Energy density and texture are two characteristics that scientists should “zero in on” to elucidate the link between UPFs and weight gain, according to Dr Nicola Guess, research programme manager for a type 2 diabetes remission trial at the University of Oxford.
“Many of the effects of UPFs on energy intake can be explained by what we know already about energy density – the fact that these foods are displacing healthy foods from our diet,” said Dr Guess, addressing an online audience at the BDA’s Are all UPFs created equal? webinar.
“I don’t think the relationship between the palatability, smell and taste of foods and energy intake is as straightforward as you might think, and, based on my reading of the literature, energy density and texture seem to be the key mediators of processing on energy intake.”
Landmark UPF study: The first randomised, controlled trial
Ultra-processed foods crashed into the media spotlight in 2019 when researchers from the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) published a study on the relationship between UPFs and the risk of weight gain.
“This trial really made people stand up and pay attention. It compared diets with 80% of calories from UPFs and 80% of calories from unprocessed foods over two weeks, and what was really shocking was the amount of extra calories people consumed on the UPF diet. It was significant enough that had it continued, they would have experienced very significant weight gain. As it was, they saw a one kilo weight change in two weeks. We don’t often see that magnitude of effect in nutrition studies,” explained Dr Guess.
As well as delivering “striking” results, this study was significant in that it was the first randomised, controlled trial to be carried out in this area. Previous evidence was based on epidemiological data.
“The challenge with observational data is that where the exposure is closely linked to confounding factors that also influence weight gain, it is very difficult to tease out these relationships…What the researchers did well in this study was to match the diets to eliminate some of the confounding factors,” said Dr Guess.
However, she said that one factor they didn’t control for was meal-related energy density – in other words, calories per gram of food.
Energy density part of the UPF equation
“The UPFs had twice as many calories per gram as the unprocessed foods, which may be why people consumed more…There is good evidence to suggest that energy density is linked to energy intake and that the less energy dense the diet, the greater the weight loss.”
Linking this to UPFs, she said: “One consequence of processing is that it might cause food to be more energy dense and therefore cause us to consume more food. This is where we need a more sophisticated understanding of the characteristics of food – processed or not – that could be causing us to overeat.”
Dr Guess gave several examples of research projects that have explored how certain processing characteristics influence energy intake, from umami and saltiness, to sweetness, aroma and – in particular – texture.
“There is a decent amount of data to suggest that the texture of foods affects how much we eat,” she noted.
Dr Guess cited a study in which researchers found that harder foods are consumed at a slower rate than softer foods and resulted in a 33% lower energy intake.
UPF classification 'does not seem to be useful'
“My conclusion is that whether a food is a UPF or not doesn’t seem to be a useful way of describing the healthiness of that food or its propensity to cause overeating,” Guess said.
Dr Duane Mellor, dietitian and senior teaching fellow at Aston University, agreed that the classification of foods as UPFs according to the NOVA system is an over-simplification and is not helpful – potentially even counter-productive – in tackling diet-related health issues.
He gave numerous examples of UPFs that contribute positively to health, from staples that are fortified with vitamin D and folic acid to dietetic foods such as infant formula for children with metabolic conditions.
“Are we mis-serving people by saying they shouldn’t eat UPFs?” he asked rhetorically.
Rather than demonising UPFs, he suggested that we need to “unpack” them.
“There is a lot of chemistry in food that we don’t necessarily understand. We need to remember that chemicals make up foods and unpick how these chemicals are digested, metabolised, processed, and used by the body and how they interact with the microbiome,” he said.
He also shared Dr Guess’ view on the importance of better understanding the structure and texture of UPFs.
“Many aspects of ultra-processing change the food matrix. A big part of this processing is about increasing palatability and improving texture…I think one of the things we need to look at is how we enhance the structure to make it nutritionally positive - in terms of digestibility,” he said.
The way forward: Back to the lab
Given how many unknowns there are around UPFs, Dr Deirdre Tobias, assistant professor in the Department of Nutrition at Harvard University, suggested it was premature for effective policy making.
“Evidence is stacking up for UPFs as a contributing factor in the obesity epidemic but certainty of the causal drivers is needed for effective population level interventions,” she told the webinar audience, adding: “That takes us back to the lab.”
Dr Tobias highlighted two principal areas of UPF research that need “disentangling”.
“We need to narrow it down as there is a lot of heterogeneity; if we are talking about a food group that accounts for 70% of our calories, that covers a lot of different foods. We need to know which foods, or nutritional features, are driving the relationship with obesity,” she said.
“We also need to establish what certainty there is that these foods cause obesity and chronic disease in our population. We are gathering that certainty and precision through research,” she added.
The policy options – which include bans, fortification, economic strategies limiting availability, labelling, education and awareness – can then follow the science, said Dr Tobias.
“It is not the first time public health has had to decide what to do with a diet related issue and it won’t be the last,” she said.
