet contained enough fiber, they had to include beverages such as fruit juice and lemonade. By comparison, the main beverage in the unprocessed diet was water. Some research suggests that people don’t feel as “full” after drinking beverages than after eating solid foods — a factor that could have led the volunteers to increase their calorie intake while on the unprocessed food diet.
“Such a recommendation could potentially be embraced across a wide variety of healthy dietary approaches, including low-carb, low-fat, plant-based, or animal-based diets,” they add.
Examples of meals
Here are some examples of the meals provided in the study:
Honey Nut Cheerios (General Mills)
Whole milk (Cloverland) with NutriSource fiber
Blueberry muffin (Otis Spunkmeyer) with margarine (Glenview Farms)
Greek yogurt (Fage) parfait with strawberries, bananas, with walnuts (Diamond), salt and olive oil
Apple slices with fresh squeezed lemon
Beef ravioli (Chef Boyardee)
Parmesan cheese (Roseli)
White bread (Ottenberg)
Margarine (Glenview Farms)
Diet lemonade (Crystal Light) with NutriSource fiber Oatmeal raisin cookies (Otis Spunkmeyer)
Spinach salad with chicken breast, apple slices, bulgur (Bob’s Red Mill), sunflower seeds (Nature’s Promise) and grapes
Vinaigrette made with olive oil, fresh squeezed lemon juice, apple cider vinegar (Giant), ground mustard seed (McCormick), black pepper (Monarch)
Mashed potatoes (Basic American Foods)
Margarine (Glenview Farms)
Corn (canned, Giant)
Diet lemonade (Crystal Light) with NutriSource fiber
Low fat chocolate milk (Nesquik) with NutriSource fiber
Beef tender roast (Tyson)
Rice pilaf (basmati rice [Roland] with garlic, onions, sweet peppers and olive oil)
Side salad (green leaf lettuce, tomatoes, cucumbers) with balsamic vinaigrette (balsamic vinegar [Nature’s Promise])
Salt and Pepper (Monarch)
FMI:You can download read the study on Cell Metabolism’s website.