“These are landmark findings that processing of foods makes a huge difference in how much a person eats” – Dariush Mozaffarian, dean of Tufts University’s Friedman School of Nutrition Science and Policy

Participants when on the ultra-processed diet on average consumed over 500 calories more a day. This diet increased body weight. The whole and minimally processed diet decreased body weight. Body fat also increased on the ultra-processed diet and decreased on the whole and minimally processed diet. The nutrient profiles of both diets were practically identical.

What is the dietary cause of overweight and obesity, which are now pandemic and increasing rapidly especially in middle- and low-income countries throughout the world? (1). A study just published in the journal Cell Metabolism (2), which has a huge impact factor of 20.55, shows that eating an ultra-processed diet rather than a minimally processed diet high in fruits, vegetables, grains, and fresh meat and fish, increases dietary energy consumption and causes increase in weight.

On publication the study and its results gained massive media coverage. Business Insider is the US financial and business news website with international editions in the major European and Asian countries, Australia and  South Africa, with a web impact said to be as high as that of the Wall Street Journal (3).In a leading story it reported “This week a ground-breaking study from the National Institutes of Health found that people on ultra-processed diets ate more calories and gained more weight than they did when offered the same amount of nutrients from less processed food” (4). The National Institutes of Health (NIH) is the lead agency of the United States government responsible for biomedical and public health research.

This finding is not new. Very many published studies using the NOVA classification, developed since 2010 at the University of Sāo Paulo in collaboration with research teams in many countries (5), show this effect. But the power and impact of the new study is that it is a randomized controlled trial (RCT), the type generally agreed to be the best for showing causal relationships.

It has been conducted byKevin Hall of the National Institute of Diabetes, Digestive and Kidney Diseases at NIH, working with 24 co-investigators, at the NIH Metabolic Clinical Research Unit.  It is the first RCT to compare the effects of consuming diets solely made up of non-ultra- processed foods  with those made up almost entirely (83 per cent of total dietary energy) of ultra-processed foods.

As explained by NIH director Francis Collins, Hall and his team used the “NOVA diet classification system. This system categorizes food based on the nature, extent, and purpose of food processing, rather than its nutrient content” (6). As set out fully by Business Insider , NOVA “classifies everything we eat as one of four categories: unprocessed or minimally processed, processed culinary ingredients, processed foods, and ultra-processed food and drink products”(4). Business Insider also gave examples of all the four NOVA categories, and included a video showing examples of some ultra-processed products and the ingredients they contain (7).

The two diets were designed so that their nutrient profiles were practically identical, with the same amounts of dietary energy, fat, carbohydrate (starches, sugars, dietary fiber), protein, and other nutrients.  The 20 participants, 10 men and 10 women in their early 30s, were treated as hospital patients, not allowed to leave the facility. They were served twice the amount of food they needed to maintain their weight, so they could eat as much as they liked until they felt full, and they exercised every day for an hour. The meals were presented attractively, as evident in the picture shown above, and participants rated each type as equally familiar and pleasant. All the participants ate one type of diet for two weeks, and then changed to the other diet for two weeks.

“The original purpose of our study was to try to answer the question of whether it’s the nutrient composition of the foods, or something about the processing of them, that’s leading people to overeat” said Hall to Sally Wodyka of Consumer Reports (8). I thought if we matched for those nutrients, there would be very little difference in calorie intake. I was wrong.”

Speaking to Mandy Oaklander for Time magazine (9), Hall said “It’s the first trial that can actually demonstrate that there is a causal relationship between something about ultra-processed foods – independent of those nutrients – that cause people to overeat and gain weight”. The result was that eating a diet made up almost entirely of ultra-processed foods evidently drives people to overeat and gain weight compared with a diet made up of whole and minimally processed foods. When on the ultra-processed diet, study participants ate an average of 508 calories more per day and ended up increasing an average of over 2 pounds (0.9 kilograms) in two weeks. When on the whole and minimally processed diet they reduced on average over 2 pounds (also 0.9 kilograms) in the same time period.

