A diet of highly processed foods causes excess energy intake and weight gain compared with an unprocessed diet, even when diets are matched for energy, sugar, fat, fibre, and macronutrients, a small but tightly controlled study* published in Cell Metabolism has shown. The researchers involved said their results showed that limiting consumption of ultra-processed foods might be an effective strategy for obesity prevention and treatment, although they noted that this might not be equally easy to achieve for everyone.
10 weight-stable healthy men and 10 women, mean body mass index (BMI) 27kg/m2 and mean age 31.2 years, were admitted to the National Institutes of Health Metabolic Clinical Research Unit for the duration of the month-long study. Researchers randomly assigned participants to two weeks of either meals made up of ultra-processed foods, or two weeks of meals comprising minimally processed foods; after two weeks people swapped over. All participants were offered three prepared meals a day, with access to bottled water and either ultra-processed or unprocessed snacks throughout the day. The ultra-processed and unprocessed meals had the same energy content and amounts of sugars, fibre, fat, sodium and carbohydrates.
As an example, an ultra-processed breakfast might comprise Honey Nut Cheerios, whole milk with added fibre, a packaged blueberry muffin, and margarine; and an unprocessed one a parfait made with plain Greek yogurt, strawberries, bananas, walnuts, salt, and olive oil and apple slices with freshly-squeezed lemon.
Participants were told they could eat as much as they wanted, and the researchers measured the quantities they consumed. They reported that people’s energy intake was greater during the ultra-processed diet than the unprocessed diet (mean difference 508kcal/day), with increased consumption of carbohydrate (280kcal/day) and fat (230kcal/day), but not protein (−2kcal/day); food preference was not a factor in this, as participants said both diets tasted good and were satisfying. The researchers suggested several possible reasons for why people ate more food on the processed diet – for example, they ate faster than they did on the unprocessed diet.
In addition, there was a strong correlation between weight change and energy intake – participants gained a mean of 0.9kg during their fortnight on the ultra-processed diet and lost a mean of 0.9kg on the unprocessed diet.
The study authors pointed out that for these participants, neither cost nor preparation time were factors in their food choices, which in the real world would also have an impact on people’s food choices. They commented: “We know there are a lot of factors that contribute to why someone might choose an ultra-processed meal over an unprocessed one. For people in lower socio-economic brackets especially, we need to be mindful of the skills, equipment, knowledge, and expense needed to create unprocessed meals… Just telling people to eat healthier may not be effective for some people without improved access to healthy foods.”
But they concluded: “Limiting consumption of ultra-processed foods may be an effective strategy for obesity prevention and treatment.”
*Hall KD, Ayuketah A, Bernstein S, et al. Ultra-processed diets cause excess calorie intake and weight gain: a one-month inpatient randomised controlled trial of ad libitum food intake. Cell Metabolism. 16 May 2019, DOI:10.1016/j.cmet.2019.05.008