Ultra-Processed Foods Cause Weight Gain

Every now and then, a landmark study comes along that definitively answers an important question, and, perhaps more importantly, lays to rest many of the theories that float around both in the scientific literature and the lay media.

It would perhaps not be superlative to note that just such a study by Kevin Hall and colleagues (presented just a few weeks ago at the 6th Biennial Canadian Obesity Summit) has now been published in Cell Metabolism.

The study examines what happens to calorie intake and body weight when people have free access to a diet largely composed of ultra-processed foods vs. a diet composed of unprocessed foods. Ultra-processed foods have been described as “formulations mostly of cheap industrial sources of dietary energy and nutrients plus additives, using a series of processes” and containing minimal whole foods.

Importantly, the two diets were carefully matched for presented calories, energy density, macronutrients, sugar, sodium, and fiber. Although protein, carbohydrate, and fat content were virtually identical, the ultra-processed foods differed substantially from the un-processed foods in the proportion of added to total sugar (∼54% versus 1%, respectively), insoluble to total fiber (∼77% versus 16%, respectively), saturated to total fat (∼34% versus 19%), and the ratio of omega-6 to omega-3 fatty acids (∼11:1 versus 5:1).

The 20 weight stable healthy participants, who spent over four weeks in an in-patient metabolic ward, were instructed to consume as much or as little of the foods offered with one diet over a two week period before switching to two weeks of the other diet (in a random cross-over fashion).

In short, during the 2nd week of eating the ultra-processed diet, subjects consumed about 500 kcal more per day than during the 2nd week of the unprocessed diet. This was accompanied by an almost 2 lb weight gain on the ultra-processed diet (whereas weight reduced by about the same measure on the unprocessed diet). This response was seen irrespective of which diet came first or of the baseline BMI of participants.

To set this study in perspective, there have long been theories about how the increased availability of ultra-processed foods may be playing a causal role in the obesity epidemic. Thus, as the authors point out, “Ultra-processed foods may facilitate overeating and the development of obesity because they are typically high in calories, salt, sugar, and fat and have been suggested to be engineered to have supernormal appetitive properties that may result in pathological eating behavior. Furthermore, ultra-processed foods are theorized to disrupt gut-brain signaling and may influence food reinforcement and overall intake via mechanisms distinct from the palatability or energy density of the food.” As plausible as these “theories” many seem, to date, this hypothesis has never been tested in a well-controlled randomised trial.

The increased caloric consumption on the ultra-processed diet was not explained by greater palatability or familiarity of the ultra-processed diet, or differences in reported hunger, fullness, satisfaction, and capacity to eat. However, the rate of eating (measured as calories or grams per minute) was higher on the ultra-processed diet (as I have noted previously, the problem with fast food is more the “fast” than the “food”).

As the authors note, “The perpetual diet wars between factions promoting low-carbohydrate, keto, paleo, high-protein, low-fat, plant-based, vegan, and a seemingly endless list of other diets have led to substantial public confusion and mistrust in nutrition science. While debate rages about the relative merits and demerits of various so-called healthy diets, less attention is paid to the fact that otherwise diverse diet recommendations often share a common piece of advice: avoid ultra-processed foods.” Clearly this study strongly supports the idea that cutting ultra-processed foods from your diet will likely help avoid excess caloric intake and subsequent weight gain.

There is however one caveat: based on the cost of ingredients obtained from a local supermarket, the weekly cost for ingredients to prepare 2,000 kcal/day of ultra-processed meals was estimated to be $106 versus $151 for the unprocessed meals. This would mean a food bill that is 50% higher for the average household. In addition, there is a time cost for meal preparation (and chewing) of the unprocessed food diet.

Be that as it may, any diet that reduces the amount of ultra-processed foods in your diet is likely to help you better manage your caloric intake and reduce inadvertent weight gain. Whether or not such a diet can be implemented at a population level remains to be seen but clearly the more dependent we remain on ultra-processed foods, the harder it will be to contain the obesity epidemic.

Berlin, Germany