NHS gastroenterology registrar Victoria Nicholas on why no single food is the villain, and the eating patterns that really shape your gut health.
Key Takeaways
No single food is "the worst" for your gut. It's your overall dietary pattern, and what you eat most of the time, that has the greatest impact on gut health.
Ultra-processed foods, excess alcohol and low-fibre diets are most consistently associated with poorer gut health as they may reduce microbial diversity and promote inflammation.
The evidence on artificial sweeteners and food additives is still emerging; some studies suggest they may affect the gut microbiome, but more high-quality human research is required.
Crowd out, don't just cut out: focus on what you can add rather than what you should avoid. Increasing fibre-rich whole foods naturally helps crowd out less nutritious options without unnecessary restriction.
The biggest dietary levers for gut health are diversity and consistency. Aim for around 30 different plant foods each week, include fermented foods and eat a variety of polyphenol-rich foods such as berries, olive oil, herbs, spices, tea and cocoa.
Victoria is a medical doctor and NHS gastroenterology registrar in London, having graduated with a first-class degree from the University of Nottingham in 2016 before completing her general medical training in North West London. She holds a Master's in Genomic Medicine from Imperial College London, deepening her understanding of the molecular basis of health and disease. Alongside her clinical work, Victoria competes as a professional padel player for Great Britain, bringing a unique dual perspective to gut health and athletic performance.
Western-style dietary patterns, which tend to be higher in ultra-processed foods, saturated fat and added sugars and lower in fibre and plant diversity, have been associated with changes in the composition and function of the gut microbiome. In contrast, diets rich in a variety of plant foods provide the fibre and polyphenols that gut microbes use to produce beneficial compounds, including short-chain fatty acids.
Importantly, this doesn't mean that one meal or one food will suddenly change your microbiome. It is the overall pattern of what you eat most of the time that is likely to have the greatest influence.
The NOVA system classifies foods according to the extent and purpose of processing, dividing them into four groups. Group 1 includes unprocessed or minimally processed foods, such as fruit, vegetables, eggs and meat. Group 2 consists of processed culinary ingredients such as oils, butter, sugar and salt. Group 3 includes processed foods made by combining foods from groups 1 and 2, such as canned vegetables, cheese and traditionally made bread. Group 4 consists of ultra-processed foods, which are industrial formulations typically containing multiple ingredients and additives to enhance taste, texture, appearance or the life-span of the food.
Studies have shown an association between dietary intake of ultra-processed foods and various adverse health outcomes including cardiovascular disease, cancer and type 2 diabetes. There are a number of explanations for this. Firstly ultra-processed foods displace nutritious whole foods from the diet, such as fruit, vegetables, fish, beans, nuts and seeds, reducing intake of beneficial compounds present in these foods such as polyphenols. Secondly there are the characteristics of the foods themselves which are generally high in salt, sugar and saturated fat with low levels of dietary fibre, micronutrients and vitamins. There may also be physical and chemical properties of ultra-processed foods which affect digestion, nutrient absorption and the feeling of fullness.
When it comes to gut health, adding beneficial foods may be more sustainable, and more effective, than creating a long list of foods to avoid.
It is more important to think about what you can add to your diet rather than what you need to remove:
These changes will gradually increase your intake of fibre and plant diversity.
The same principle applies to less nutritious foods. If you fill more of your plate with fibre-rich plants, whole grains, legumes, nuts, seeds and other minimally processed foods, these foods naturally begin to "crowd out" some of the ultra-processed and lower-fibre options in your diet, without the need for strict rules or complete avoidance.
A diverse range of plant foods is one of the simplest ways to support a diverse gut microbiome. Different types of bacteria thrive on different fibres, so eating a wide variety of plants provides a broader range of substrates for your gut microbes.
A practical target is to aim for around 30 different plant foods per week. This isn't a magic number or an official dietary requirement, but it is a useful way to encourage variety. Think beyond fruit and vegetables to include whole grains, beans, lentils, nuts, seeds, herbs, spices and other plant foods.
In a small clinical trial, increasing fermented food intake (including foods such as yoghurt, kefir, kimchi and other fermented vegetables) was associated with increased gut microbiome diversity and reductions in several markers of inflammation. More research is needed to understand whether these effects are consistent across different foods and populations, but they are a promising addition to a gut-friendly diet.
Polyphenols are naturally occurring plant compounds found in foods such as berries, cocoa, tea, coffee, olive oil, herbs, spices and many colourful fruits and vegetables. They have antioxidant effects and can be metabolised by gut bacteria.