Participants when on the ultra-processed diet on average consumed over 500 calories more a day. This diet increased body weight. The whole and minimally processed diet decreased body weight. Body fat also increased on the ultra-processed diet and decreased on the whole and minimally processed diet. The nutrient profiles of both diets were practically identical.

Speaking to Maria Godoy of US National Public Radio (10), Dariush Mozaffarian, dean of Tufts University’s Friedman School of Nutrition Science and Policy, said “These are landmark findings, that processing of foods makes a huge difference in how much a person eats. Putting people in a controlled setting and giving them their food lets you really understand biologically what’s going on, and the differences are striking”.

Barry Popkin of the University of North Carolina at Chapel Hill, who like Dariush Mozaffarian was not involved in the NIH study, speaking to Anahad O’Connor of The New York Times (11) and to Maria Godoy (10), said “The difference in weight gain for one group and weight loss for the other during these two periods is phenomenal. This is a very important study and a major challenge to the global food industry and the food science profession”.

“I wouldn’t have expected the results to be this striking” Kathleen Page, an endocrinologist specializing in diabetes and childhood obesity at the University of Southern California, also not involved in the study, speaking to Emily Baumgaertner of the Los Angeles Times (12), she said of the participants “To consume that much more food and not even call it more palatable or pleasant – I don’t think they realise they’re doing it.”

Ultra-processed foods on average contain more sugar and fat than other types of food. But the new study shows that this is not the only reason for weight increase. This is a remarkable and important finding.  So what is it about ultra-processed foods that causes weight increase? One theory is that these foods are soft and easy to eat, and the participants did eat them faster. As Hall explained to Mandy Oaklander (9), according to this theory “you’re not giving your gut enough time to signal to your brain that you’ve had enough calories and that you’re full and to stop eating. By the time the brain gets that signal, it’s too late—you’ve already overeaten.”

Another theory is that the methods of processing used to make ultra-processed foods, plus the use of additives, is the cause. A third and related theory is that many ultra-processed foods are designed to be over-consumed, and even quasi-addictive, as industry executives confessed to Michael Moss for his 2013 book Salt Sugar Fat (13).

In support of the related second and third theories, Hall said to Maria Godoy “One thing that was kind of intriguing was that some of the hormones that are involved in food intake regulation were quite different between the two diets as compared to baseline” (10). When the participants were eating the minimally processed diet, they had higher levels of the hormone PYY (peptide tyrosine tyrosine), secreted by the gut, which reduces appetite, and lower levels of ghrelin, a hunger hormone. This could well be the physiological reason why they consumed less dietary energy.  On the ultra-processed diet, participants had lower levels of the appetite suppressing hormone and higher levels of the hunger hormone.

“This is not about willpower – we’re living in a manipulated food environment,” said Ashley Gearhardt, Associate Professor of Psychology specialising in food addiction at the University of Michigan, also not involved in the study, speaking to Katherine Wu of the US Public Broadcasting Service (14). “Ultra-processed foods are unique in ways that we are only just starting to understand. I can’t think of another study that has been this well controlled for so long. This allows us to make much more confident interpretations of what these foods are really doing.”

Carlos Monteiro, leader of the Sāo Paulo team that originated and developed the NOVA system in consultation with co-investigators in other countries that has enabled Hall’s study, also not involved in the project, said to Discover magazine (15) that Hall “gave us two gifts. One is showing that there is a causal relationship between ultra-processed foods and weight gain. The second is that reformulation will not work”. This is important, because a strategy now proposed by manufacturers of highly profitable ultra-processed foods is to adjust their formulations, to contain more protein and dietary fibre and less sugar and/or fat and/or salt. But now it is evident that not just the nutrient profiles of ultra-processed foods cause weight gain, overweight and over time, obesity. Reformulated, they would remain ultra-processed.

