The impact of ultra processed foods on the gut

What are UPFs? 

Ultra-processed foods are foods that have been formulated largely from substances extracted from foods, refined ingredients and additives, using industrial processes not typically replicated in a home kitchen. They are often designed to be convenient, ready-to-eat or ready-to-heat, highly palatable, heavily marketed and long-lasting on the shelf. 

That said, it is important not to confuse all processing with poor health. Most foods are processed in some way. Washing, chopping, freezing, pasteurising, fermenting and canning are all forms of processing, and many are useful or even beneficial. The concern around UPFs is not simply that they have been processed, but that many are produced in ways that alter the original food structure, concentrate refined ingredients, add cosmetic additives and make it easier to consume large amounts quickly

Many people assume UPFs are limited to fast food, sweets and fizzy drinks, but in reality they often include everyday items that have become standard in modern diets. In the UK, common contributors include packaged breads and buns, breakfast cereals, confectionery, biscuits, cakes, ready meals, processed meat products, savoury snacks and soft drinks. Some low-fat spreads, flavoured yoghurts and sweetened drinks can also fall into this category. This is partly why the topic can feel confusing. UPFs are not always obvious, and they do not all look like “junk food”. For many people, they are simply normalised parts of a busy routine. 

Why is gut health important? 

Gut health is often reduced to digestion alone, but its role is far broader. The gut is involved in nutrient absorption, immune activity, barrier function and communication with the brain through what is often called the gut-brain axis. The trillions of microbes that live in the gastrointestinal tract help produce metabolites such as short-chain fatty acids, support immune signalling and interact with many aspects of health. 

Diet is one of the most important modifiable influences on the gut microbiome. In particular, fibre-rich diets and a wide variety of plant foods appear to support greater microbial diversity and the growth of bacteria associated with favourable metabolic functions. Data from the American Gut Project found that people eating more than 30 different plant foods per week tended to have more diverse gut microbiomes than those eating fewer than 10. 

How do UPFs impact my gut health? 

This is where the evidence becomes especially interesting, but also where caution is needed. There are several plausible ways in which diets high in UPFs may affect gut health.

First, UPF-heavy diets often displace foods that nourish the microbiome. Many UPFs are lower in fibre and plant diversity, meaning they provide less of the material that beneficial gut bacteria ferment into short-chain fatty acids. In UK dietary data, higher UPF intake is associated with poorer nutritional quality overall.  

Second, some additives commonly found in UPFs, including certain emulsifiers and sweeteners, have been linked in experimental studies to changes in gut microbiota composition, intestinal permeability and inflammatory processes. A 2024 review concluded that there is increasing evidence associating diets high in UPFs and food additives with gut disease, including inflammatory bowel disease, colorectal cancer and irritable bowel syndrome, while also emphasising that much of the mechanistic evidence comes from animal and preclinical studies rather than long-term human trials.

Third, processing can alter the food matrix. This refers to the physical structure in which nutrients are packaged within a food. The way a food is processed can change how quickly it is eaten, digested and absorbed, and how full it leaves us feeling. In other words, two foods may look similar on a nutrition label but behave differently in the body because their structure has been altered. 

That said, the current evidence does not prove that all UPFs directly damage the gut, nor that every additive has the same effect in every person. SACN’s 2025 evidence update concluded that associations between higher consumption of processed and ultra-processed foods and adverse health outcomes are concerning, but it remains unclear how much of that risk is due to processing itself versus the fact that many UPFs are high in calories, saturated fat, salt and free sugars. 

So, the most evidence-based conclusion is this: high intakes of UPFs may impair gut health partly because they displace fibre-rich whole foods, and partly because some formulations and additives may interact directly with the microbiome and gut barrier. 

What about sweeteners? 

Sweeteners are often discussed separately in the UPF conversation, particularly because many low- or no-sugar products still fall under the ultra-processed umbrella. The evidence here is mixed. A 2022 randomised controlled trial found that some non-nutritive sweeteners, including saccharin and sucralose, altered aspects of the gut microbiome and glycaemic responses in a person-specific way, even when consumed below the acceptable daily intake. 

However, this is not the whole story. A 2025 long-term randomised controlled trial found that within an otherwise healthy, sugar-reduced diet, sweeteners and sweetness enhancers supported weight-loss maintenance and were associated with gut microbiota shifts interpreted by the authors as beneficial in adults with overweight or obesity. 

