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Beyond ‘calories in, calories out’ in treating obesity 

9th August 2023

The impact on calorie intake of portion size, energy density and plant-based and ultra-processed foods was discussed at the American Diabetes Association’s 83rd Scientific Sessions held in San Diego recently. Recent research findings on eating behaviour could help tackle the mounting challenge of obesity more successfully. Dr Susan Aldridge reports. 

Eating behaviour is complex and food intake is affected by several factors, including energy density, portion size and variety, which are relatively well researched. Palatability, macronutrient content, processing and food form also influence food intake, but less is known about these factors. Dr Barbara Rolls of Penn State University said: “Research shows that intake of almost all foods, even fruit and vegetables, is affected by how much is served. People tend to assume that if they overeat in one meal, they will compensate by eating less at another.” This is not so, according to a study she was involved with, where adult participants accumulated an extra 4,000 to 5,000 calories during an 11-day exposure to a diet where food portions were increased by 50%. This lack of a compensatory response was also found in research on young children, previously believed to have an innate ability to self-regulate their food intake.

Behavioural strategies are therefore needed to help people address the issue of portion size for managing food intake. For instance, they can simply be encouraged to eat less, be advised on what specific foods to downsize or consume pre-portioned foods to reduce exposure to large servings. When these three approaches were tested in a clinical trial, those in the pre-portioned group did significantly better than those in the other two groups, maybe because they didn’t have to ‘learn’ anything. Then there was the usual weight-loss plateau and all three ended up losing around 6% of their body weight. 

Focus on energy density

Dr Rolls noted that there was a lot of action on portion size – political activity and initiatives around providing information, including labelling and MyPlate (a nutrition guide published by the United States Department of Agriculture that guides people on how much to eat of each food group) tools. “We need to modify environmental cues, give more choice on portion size and provide ‘to-go’ containers at the start of a meal, which has shown some success,” she said. “But if we could reduce the energy density of the foods people were eating, we wouldn’t have to worry so much about portion size.”

Fat and sugar reduction can, of course, reduce the energy density of food. So, too, can adding water, an approach that is only now coming to the fore. Dr Rolls illustrated this with two trays of food, matched for calories and macronutrient composition, but with the energy density of one reduced by increasing the proportion of water-rich vegetables. “Here’s the surprise – but one which has been replicated in many labs – people tend to eat similar volumes of foods, so if the energy density is less, they will be eating fewer calories spontaneously, and there is no sign of a compensatory response.”

A clinical trial has shown that this approach led to significant weight loss and Dr Rolls’ team also did a secondary data analysis on the PREMIER trial, which compared the DASH (Dietary Approach to Stop Hypertension) diet with advice/counselling following national guidelines on hypertension control. This analysis revealed that energy density was the key factor in weight loss. In another study in children, lower and higher energy diets and a control diet were compared. Those provided with the higher energy density foods ate more and those with lower energy density ate less. 

To show how powerful these effects are, Dr Rolls cited a study on how energy density and portion sizes affected the intake of popular foods, such as chicken, macaroni cheese and apple sauce, in preschool children. The study had a three by two design, with three portion sizes – 100%, 150% and 200% – and higher and lower energy densities – 100% and 142%. These combinations varied in calorie content from 455 to 1290 kcals and the meals all looked similar. The results showed that portion size and energy density combine to affect intake. This meant that between the smallest lower energy density meal and the largest higher energy density meals, there was a 79% higher accumulated calorie intake. “So energy density and portion intake act independently, and can combine and have very powerful effects,” said Dr Rolls. “Eating behaviour is complex and we’ve shown that individual factors can affect intake.” 

Strategies for reducing food intake according to energy density include increasing portions of palatable vegetables and fruit and lower energy-dense foods. Portion sizes of energy-dense foods can also be controlled with strategic use of the MyPlate tool. “We’ve done studies to show this works and it’s very effective to show people how much they can still eat if they lower the energy density of their diet,” Dr Rolls concluded.  

Animal or plant?

Plant-based diets have gained in popularity in recent years, but do they help people lose weight? Christopher Gardner, Rehnborg Farquhar Professor of Medicine at Stanford University, did a quick comparison. Animal foods have more protein and are more energy dense, while plant foods have more carbohydrate and more water. When it comes to nutrients, animal foods contain more calcium, zinc and iron, but plants have more fibre, unsaturated fat and antioxidants. 

On the down side, animal foods have more saturated fats and cholesterol, while plants are the source of added sugars and refined grains. Finally, when it comes to healthy foods, there are fish and yogurt in the animal domain; broccoli and chickpeas in the plant domain. And, on the unhealthy side, breaded chicken and processed red meat, and sodas and pastries. 

Translating this into clinical trials, Professor Gardner cited work at Harvard on healthier and less healthy plant-based diets, which showed that the healthier version is associated with less diabetes. There is also work with the Zoe group in the UK, revealing the striking microbiome differences between healthy and unhealthy animal and plant-based diets.