Meanwhile for consumers “The best advice is to limit your intake of ultra-processed foods as much as possible,” said Qi Sun, assistant professor of nutrition at the Harvard TH Chan School of Public Health, speaking to Sally Wadyka (8). “What all healthy diets have in common is they emphasize eating whole, unprocessed foods”.  NIH director Francis Collins agrees: “It appears that a good place to start in reaching or maintaining a healthy weight is to… work to eliminate or at least reduce ultra-processed foods in your diet in favor of a balanced variety of unprocessed, nutrient-packed foods”(6).


  1. Monteiro C, Moubarac J-C, Cannon G, et al. Ultra-processed products are becoming dominant in the global food system. Obesity Rev 2013, 14, supp 2, 14-21. doi: 10.1111/obr.12107
  2. Hall K, Ayuketah A, Brychta R et al. Ultra-processed diets cause excess calorie intake and weight gain: an inpatient randomised controlled trial of ad libitum food intake. Cell Metabolism 2019, 30, 1-10. doi: 10.1016/j.cmet. 2019.05.008
  3. Carr D. Ezra Klein is joining Vox Media as web journalism asserts itself. The New York Times, January 26 2014. https://www.nytimes.com/2014/01/27/business /media/ezra-klein-joining-vox-media-as-web-journalism-asserts-itself.html
  4. Breuck H. Processed foods make us fatter, lead to cancer, and are linked with early death. But what exactly is a processed food? Business Insider, 16 May 2019. https://www.businessinsider.com/what-is-processed-food-2019-5
  5. Monteiro C, Cannon G, Moubarac J-C. et al. The UN Decade of Nutrition, the NOVA food classification, and the trouble with ultra-processing. Public Health Nutr 2018, 21, 1, 5-17. doi:10.1017/S1368980017000234  
  6. US National Institutes of Health, 21 May 2019. Collins, F. Ultra-processed diet leads to extra calories, weight gain. https://directorsblog.nih.gov/ 2019/05/21/ultra-processed-diet-leads-to-extra-calories-weight-gain/
  7. Business Insider,16 May 2019. This stunning visualization breaks down all the ingredients in your favorite processed foods. https://www.businessinsider.com/ingredients-processed-foods-red-bull-twinkies-doritos-kraft-cheese-2016-10
  8. Wadyka S. Processed foods are bad for weight loss, study shows. Consumer Reports, 16 May 2019. https://www.consumerreports.org/packaged-processed-foods/processed-foods-are-bad-for-weight-loss/
  9. Oaklander M. What eating processed foods for two weeks does to your body. Time, 16 May 2019. http://time.com/5589702/processed-foods-weight-gain-diet/?utm_source=time.com&utm_medium=email&utm_campaign=the-brief&utm_content=2019051710am&xid=newsletter-brief
  10. Godoy M. It’s not just salt, sugar, fat. Study finds ultra-processed foods drive weight gain. The Salt (National Public Radio). 16 May 2019 https://www.npr.org/sections/thesalt/ 2019/05/16/723693839/its-not-just-salt-sugar-fat-study-finds-ultra-processed-foods-drive-weight-gain
  11. O ‘Connor A. Why eating processed foods might make you fat. The New York Times, 16 May 2019. https://www.nytimes.com/2019/05/16/well/eat/why-eating-processed-foods-might-make-you-fat.html
  12. Baumgaertner E. Eating ultra-processed foods will make you gain weight. Here’s the scientific proof. Los Angeles Times, 16 May 2019. https://www.latimes.com /science/la-sci-processed-foods-weight-gain-clinical-trial-20190516-story.html
  13. Moss M. Salt Sugar Fat. How the Food Giants Hooked Us. New York: Random House, 2013. eISBN: 978-0-679-60477-8
  14. Wu K. Ultra-processed foods make us eat more, and it’s not about their nutritional makeup. US Public Broadcasting Service, 16 May 2019. https://www.pbs.org/ wgbh/nova/article/ultra-processed-foods-weight-gain/
  15. Groves A. Processed foods, regardless of nutrition, still cause weight gain. Discover, 17 May 2019. http://blogs.discovermagazine.com/crux/2019 05/17/ultra-processed-foods-weight-gain-nutrition-diet/#.XOAz0chKjIU/