Taken together, this suggests there is no simple “sweeteners are good” or “sweeteners are bad” answer for gut health. Their effects may depend on the specific sweetener, the dose, the individual and the wider dietary context. For some people, they may help reduce free sugar intake; for others, they may contribute to gastrointestinal symptoms or lead to a false sense of healthfulness around otherwise poor-quality products. 

Sugar alcohols such as sorbitol and xylitol deserve a separate mention, as they can contribute to bloating, wind or diarrhoea in some people, especially at higher intakes. That does not mean they must always be avoided, but moderation is sensible. 

Are all UPFs equal? 

No, and this is one of the most important points. UPFs are a broad category. Soft drinks, processed meats, packaged cakes, fortified plant milks, wholegrain breakfast cereals and some convenience breads may all fall under the same NOVA group, despite being nutritionally and functionally very different. 

At population level, higher UPF intake is consistently associated with poorer diet quality and worse health outcomes. However, at an individual food level, there is a clear need for nuance. A blanket message that all UPFs are equally harmful is not supported by the evidence. Some UPFs also play an important role in modern diets. Fortified plant milks can support micronutrient intake, baby formula is essential where breastfeeding is not possible or chosen, and some wholegrain cereals or convenient staple foods may be nutritionally useful and affordable. Public health messaging should reflect this reality. 

Is there a safe level to consume? 

There is no official UK threshold for a “safe” level of UPF intake. No guideline states that a certain percentage of the diet can come from UPFs without consequence, or that a specific cut-off is universally harmful. 

This is partly because the category is so broad, and partly because the science is still developing. Much of the evidence linking UPFs to poorer health outcomes is observational, meaning it can show associations but cannot always prove cause and effect. That is especially relevant for gut health, where long-term human intervention data are still limited. 

The most practical evidence-based message is not to aim for zero, but to reduce reliance on UPFs overall, especially those high in free sugars, saturated fat and salt, while focusing on improving the quality of the whole diet. 

How can I reduce UPFs in my diet? 

Reducing UPFs does not need to mean cooking every meal from scratch or throwing out every packaged product in your kitchen. In fact, an all-or-nothing mindset is rarely helpful. A more sustainable approach is to make minimally processed, fibre-rich foods the default more often. Start by building meals around foods such as vegetables, fruit, beans, lentils, wholegrains, nuts, seeds, eggs, fish and natural yoghurt. Where possible, try to add rather than restrict: more beans and lentils in sauces, more wholegrains at breakfast, more vegetables at lunch, and more variety across the week. This helps support both diet quality and gut health.  

Practical swaps can also make a big difference. Overnight oats or plain porridge can replace heavily sweetened breakfast cereals. Plain Greek yoghurt with fruit can replace flavoured yogurts with long ingredient lists. Beans, lentils or homemade fillings can reduce reliance on processed meats. Homemade popcorn, nuts or oat-based snacks can be useful alternatives to packaged savoury snacks and bars.  

Planning helps too. Shopping with a list, checking cupboards before you go, keeping a few simple staple recipes on rotation, and batch-cooking where possible can all reduce the need to default to convenience foods at every mealtime. For many people, convenience foods are tied to budget, time, family life, shift work, illness or access. Nutrition advice should acknowledge that reality. The goal is not to eliminate every UPF from your diet, but to create a pattern where minimally processed, fibre-rich foods appear more often than they do now. 

To learn more about other gut health topics, visit Symprove’s Gut Hub. 

Sources 

Ultra-Processed People. Chris van Tulleken. (2023). Random House. 

Ultra-processed foods. Food Standards Agency. (2024). Link. 

Ultra-processed food exposure and adverse health outcomes: umbrella review of epidemiological meta-analyses. BMJ. (2024). Link.  

The UN Decade of Nutrition, the NOVA food classification and the trouble with ultra-processing. Public Health Nutrition. (2018). Link. 

Intake of Ultra-processed Foods Is Associated with an Increased Risk of Crohn’s Disease: A Cross-sectional and Prospective Analysis of 187 154 Participants in the UK Biobank. Journal of Crohn’s and Colitis. (2022). Link. 

Gut Microbiota Differences According to Ultra-Processed Food Consumption in a Spanish Population. Nutrients. (2021). Link. 

American Gut: an Open Platform for Citizen Science Microbiome Research. MSystems. (2018). Link. 

Processed foods and health: SACN's rapid evidence update summary. Office for Health Improvement and Disparities. (2025). Link. 

Ultra-processed foods and food additives in gut health and disease. Nature Reviews Gastroenterology & Hepatology. (2004). Link.