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expert reaction to study looking at processed food, calorie consumption and weight gain

Research published in Cell Metabolism found in diets matched for macro-nutrients and calorie content people consumed more food and gained weight on an ultra-processed diet.


Dr Ian Johnson, Nutrition researcher and Emeritus Fellow, Quadram Institute Bioscience, said:

“This is a fascinating and painstaking piece of research but, because of the many differences in the types of food provided by the unprocessed and ultra-processed diets, it is also very hard to interpret.  For example, although the two diets were carefully matched for energy, nutrients and fibre, the unprocessed meals contained far more fruits, vegetables and whole-grain cereals than the processed meals, and the physical properties of the dietary fibre they provided also differed very significantly.

“This study raises many interesting questions but with so many variables at work I am not convinced that it is useful to attribute the differences in energy consumption simply to the rather broad concept of ‘ultra-processing’.”


Prof Bob Rastall, Professor of Food Biotechnology, University of Reading, said:

“As the study tested was ad libitum consumption, all this research actually shows is that people preferred the processed foods over the supposedly ‘unprocessed’ varieties.  Why would anyone be surprised at this?  They ate more of the ‘ultra-processed’ food, resulting in a higher energy intake and gained weight as a result.  To investigate the effect of food processing you would need nutritionally matched foods in each group and to control the amount eaten – then the only difference would be the degree of processing.

“The study also throws up another troubling issue in relation to the terminology used.  There is no such thing as ‘unprocessed food’, as all food goes through some kind of processing to ensure that it gets from the farm to your fork.”


Dr Gunter Kuhnle, Associate Professor in Nutrition and Health, University of Reading, said:

“This is a well-designed and well conducted study with interesting, although perhaps not surprising, outcomes.  It seems that participants found ultraprocessed food more palatable, ate more quickly and consequently more – possibly because it took longer for them to feel full.  A very interesting outcome of the study is the cost-per-energy: the ultaprocessed diet was considerably cheaper (66%) than the unprocessed control diet, and this is likely to have implications from a public health point of view.

“What is more difficult however is the interpretation of the results.  ‘Processed food’ has become a catch-all phrase to describe ‘unhealthy’ food, when most foods are processed – and processing is usually important for palatability, safety and preservation.  According to the NOVA classification, which has been used by the authors to categorise foods, chilling, freezing or packaging is already a processing step – and for example butter, cheese or bread are processed foods.  The ‘ultraprocessed’ category is somewhat arbitrary, as it categorises foods not by the actual processing steps used, but rather the intended outcomes of these processes (“The overall purpose of ultra-processing is to create branded, convenient, attractive and highly profitable food products designed to displace all other food groups.”).

“For example pre-prepared frozen meals are categorised as ‘ultra-processed’, even though their production might have fewer processing steps than foods in the ‘processed’ category.  As the author acknowledge, the NOVA classification has therefore been extensively criticised.”



‘Ultra-processed diets cause excess calorie intake and weight gain: an inpatient randomized controlled trial of ad libitum food intake’ by Kevin D. Hall et al. was published in Cell Metabolism at 16:00 UK time on Thursday 16 May 2019.


Declared interests

Dr Ian Johnson: “No conflicts to declare.”

Prof Bob Rastall: “I don’t have any specific interests to declare – the department works very closely with the food industry of course in terms of student placements, our industry advisory board and research contracts.”

Dr Gunter Kuhnle: “No DOI.”

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