Plant-based diets

Professor Gardner was lead investigator in the SWAP MEAT (Study With Appetising Plant food Meat Eating Alternative Trial), which was the first significant study of plant-based meat products. In this 16-week crossover trial, 36 adults ate two servings per day of a Beyond Meat (a plant-based meat alternative) product or an organic meat product. As far as nutrients are concerned, the two were very similar. Those on the plant-based meats lost a small but significant amount of weight and lowered their LDL-cholesterol. 

Professor Gardner has also been involved in a comparison study of the impact  on HbA1c of a low-carb, ketogenic diet versus Mediterranean diet. On the keto diet, to get to the required 5% carbohydrate, you need to get rid of fruits, legumes (including beans) and whole grains, which can all be kept in the Mediterranean diet. Both diets avoid added sugars and refined grains, and include non-starchy vegetables – namely, those grown above ground. The central question in the trial was, after eliminating added sugars and refined grains, are there additional benefits to eliminating legumes, fruits and whole grains? 

This was a crossover trial with 20 participants in each arm with prediabetes and diabetes. “Both lowered their HbA1c but there was no difference between them,” said Professor Gardner. “They did it by getting rid of the added sugars.” Other measures, such as lipids, glucose and weight, also improved. However, LDL-cholesterol was worse on the keto diet, which is quite commonly seen because of its high fat content. 

In conclusion, does it matter whether you go for an animal or plant-based diet? On the basis of research so far, Professor Gardner recommends the best of both. “Choose a whole-food, plant-based Mediterranean diet, eliminate added sugar and refined grains, and add yogurt, fish and eggs.”

The challenge of ultra-processed foods

The NOVA classification groups all foods according to their degree of processing – namely unprocessed or minimally processed; processed culinary ingredients, such as sugar or butter; processed food and ultra-processed foods. This last group consists of combinations of ingredients, including either original or chemically modified foods from the fractioning of original foods and various additives. These combinations are designed to be palatable – or even ultra-palatable – convenient and low cost. Therefore, they are ubiquitous and widely consumed, but what is their impact on people’s health? 

“There is a link between consumption of ultra-processed foods, regardless of nutrient content, and obesity, type 2 diabetes, cancer and all-cause mortality,” said Dr Kevin Hall, a researcher at the National Institute of Diabetes and Digestive and Kidney Diseases. These foods are high in fat, sugar and salt and low in fibre, which is what drives people to overeat them. A National Institutes of Health study prepared nutrient-identical, ultra-processed and unprocessed versions of a diet and used them in a two-week crossover study, where leftovers were analysed to see how many calories were consumed of each version. “To feel satisfied, participants needed to eat 500 calories more on the ultra-processed version, which suggests that ultra-processed diets lead to increased intake,” said Dr Hall. Moreover, those on the unprocessed diet lost weight, while those on the ultra-processed version gained both weight and fat. 

However, there was individual variability in how much more those on the ultra-processed version actually ate and those who ate more calories, unsurprisingly, gained more weight. But some people ate the same number of calories on both diets, so they were not susceptible to whatever it is about the ultra-processed diet that is so appealing. 

Digging deeper, more fat and carbohydrate were being consumed in the ultra-processed diet and this was done by eating more calories in the main meals, but not in the snacks. “This is surprising because it’s usually the snacks that we think of as driving overeating,” said Dr Hall. “Everyone has an opinion about what is causing overconsumption of ultra-processed foods. We now need to do a study where we start to take apart these hypotheses.”  

He reviewed various hypotheses. First, maybe people just like ultra-processed meals more. However, in the study above, there were no differences in pleasantness of familiarity. Second, the ultra-processed foods were eaten more quickly, which might lead to eating more calories. Finally, when beverages – added to balance the fibre content of the two meal versions – were taken out of the analysis, the ultra-processed version had much higher energy density, which made the eating rate more complicated because people were then eating more slowly, but were also consuming more calories per minute.  

Further studies, however, suggest that energy density may be only part of the hypothesis around the appeal of ultra-processed foods. Hyper-palatability may also play a part. This is defined as the presence of certain proportions of fat, simple sugars and salt in ultra-processed foods, which drive over-consumption.

Analysis of foods in the ultra-processed versus unprocessed diet trial, described above, showed that 70% of the calories from the former actually fell into the hyper-palatability category. A new study is therefore underway involving four test diets, differing in hyper-palatability and energy density, to tease out the impact of these factors on energy intake. “We don’t know the answer yet, so stay tuned,” Dr Hall concluded.  

To learn more, enrol on the EASD e-Learning course ‘Lifestyle intervention’.

Any opinions expressed in this article are the responsibility of the EASD e-Learning Programme Director, Dr Eleanor D Kennedy.