A clinical trial suggests that adopting a diet centered on minimally processed, homemade foods can result in approximately double the weight loss compared to a diet composed of ultra-processed meals and snacks.
Food items are typically classified as ultra-processed when they contain ingredients rarely found in home kitchens, such as high-fructose corn syrup. They may also include additives like flavorings and thickeners, designed to enhance palatability or visual appeal.
Numerous observational studies have associated the consumption of ultra-processed foods with negative health outcomes, including cardiovascular issues, type 2 diabetes, and weight gain. However, these studies have not definitively determined whether the unhealthy effects are attributable to the specific ingredients in ultra-processed foods or to the processing methods themselves. This has led to ongoing debate, especially concerning foods high in sugar, salt, or fat, like cookies and ready-made microwave meals.
To gain a clearer understanding of this effect in the context of weight management, researchers led by Samuel Dicken at University College London conducted a trial. This study involved 55 participants who were either overweight or had obesity. They were randomly assigned to follow either an ultra-processed diet or a minimally processed diet.
While the term “ultra-processed foods” often brings to mind items like pizza and chips, the researchers ensured that both dietary approaches adhered to the UK’s Eatwell Guide. This guide promotes a balanced diet, emphasizing the daily consumption of at least five portions of fruits and vegetables, alongside various protein sources such as beans, fish, eggs, and meat.
Crucially, both diets were carefully matched to contain similar quantities of fat, sugar, salt, and carbohydrates. The study delivered food directly to participants, marking it as the first trial to compare these dietary patterns under real-world conditions, outside of a clinical setting.
The ultra-processed diet group received items such as breakfast cereals, protein bars, chicken sandwiches, and ready-made lasagnes, albeit versions formulated to be lower in fat and salt. Dicken noted that these are the types of foods often advertised with prominent nutritional health claims in supermarkets.
In contrast, the minimally processed diet consisted of homemade options, including overnight oats, chicken salad, bread baked from scratch, and spaghetti bolognese. Participants in both groups were provided with sufficient food to meet approximately 4000 calories daily and were instructed to eat as much as they desired. The study structure involved participants following one diet for eight weeks, followed by a four-week break, and then switching to the other diet for another eight weeks.
Participants were informed that the study aimed to investigate the health effects of balanced meals prepared in different ways, rather than focusing specifically on weight loss. Despite this framing, both diets contributed to participants losing weight. The minimally processed food diet was associated with an average weight reduction of 2 percent, while the ultra-processed diet resulted in a 1 percent reduction.
Dicken commented on these findings, stating, “We saw more weight loss on the minimally processed diet, and it’s not just that, we also saw greater fat loss and also a greater reduction in craving.”
Beyond weight changes, the researchers examined other health indicators. The minimally processed diet was found to reduce body fat content and its levels in the bloodstream. Interestingly, the ultra-processed diet was linked to lower levels of low-density lipoprotein, often referred to as “bad” cholesterol.
However, Ciarán Forde of Wageningen University in the Netherlands raised a point regarding the calorie density. He suggested that the ultra-processed diet may have been more calorie-dense, potentially explaining the observed difference in weight loss. Forde emphasized that the fundamental question remains unaddressed: “what type of processing or ingredient is driving the observed effects.”
Forde also pointed out that weight loss among participants was perhaps unsurprising, given their initial overweight or obese status and the subsequent adoption of a healthy diet. This context suggests that the weight-loss outcomes might not be directly generalizable to the broader population.