One problem in determining just what about ultra-processed food is bad is that such foods are typically low in nutrients, so it’s hard to know whether the problems UPFs may cause is due to nutritional deficiencies, or to something about the processing itself.
A paper by Dicken et al. was published early this month in Nature Medicine and has addressed this by comparing “healthy” ultra-processed foods (UPFs) and minimally-processed foods (MPF) on measures of weight loss and related measures. All of the foods provided to the participants, both UPFs and MPF, met the standards for “healthy diet” in the UK (the “Eatwell guide”).
The study had a “crossover” design, such that everyone in the study did 8 weeks of each diet, with a “wash out” period in between. This design reduces effects of individual differences in vulnerability or response to the diets.
As this was a study containing all “healthy” foods, there were only a few differences. The minimally processed foods led to more weight loss, and less body fatness, although the usual metabolic measures (cholesterol, blood pressure etc.) were similar. The most interesting thing to me, though, were the reports of “adverse symptoms/events”. When on the ultra-processed “healthy” diet, participants reported gut symptoms, especially constipation and gastrointestinal reflux (GERD) as well as mood symptoms and especially fatigue, at much higher rates than when they were on the minimally processed diet.
This indicates that anyone concerned about reducing gut and mood symptoms should avoid ultra-processed foods, even if they claim to be “healthy” (such as protein bars etc.).