The volunteers were fed two different types of food: ultra-processed, i.e. containing flavourings and other additives (sausages, hot dogs, sweet yoghurts), and minimally processed, containing almost none (vegetables, porridge, grilled meat). As a result, each participant in the first group gained an average of one kilogram in just two weeks, while those in the second group lost weight. Recently, nutritionists have become increasingly concerned about so-called ultra-processed foods, a term coined by Brazilian professor Carlos Monteiro to describe ready-made foods that contain various flavour additives, flavourings, flavour stabilisers and the like. Previous studies involving thousands of people have shown that eating such foods is associated with health problems, including an increased risk of cancer, obesity and even premature death. However, scientists have not yet been able to establish a causal link between eating such foods and harm. Experts from the US National Institute of Diabetes and Digestive and Kidney Diseases have attempted to do so. In their new study, they compared the condition of people after two types of diets: one based on ultra-processed foods and the other on minimally processed foods. The latter may consist of frozen or dried foods, cooked or vacuum-packed, but should not contain any additives, including sugar, salt and oil. Twenty healthy volunteers participated in the experiment – 10 men and 10 women. They had to live in a clinical centre at the National Institute of Health for a month and eat only what the scientists offered them. One group was given two weeks of ultra-processed food – cereals, sweet yoghurts, canned pasta, hot dogs and so on. The second group was given minimally processed food: oatmeal, steamed vegetables, grilled chicken, and vegetable salads. Nutritionists ensured that in both cases, the diet contained the same amount of calories, sugar, fat, dietary fibre, and sodium. The participants in the experiment could eat as much as they wanted. After two weeks, the groups switched diets. The researchers determined how much was eaten by weighing everything left on the table after each meal, down to every portion of ketchup that didn’t make it onto a hot dog. The results showed that those who ate ultra-processed foods consumed an average of 500 more calories per day than their peers over the two-week period. Each of them gained an average of 900 grams. At the same time, those who ate minimally processed foods lost about the same amount of weight. The top graph shows the number of kilocalories per day consumed by the participants in the experiment. The bottom graph shows the weight they gained during the experiment. The X-axis shows the days of each diet. Picture from Hall et al. Ultra-Processed Diets Cause Excess Calorie Intake and Weight Gain: An Inpatient Randomised Controlled Trial of Ad Libitum Food Intake, published in the journal Cell Metabolism. Blood tests of the subjects showed that people on an ‘ultra-processed’ diet had higher levels of the appetite-suppressing hormone PYY. Levels of the appetite-stimulating hormone ghrelin were lower. However, the scientists were unable to determine how these changes were related to the quality of food processing. Scientists suggest that people ate more ultra-processed foods because, first, they are bright and attractive. Second, such foods are softer, making them easier to chew and swallow. When people eat quickly, they tend to eat more because they miss the moment when they feel full. In addition, despite the efforts of scientists to make both types of diets equal in calories and nutrients, ultra-processed foods usually contain less protein. At the same time, people feel full after eating a certain amount of protein, so they need to eat more of this type of food to get the recommended amount. Thus, scientists have concluded that highly processed foods lead to weight gain, even if they contain a balanced amount of protein, fat and carbohydrates.