TW: This post (and the articles linked from it) contain specific weight numbers, o-words and some fatphobic/healthist language.
Last week, the journal Cell Metabolism published a study on the effect of ultra-processed foods on weight. If you were only to look at the media and “nutrition Twitter,” you would’ve thought they’d found some sort of smoking gun. Science quotes a researcher (not involved in the study) calling it “a landmark first,” while Gizmodo claims that it “confirms your worst suspicions about processed foods.” Sociologist Sarah Bowen tweeted, “Michael Pollan is vindicated; processed foods seem to be bad.”
So what does the study really say? Do we really have to swear off ultra-processed foods forever, or is this yet another case of the media misrepresenting the science?
What Actually Happened in the Study
While the research on ultra-processed food thus far has been based on observational studies—in other words, observing people’s eating patterns along with different outcomes and trying to draw correlations—this study is the first clinical trial to compare an “ultra-processed” diet with an “unprocessed” diet.
Twenty adults (10 men, 10 women) were admitted to a metabolic ward for 28 days—half of them ate an “ultra-processed” diet for two weeks, while the other half ate an “unprocessed” diet, and then they switched for the remaining two weeks. “Ultra-processed” and “unprocessed” were defined based on the NOVA system.
For both diets, the meals were presented three times a day, and people were given an hour to eat however much they wanted. Snacks for the day were placed in boxes in each subject’s room, so they could have them at any time. Both diets provided a similar amount of calories, carbohydrates, fat, and protein. The meals were also matched for fibre, sugar, and sodium, but this was not the case for the snacks—the unprocessed snacks, which were basically fruit and nuts, were much higher in fibre and sugar, while the processed snacks (Goldfish crackers, baked chips, applesauce, etc.) were much higher in sodium. Overall, the ultra-processed diet was higher in saturated fat, added sugar, and insoluble fibre, and lower in omega-3s compared to the unprocessed diet as well. Photos of meals and snacks from both menus were included in the study.
In the end, the subjects tended to eat more while on the ultra-processed diet than the unprocessed diet in terms of calories, carbohydrates, and fat, and they also tended to eat faster. While on the ultra-processed diet, average weight gain was 0.9 kg (2 lb), and with the unprocessed diet, average weight loss was 0.9 kg (2 lb). The change in weight was about half from body fat and half from fat-free mass in both cases, and the researchers noted that fluid shifts due to the differences in sodium in the two diets likely played a role.
Subjects rated their hunger, fullness, satisfaction and appetite similarly between the two diets; pleasantness and familiarity of the two diets were given similar ratings as well. Energy expenditure, blood glucose levels, and insulin sensitivity were not statistically different between the two diets either.
What the Study Doesn’t Say
We don’t really know why the subjects ate more calories and ate faster on the ultra-processed diet compared to the unprocessed diet, but that hasn’t stopped people (including the researchers) from blaming it on the processing. In terms of eating speed, this makes sense—processing can make foods easier to eat and digest. Looking at the photos of the meals, I can imagine it taking less time for me to eat a sandwich on white bread than chewing through a large, leafy salad.
One thing that I find interesting is that the amount of food eaten by the subjects was reported as calories, not volume. When I compare the meals between the two diets, it looks like there is a lot more food in the unprocessed meals in order to be able to match the amount of calories and nutrients available.
For example, here is lunch on Day 4 for the ultra-processed and unprocessed diets, respectively (which Science conveniently put side-by-side):
While I personally could see myself eating the two hot dogs with the condiments, drinking the juice and eating some (if not all) the chips and yogurt, the lunch on the right looks overwhelming to me, and I would likely end up leaving a bit of everything, so I definitely wonder how food volume and/or the way the food was presented might have played a role.
On a related note, even though the study title claims that this was ad libitum intake, the meals were provided at set times and subjects were given a limited (though generous) amount of time to eat. Could this structured eating schedule have affected intake, especially given the larger volumes of food presented in the unprocessed meals?
Finally, although subjects with active eating disorders or strict dietary concerns were excluded from the study, I wonder how disordered eating or relationship with food (i.e. internalized food rules) might have affected what and how much was eaten.
What Does This Study Mean for the Real World?
Even the study authors admit that “the inpatient environment of the metabolic ward makes it difficult to generalize [the] results to free-living conditions” amongst other limitations, like the small number of subjects, the short length of the trial, the lack of a run-in period before the study, and the lack of a washout period between diets.
What upsets me the most about how this trial has been picked up by the media is the way that it has perpetuated fatphobia and demonization of foods. Headline after headline claims that this study proves that “ultra-processed food makes us fat” or “causes obesity”. The average baseline weight of the participants was 78.2 kg, so a 0.9 kg increase represents a 1% weight gain. The subjects were considered to be “weight stable” at baseline, which was defined as a < 5% weight change within the past six months. Even though the change in weight seen in the study was statistically significant, it would not be considered to be clinically significant. It should also be noted that the study didn’t show any significant changes in health between the ultra-processed and unprocessed diets.
What also worries me is that this study could be co-opted to justify mindful eating as a weight loss intervention. One of the most common misconceptions of mindful eating is that it’s about eating more slowly. In The Mindful Eating Workbook, I talk about the idea of “sensing, not slowing.” Mindful eating is defined as being aware of the eating experience, without judgment. Though many people end up eating more slowly in order to help them engage their senses to become more present, it isn’t absolutely necessary to eat slowly in order to be mindful, and mindful eating is meant to be a weight-inclusive practice.
While I respect that this study is the first of its kind, I think it’s too early to say that it has proved anything, let alone drive any changes in practice or policy. Even if down the road a more powerful study with more subjects, a longer intervention, and better-matched meals really showed that ultra-processed foods were detrimental to health, the solution shouldn’t be about telling individuals to avoid ultra-processed foods. Many (including the study authors) have commented that the financial and time cost of adopting an unprocessed versus ultra-processed diet may make it out of reach for many folks; the authors estimate that the weekly cost of ingredients would be $106 for the ultra-processed diet, versus $151 for unprocessed—a difference of over 40%. And we haven’t even gone into the psychological and behavioural consequences of deprivation, or looked at eating patterns that fall between the two extremes.
Solutions that would actually make an impact on people’s health will likely require systemic and societal change that addresses issues like poverty and oppression.The 'best diet' can't truly improve health if it doesn't also address poverty and oppression. Click To Tweet