While the quantity and quality of processed foods in our diets continue to be a major focus in articles and discussions about diet, the role of ultraprocessed foods is now gaining more attention. A new study published in Cell Metabolism is bringing the topic of ultraprocessed back in the news­­. This time, researchers are examining the link among ultraprocessed foods, calorie intake and weight gain. You may have seen the headlines, but what about the findings? Let’s take a closer look.

How Was the Study Done?

This study was a randomized control trial (RCT) conducted on twenty healthy people (ten males and ten females). Study participants were between the ages of 18 and 50 (the average age was 31), had a body mass index (BMI) above 18.5 (the average BMI was 27) and had body weights that had remained stable during the six months prior to the study. For 28 days, the participants lived at the National Institutes of Health facility in Bethesda, Maryland, where all their daily food and drinks were provided for them. The study assessed participants’ food intake and the resulting changes in their body weights.

Two diets were tested in the study: one consisting entirely of ultraprocessed foods and the other containing only unprocessed foods. The diets were equal in total calories, macronutrients (fat, carbohydrates and protein), sugars, sodium and fiber. To construct the meal plans, study authors used the NOVA classification system, which characterizes food processing in the following four groups: (1) unprocessed or minimally processed foods, (2) processed culinary ingredients, (3) processed foods, and (4) ultraprocessed foods. In other words, the “unprocessed” diet was put together with foods from group 1 and the “ultraprocessed” diet was made with foods from group 4. Randomly, ten people were assigned to begin with the ultraprocessed diet for 14 days, while the other ten received the unprocessed diet. After 14 days, the participants switched diets.

This was a controlled feeding study, but participants were told they could eat as much or as little as they wanted. Up to 60 minutes was given to eat each meal, whereas snacks and water were made available in unlimited amounts all day long.

If you want to know more about the types of foods used in the study and what the meals looked like, you can find details here.

What Were the Results?

Overall, the study found that:

1. Participants ate more calories while eating the ultraprocessed diet (around 500 more per day).
2. Participants gained weight (about two pounds) on the ultraprocessed diet.
3. Participants lost weight (about two pounds) on the unprocessed diet.

Increases in calorie intake on the ultraprocessed diet were attributed to carbohydrates (about 280 more calories per day) and fat (about 230 more calories per day). Calories from protein were the same on each diet.

In addition to total calorie consumption, the amount eaten at each meal was also documented. When eating the ultraprocessed diet, people ate significantly more at breakfast (about 124 more calories per day) and lunch (about 213 more calories per day). While people ate more calories at dinner (about 66 more calories per day) and from snacks (about eight more calories more per day), these results weren’t statistically significant.

Consumption of fiber and total sugar was the same on each diet, but there were other significant differences observed on the ultraprocessed diet:

• Fat consumption was higher at all three meals, but not from snacks.
• Carbohydrate consumption was higher at breakfast and lunch but not at dinner or from snacks.
• Protein consumption was lower at lunch, but the same for other meals and snacks.
• Sodium consumption was higher.

Study Strengths and Limitations

While it’s easy to take these results and apply them broadly, it’s important to know that this study was designed to determine the differences in calorie intake between two different diets in a controlled environment. The study authors identify proposed theories and mechanisms for how ultraprocessed foods might impact calorie consumption and health, while also stating that there is a lack of RCT evidence assessing the impact of ultraprocessed foods on health. Addressing this void in the literature is a major strength of the study.

Overall, the study was well done, both in design and execution. The diets were matched for important nutritional components and the authors went to great lengths to provide a controlled setting.

The study did have some limitations, including:

Participants were informed of key study details. Study participants were told of the study’s objective to learn how processed and unprocessed foods impact the amount of food they eat and their health.

The cost of food Everyone has a different budget, including what we allocate for food. The unprocessed diet used in this study was more expensive (estimated to cost $151/week) than the ultraprocessed diet (estimated to cost $106/week).

The cause of intake differences The two diets were matched closely for nutrition with the objective to study calorie intake differences. While calorie intake differences between diets were observed, further analysis and study is needed to determine underlying causes.

Surroundings Feeding studies in confined settings over long periods of time require people to adjust their normal routines. Living for a month in a foreign environment could impact us in a variety of ways.

Fiber While the fiber provided by each diet was technically equivalent, it was provided through different means. On the ultraprocessed diet, fiber was provided in a beverage. On the unprocessed diet, it was provided through food. The impact of fiber delivery through food versus drink is hard to assess. On both diets, the amount of fiber consumed was about three times what is typical in the American diet. Without baseline diet intake, it’s hard to tease out the effect of fiber on these findings.

Closing Thoughts

It’s much easier to think in black and white. Shades of grey just make everything more complicated, leaving too much room for interpretation. Such is the nature of food. We all eat, we just go about it in different ways and for various reasons. But there is no doubt that the “quality” of food matters. The question is: What is the right mix of processed and unprocessed foods in a healthy diet?

Processed foods have benefits, but they should not be relied upon to provide all of the calories and nutrients we need. No one is suggesting they should. Looking at the results from this single study in a vacuum might lead one to believe that all processed foods should be avoided completely. Seeking nutrition from whole foods first is the best approach, but a healthy diet need not be a diet of only unprocessed foods.