People who followed a completely unprocessed food diet ate more than 50% greater amounts of food than those eating only ultraprocessed foods (UPFs) but still consumed about 330 fewer calories per day on average, a new analysis showed. 

“We wanted to know whether people preferentially selected certain foods with a particular composition, and whether this might explain why people tend to consume fewer calories when they’re offered an unprocessed diet,” Jeff Brunstrom, PhD, School of Psychological Science, University of Bristol, Bristol, England, told Medscape Medical News.

To find out, Brunstrom and his colleagues did a post hoc analysis of a randomized controlled trial (RCT) that compared results of a UPF diet with those of an unprocessed food diet.

“The most surprising finding [of our study] wasn’t that people consumed more calories on a UPF diet,” because the RCT already found that, he said. “Rather, our study found that despite consuming more calories on the UPF diet, [the] volunteers consumed a larger amount (by weight) of food on the unprocessed diet.”

The study was published online in The American Journal of Clinical Nutrition.

Nutritional Intelligence

The study was prompted by the “popular narrative” that our diet is governed by the agents in our food system who manufacture and supply it, Brunstrom explained. “As such, we tend to blame the food industry and cast ourselves as passive recipients of the foods they provide.”

Brunstrom argues, “This is a mistake; humans have agency, and there’s an underlying sophistication to the dietary decisions we make. In particular, we respond to and learn about food composition (eg, protein content, calorie content, etc.) and discriminate and choose foods on this basis.” This preferential selection is referred to as “nutritional intelligence.”

The authors of the current study predicted that the participants of the previous RCT “selected meal components that produced a more balanced combined (meal-level) blend of carbohydrate and fat than the blend found in the individual components from which the meals were derived.”

In addition, they hypothesized that an unprocessed food diet might promote lower energy intake because the meal components may be selected for nutritional qualities (eg, fruits and vegetables) and not for calories — a process called “micronutrient seeking.”

To assess whether participants selected meal components with specific nutritional characteristics (as hypothesized) and how this affected energy intake, the researchers did a post hoc analysis of a 28-day RCT trial in which 20 participants (50% women) followed a 2-week ad libitum UPF diet followed by a 2-week ad-libitum diet of unprocessed food, or vice versa. Participants had a mean age of 31 and a mean BMI of 27.

At breakfast, the ultraprocessed and unprocessed components had a similar carbohydrate-to-fat “blend index” and did not differ significantly, with a weighted mean of 0.485 for ultraprocessed components and 0.494 for unprocessed ones. A blend index can vary from 0 (contains carbohydrate but no fat energy or contains fat but no carbohydrate energy) to 1 (contains equal amounts of carbohydrate and fat energy).

With lunch and dinner, however, the researchers saw a marked difference. In both cases, the ultraprocessed components had a significantly higher carbohydrate-to-fat blend index than the unprocessed components, with a blend index difference of 0.22 at lunch and 0.24 at dinner.

Overall, participants consumed 15.3% more calories in the ultraprocessed diet, equivalent to an additional 330 kcal per participant per day. However, the opposite was true with mass: The unprocessed diet encouraged participants to consume 57% more nonbeverage food, but their total calorie intake remained lower than on the UPF diet.

“Unprocessed meals may reduce energy intake because: 1) they have a less balanced carbohydrate-fat blend, and 2) they promote a form of nutritional intelligence whereby a compromise is struck between consuming calories and consuming micronutrients, which we refer to as ‘micronutrient deleveraging,’” the authors suggested.

With respect to components consumed, when recommended dietary allowance values were summed, the unprocessed diet delivered 36% more micronutrients than the ultraprocessed diet.

Overall, Brunstrom said, “Our findings support the longstanding advice to eat lots of fruit and vegetables. In the absence of ultraprocessed alternatives, this might be easier than people suspect.” 

‘Compelling New Framework’

Commenting on the study for Medscape Medical News, Kelly Issokson, RD, senior dietitian with the University of California San Francisco, said the findings are consistent with prior studies showing that greater intake of UPFs is associated with increased energy intake and weight gain.

While the findings supported the authors’ hypotheses about why this might be so, she said that “additional studies are needed to confirm these results and further elucidate the mechanisms linking food processing, macronutrient ratios, reward signaling, and energy intake. Larger and longer-term trials in diverse populations would improve generalizability.”

Martin Warren, PhD, chief scientific officer and group leader in synthetic biology and biosynthetic pathways at Quadram Institute in Norwich, England, also commented, “While the findings offer a compelling new framework for understanding how modern foods may bypass natural dietary constraints, they are based on post hoc analyses of a small, short-term study and rely on assumptions about optimal macronutrient balance that remain untested. The work therefore opens important new questions about food design and public health, but calls for confirmation in larger, purpose-built trials before firm policy conclusions can be drawn.”

If confirmed, he said, “[T]hese results suggest that effective dietary strategies should focus less on calorie restriction and more on restoring natural nutritional trade-offs.” In practice, this could mean designing meals where micronutrients are delivered primarily through low-energy-dense foods, such as vegetables, “rather than being bundled with calories via fortification and processing.”

For public health and food policy, he added, “Limiting the routine co-formulation of fat and carbohydrate in highly processed foods, and prioritizing minimally processed, component-based meals in schools, hospitals, and workplaces, could help reduce passive overconsumption without relying on individual restraint.”

The study was supported by grants awarded to individual researchers. Brunstrom is partly supported by the National Institute for Health and Care Research (NIHR) Bristol Biomedical Research Centre and NIHR Applied Research Collaboration. Issokson is on the National Board of the Crohn’s & Colitis Foundation and serves as a paid consultant/speaker to Ironwood, Takaeda, and Ajinomoto Cambrooke; she has also received grant funding from the Helmsley Charitable Trust. Warren declared no associated interests with this paper and no sponsorship by any food company. 

Marilynn Larkin, MA, is an award-winning medical writer and editor whose work has appeared in numerous publications, including Medscape Medical News and its sister publication MDedge, The Lancet (where she was a contributing editor), and Reuters Health.