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Study links minimally processed diet to greater weight loss than UPF

A crossover trial finds differences in weight, fat loss, and appetite between minimally and ultra-processed diets, despite both adhering to official UK nutritional guidelines.

The results of the eight-week trial were published by Dicken et al in Nature Medicine in early August.

Researchers enrolled 55 adults in England with high habitual ultra-processed food (UPF) intake. All participants followed minimally processed food (MPF) and UPF diets in a random order, with all meals provided. Each subject received 4,000 kcals worth of food per day and was instructed to eat until they were full.

The UPF diet consisted of reformulated, nutritionally improved foods commonly available in UK supermarkets, while the MPF diet was composed of minimally processed culinary preparations. In both cases, the diet aligned with the National Health Service’s Eatwell Guide, which suggests a “healthy, balanced diet”.

Participants lost weight on both diets, but more so in the MPF condition. The mean reduction in body weight was 2.06% on the MPF diet versus 1.05% on the UPF diet; a statistically significant difference. Fat mass, body fat percentage, and visceral fat also decreased to a greater extent on the MPF diet.

UPFs are defined by the Nova classification as industrial formulations containing additives and ingredients not used in home cooking, and account for over 50% of energy intake in the UK. They have been linked to obesity and other adverse health outcomes in observational studies, although evidence from intervention trials remains limited.

Two earlier trials, including one led by Kevin Hall in 2019, found higher energy intake on UPF diets under controlled metabolic ward conditions. The present study builds on that work by evaluating health impacts under conditions more reflective of the real world while adhering to national dietary guidance.

“These findings highlight the importance of food processing in public health policy and dietary guidance in addition to existing recommendations,” the authors concluded.

Appetite, fat loss, and health outcomes

Fat mass decreased by an average of 0.98 kg on the MPF diet. Body fat percentage, visceral fat, and total body water mass also declined significantly more than during the UPF phase. In contrast, there were no significant differences in muscle mass, bone mass, or fat-free mass between diets.

Participants also reported improved appetite control while in the MPF condition. Cravings for savoury foods, difficulty resisting desired foods, and perceived control over eating were all significantly improved while consuming MPFs, compared to UPFs. While self-reported energy intake declined on both diets relative to baseline, the reduction was greater on the MPF diet.

The MPF diet led to improvements in triglycerides and HbA1c, markers associated with cardiovascular and metabolic disease risk. Triglycerides are blood fats linked to heart disease, while HbA1c reflects long-term blood glucose levels and is used to assess diabetes risk.

The UPF diet resulted in lower low-density lipoprotein cholesterol (LDL-C), often referred to as “bad” cholesterol; a result that seems to contradict typical expectations about UPF consumption.

Systolic and diastolic blood pressure declined significantly only on the MPF diet. Other cardiometabolic biomarkers did not differ significantly between diets.

Adherence was high across both diets, but participants rated MPF meals lower in taste and flavour. There were no significant differences in ratings of satisfaction, portion size, or overall meal quality.

Possible explanations for the greater fat and weight loss observed on the MPF diet are a combination of lower energy density, improved appetite regulation, and slower eating rates. The MPF diet may have promoted satiety more effectively, leading to a greater energy deficit.

Previous research suggests that UPFs are consumed more quickly and require fewer chews, which can delay satiety signals and increase intake. Branding, packaging, and habitual eating cues may also contribute to overeating.

Expert perspectives highlight trial design challenges

Several experts urged caution in interpreting the findings. Nutrition scientist Dr Nerys Astbury, associate professor at the University of Oxford, said the crossover design introduced “order effects” that may have biased results.

“To properly examine possible health impact of consuming diets containing UPF, much longer methodologically rigorous trials in free-living conditions are needed – where the effects of consuming diets containing variable amounts of UPF can be compared in real world settings, outside the controlled environment of a lab or clinic,” Astbury advised.

Nicola Guess, clinical and academic dietitian and senior lecturer at King’s College London, highlighted the potential impact of unblinded participants and study staff in an otherwise mostly positive commentary.

Although participants were not told which diet they were receiving, the foods’ appearance and branding likely made the distinction clear. Behavioural responses, even unconscious ones, may have influenced eating patterns and outcomes.

“The difference in weight loss between unblinded and blinded participants in weight loss trials is probably at least 1% of body weight ... which is enough to explain the difference between the UPF and MPF group here. Am I saying it did? No, but can we at least acknowledge it could have?” queried Guess.

While the authors of the study concluded that their findings necessitate stakeholders to “align and focus on wider actions to improve the food environment (for example, taxes and subsidies), to enable affordable, available and desirable healthy diets for all,” some experts also saw the results as evidence that UPFs are not necessarily harmful.

For example, Gunter Kuhnle, professor of nutrition and food science at the University of Reading, noted that “the study confirms a large consensus among nutrition and food scientists that ultra-processed foods are not inherently unhealthy and can be an important part of a healthy and balanced diet.”