Does eating ultra-processed foods hinder weight loss?

You might have read stories about ultra-processed foods (UPFs) in the news recently, particularly in relation to weight loss.

This has been sparked by a recent trial published in the journal Nature Medicine.

The UK study compared the effects on weight loss of eating a diet mostly made up of UPFs like shop-bought lasagne with a diet of mostly minimally processed foods (MPFs) like homemade lasagne.

Both diets contained the same calories, fat, sugar and fibre. The only difference was how the food was made – ultra-processed or not.

It found those eating mostly UPFs lost half as much weight as those eating mostly MPFs. The question the researchers went on to look at is why? 

What are ultra-processed foods? 

UPFs are defined as containing ingredients not typically found in home kitchens, such as emulsifiers, preservatives and artificial flavours. 

Think of the difference between an apple and apple-flavoured cereal.

Examples include:  

• pre-packaged bread, ready meals, breakfast cereals, bars and snacks
• fizzy drinks
• processed meats like sausages and ham  
• flavoured yogurts and ice cream.

What did the researchers investigate?

The trial, called UPDATE, was led by researchers at University College London.

It aimed to compare how much weight change people experienced on UPF and MPF diets when the diets were nutritionally the same and the recommendations on how many calories to eat were the same too. 

The study involved 43 adults with a BMI between 25 and 40 whose regular diet was made up of 50 per cent UPFs.  

Each followed 2 8-week diets with a rest period of 4 weeks in between each where they returned to their usual diet.  

• Diet 1: at least 80 per cent UPFs, such as ready meals, cereals and snack bars.  
• Diet 2: at least 80 per cent MPFs including fresh vegetables, oats, whole grains and unprocessed meats.

Meals were delivered to participants in a random order and the researchers recommended eating 2,000 calories a day in line with the NHS Eatwell Guide.  

But the participants received extra food and could eat as much as they wanted to up to 4,000 calories per day on either diet. 

• Read more about ultra-processed foods and how they're categorised here

Do UPFs help or hinder weight loss? 

The trial found those on the UPF diet lost less weight – weighing an average of 1.05 per cent less at the end of the trial than the start – compared to those on the MPF diet who weighed an average of 2.06 per cent less.  

While this was a small amount of weight loss, the researchers pointed out that this was only over 8 weeks.
  
They estimated that if the same weight loss was seen over 1 year, men on the MPF diet could lose around 13 per cent of their weight and women 9 per cent, compared to just 5 per cent and 4 per cent for men and women on the UPF diet. 

The key reason those on the UPF diet experienced less weight loss was because they ate more calories, the researchers noted.

Participants consumed significantly fewer calories on the MPF diet (290 kcal/day less than their usual baseline amount) compared to the UPF diet (120 kcal/day less), despite being matched for nutrients. 

Other results showed: 

Those on the UPF diet: 

• Had lost some weight but saw no significant reduction in fat mass.
• Had more digestive issues such as constipation, reflux, fatigue and sleep-related problems.
• Rated the diet as more flavourful and easier to prepare, which may have influenced their eating behaviour.
• Had lower blood levels of bad (LDL-C) cholesterol, which is linked to lower risk of heart and circulatory disease – however this may be because in this study efforts were made to meet healthy-eating guidelines such as using fortified cereals and plant-based alternatives with potentially added fibre or plant sterols. 

Those on the MPF diet: 

• Had lost body fat and visceral fat (the harmful fat that sits around the organs). 
• Had a lower BMI (body mass index) - indicating weight loss.
• Reported fewer cravings, were better able to resist temptation and felt fuller for longer. 
• Had lower blood triglyceride levels, which is linked to a lower risk of heart and circulatory disease. 

What do the researchers say? 

The greater weight, BMI and fat mass loss, and fewer cravings on the MPF diet suggest it is the best to follow – rather than a UPF diet – when trying to stick to national guidance on eating well, the researchers say. 

They suggest the reason those on the UPF diet reported more cravings and less self-control is possibly because of the way UPFs are made and packaged: 

• UPFs can be easy to overeat as they’re often soft, sweet and require less chewing, which could lead to eating faster and missing the signal to stop eating when full. 
• UPF supermarket packaging with labels like ‘healthy’ could lead to people eating more. 

How good was the research? 

The study was well-designed and generally reliable. It used a cross-over design, meaning each participant tried both diets in a random order. This helped reduce bias - even though the group was small, with just 43 people. 

Both diets were matched for nutrients and followed the NHS Eatwell Guide. The only difference was how the food was made: one diet used homemade meals (MPF), the other used packaged supermarket foods (UPF). This setup helped reflect real-life eating habits. 

Meals were provided free and ready-prepared, which made things easier for participants. But it also meant they did not have to make everyday food decisions, something that could affect how much and how fast they ate. That’s not quite how things work in real life. 

Participants kept food diaries to track what they ate, but eating speed was not directly measured. And while the trial lasted 8 weeks - long enough to spot short-term changes - it may not be enough time to show lasting effects on health or weight.

How good was the media coverage? 

News outlets like LBC, ITV, and MSN explained the study clearly and got most of the facts right.  

ITV even shared stories from people who took part in the trial who said they felt better, slept better, and had more energy after eating less processed food. 

ITV’s headline made the study sound more dramatic than it was. It said people lost weight faster, but the study actually looked at how much weight people lost on two different diets, not how quickly.  

All three outlets said the diets were made entirely of one category of food (UPF or MPF), which was not the case. Both diets were only made up of 80 per cent of the target food, with room for flexibility.  

Overall, the media did a good job sharing the main message: eating fewer ultra-processed foods can help with weight loss and cravings. 

The BHF’s verdict 

The research adds to growing evidence that how food is processed matters when trying to keep to a heathy weight, not just nutrients.  

If your food has been processed such as has had additives added or has been reconstituted from powders, or reshaped using industrial techniques, it’s more likely to affect your body in ways fresh or minimally processed foods do not.  

These changes can impact how full you feel, how your blood sugar responds, and even how much you end up eating.  

However, not all UPFs are equal. Some, like fortified yogurts and wholegrain cereals can be part of a balanced diet. 

Following the NHS Eatwell Guide remains a sound approach to maintaining a healthy weight, but it’s worth considering not just what’s in your food, but how it’s made.

What to read next...