Eating minimally processed meals doubles weight loss even when ultraprocessed foods are healthy, study finds

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People in the United Kingdom lost twice as much weight eating meals typically made at home than they did when eating store-bought ultraprocessed food considered healthy, the latest research has found.
'This new study shows that even when an ultraprocessed diet meets nutritional guidelines, people will still lose more weight eating a minimally processed diet,' said coauthor Dr. Kevin Hall, a former senior investigator at the US National Institutes of Health who has conducted some of the world's only controlled clinical trials on ultraprocessed foods.
'This (study) is the largest and longest randomized controlled clinical trial of ultraprocessed foods to date,' Hall added.
Hall's past research sequestered healthy volunteers from the world for a month at a time, measuring the impact of ultraprocessed food on their weight, body fat and various biomarkers of health. In a 2019 study, he found people in the United States ate about 500 calories more each day and gained weight when on an ultraprocessed diet than when eating a minimally processed diet matched by calories and nutrients.
The weight loss from minimally processed food in the new study was modest — only 2% of the person's baseline weight, said study first author Samuel Dicken, a research fellow at the department of behavioral science and health and the Centre for Obesity Research at University College London.
'Though a 2% reduction may not seem very big, that is only over eight weeks and without people trying to actively reduce their (food) intake,' Dicken said in a statement. 'If we scaled these results up over the course of a year, we'd expect to see a 13% weight reduction in men and a 9% reduction in women.'
Men typically have more lean muscle mass than women, which along with testosterone often gives them a quicker boost over women when it comes to weight loss, experts say.
The study, published Monday in the journal Nature Medicine, provided free ultraprocessed or minimally processed meals and snacks to 55 overweight people in the UK for a total of eight weeks. After a short break, the groups switched to the opposite diet for another eight weeks.
Study participants were told to eat as much or as little of the 4,000 daily calories as they liked and record their consumption in a diary. By the end of the study, 50 people had spent eight weeks on both diets. While the number of participants may seem small at first glance, providing 16 weeks of food and implementing randomized controlled clinical trials can be costly.
For the first eight weeks, 28 people received daily deliveries of minimally processed meals and snacks, such as overnight oats and homemade spaghetti Bolognese.
Minimally processed foods, such as fruits, vegetables, meat, milk and eggs, are typically cooked from their natural state, according to NOVA, a recognized system of categorizing foods by their level of processing.
Concurrently, another 27 people received a daily delivery of ultraprocessed foods — such as ready-to-eat breakfast bars or heat-and-eat lasagna — for eight weeks.
Ultraprocessed foods, or UPFs, contain additives never or rarely used in kitchens and often undergo extensive industrial processing, according to the NOVA classification system.
Because ultraprocessed foods are typically high in calories, added sugar, sodium, and saturated fat and low in fiber, they have been linked to weight gain and obesity and the development of chronic conditions including cancer, cardiovascular disease, type 2 diabetes and depression. Such foods may even shorten life.
Researchers in this study, however, did something unusual, said Christopher Gardner, Rehnborg Farquhar Professor of Medicine at Stanford University in California who directs the Stanford Prevention Research Center's Nutrition Studies Research Group.
'They tried to make a healthy ultraprocessed diet by picking ultraprocessed foods with the recommended number of fruits, veggies and fiber and lower levels of salt, sugar and saturated fats,' said Gardner, who was not involved in the study.
Both the ultraprocessed and the minimally processed meals had to meet the nutritional requirements of the Eatwell Guide, the UK's official government guidance on how to eat a healthy, balanced diet. The United States has similar dietary guidelines, which are used to set federal nutritional standards.
'This is a very solid study, matching dietary interventions for nutrients and food group distribution, while varying only the contribution of ultra-processed foods,' said Dr. David Katz, a specialist in preventive and lifestyle medicine, in an email. Katz, who was not involved in the study, is the founder of the nonprofit True Health Initiative, a global coalition of experts dedicated to evidence-based lifestyle medicine.
The study's goal was weight loss, which often comes with improved cardiovascular readings, such as lower blood pressure, cholesterol and blood sugar levels.
That happened, but in rather odd and surprising ways, said Marion Nestle, the Paulette Goddard professor emerita of nutrition, food studies and public health at New York University, who was asked to write an editorial to be published with the study.
Instead of gaining weight, people on the ultraprocessed diet chose to eat 120 fewer calories a day, thus losing a small amount of weight. People on the minimally processed diet, however, ate 290 fewer calories a day, thus losing even more weight and some body fat as well.
'One possible explanation is that (people on the minimally processed diet) did not like the 'healthy' meals as much as their usual diets,' Nestle, who was not involved in the research, wrote in the editorial.
'They deemed the minimally processed diet less tasty,' Nestle said. 'That diet emphasized 'real' fresh foods, whereas the ultra-processed diet featured commercially packaged 'healthy' ultra-processed food products such as fruit, nut, and protein bars; sandwiches and meals; drinking yoghurts, and plant-based milks.'
Less than 1% of people in the UK follow all of the government's nutritional recommendations, according to the study, often choosing ultraprocessed foods as the basis of their normal daily intake. In the US, nearly 60% of an adult's calorie consumption is from ultraprocessed foods.
'People in this study were overweight or obese and were already eating a diet high in all kinds of ultraprocessed foods,' Gardner said. 'So the ultraprocessed diet in the study was healthier than their typical normal diet. Isn't that an odd twist?'
People on the minimally processed diet had lower levels of triglycerides, a type of fat in the blood linked to an increased risk of heart disease and stroke, but other markers of heart health didn't vary much between the two diets, according to the study.
There was one notable exception: low-density lipoprotein, or LDL, known as 'bad' cholesterol because it can build up in arteries and create blockages to the heart.
'Surprisingly, LDL cholesterol was reduced more on the ultra-processed diet,' said dietitian Dimitrios Koutoukidis, an associate professor of diet, obesity and behavioral sciences at the University of Oxford, who was not involved in the study.
'This might imply that processing is not as important for heart health if the foods already meet the standard UK healthy eating guidance,' Koutoukidis said in a statement. 'Further research is needed to better understand this.'
According to Hall, the results fit quite nicely with preliminary results from his current study that is still underway. In that research, Hall and his team measured the impact of four configurations of ultraprocessed foods on the health of 36 volunteers. Each lived for a month in the Metabolic Clinical Research Unit of the National Institutes of Health Clinical Center in Bethesda, Maryland.
'When you modify an ultraprocessed diet to have lower energy (calorie) density and fewer highly palatable foods, you can offset some of the effects of ultraprocessed foods in causing excess calorie intake and weight gain,' Hall said.
In other words, choose healthier foods regardless of the levels of processing.
'People don't eat the best ultraprocessed foods, they eat the worst ones, so the take home here is to follow the national guidelines for nutrient quality,' Gardner said.
'Read your nutrient label and choose foods that are low in salt, fat, sugar and calories and high in fiber, and avoid foods with too many additives with unpronounceable names. That's the key to a healthier diet.'
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