A gut-friendly diet doesn't need to be complicated. A Mediterranean-style dietary pattern is a useful model, combining a wide range of plant foods with whole grains, legumes, nuts, seeds, olive oil and fish. The goal is to create a pattern that is diverse, enjoyable and sustainable, rather than perfect. Aim to build meals around:
You don't need to eat all of these at every meal. Instead, focus on variety across the week.
Fibre needs water to do its job. As fibre moves through the gut, it draws in water to help form soft, well-formed stools and supports normal gut transit. Without adequate fluid, a high-fibre diet can actually lead to constipation.
There's no need to obsess over exact litres. A practical guide is to drink enough that you're rarely thirsty and your urine is pale yellow most of the day.
If you have persistent or worsening symptoms, it's important to seek medical advice rather than just changing your diet.
Speak to your GP if you have:
Your GP can help determine whether further investigation is needed and whether symptoms could be related to conditions such as coeliac disease, inflammatory bowel disease or other gastrointestinal disorders.
No single food is inherently "the worst" for gut health. Instead, dietary patterns appear to matter most. Diets high in ultra-processed foods, low in fibre and rich in added sugars and excess alcohol have been associated with less favourable gut microbiome profiles and poorer overall health. Rather than eliminating individual foods, focus on eating a varied, fibre-rich diet most of the time.
The evidence is mixed and still evolving. Some laboratory and human studies suggest certain non-sugar sweeteners may alter the gut microbiome in some people, while others have found little or no effect. Currently there is not enough evidence to conclude that all artificial sweeteners are harmful to gut health. If you consume them regularly, it may be sensible to moderate your intake while prioritising whole, minimally processed foods.
Sugar isn't harmful in isolation, and there is no need to eliminate it completely. However, diets consistently high in added sugars often contain fewer fibre-rich foods, fruits and vegetables, which are important for supporting a healthy gut microbiome. Enjoy sugary foods in moderation while making sure the majority of your diet is made up of diverse, whole foods.
Some additives commonly found in ultra-processed foods, including certain emulsifiers and non-sugar sweeteners, have been investigated for their potential effects on the gut microbiome and intestinal barrier. However, the evidence varies considerably between different additives, and much of the mechanistic research comes from laboratory and animal studies. It is therefore too early to conclude that these ingredients directly cause gut inflammation in humans.
The gut is one of the first parts of the body exposed to alcohol after it is consumed. Regular excessive alcohol intake has been associated with changes in the gut microbiome, disruption of the intestinal barrier and altered interactions between gut microbes and the immune system. These changes may contribute to inflammation and increased intestinal permeability.
This doesn't mean that an occasional drink will damage your gut. As with many aspects of nutrition, the amount and frequency matter. The potential risks are primarily associated with higher and more frequent alcohol consumption.
Current UK guidance is to drink no more than 14 units of alcohol per week on a regular basis and to spread this over 3 or more days if you do drink regularly.
Dietary fibre is a type of carbohydrate that is not digested or absorbed in the small intestine. It reaches the large intestine, where some types are fermented by gut bacteria.
Some types of fibre are fermented by gut bacteria to produce short-chain fatty acids (SCFAs), including acetate, propionate and butyrate. These metabolites play important roles in the gut and wider body. Butyrate, for example, is an important energy source for cells lining the colon and is involved in maintaining intestinal barrier function and regulating immune responses.
Some types of dietary fibre also meet the definition of a prebiotic: a substance that is selectively utilised by microorganisms in a way that confers a health benefit.
Higher intakes of dietary fibre, particularly from whole grains and cereals are associated with a reduced risk of mortality and a whole range of diseases. In the UK, adults are generally advised to aim for around 30g of fibre per day.
Excess added sugar is not necessarily "bad for your gut" in isolation, and there is no need to avoid sugar completely. However, diets consistently high in added sugars are often low in fibre and a less diverse range of plant foods.
Some experimental research suggests that high-sugar dietary patterns may influence the gut microbiome and intestinal barrier, but the evidence in humans is still developing. One of the most likely ways that excess sugar affects gut health is by crowding out more nutritious foods. If sugary drinks, sweets and desserts make up a large proportion of the diet, there may be less room for fibre-rich whole grains, legumes, fruit, vegetables, nuts and seeds.
The takeaway isn't that you need to eliminate sugar. It's about the overall pattern: enjoy sweet foods if you like them, while making sure that the majority of your diet contains a diverse range of fibre-rich, minimally processed foods.
Non-sugar sweeteners (NSS) are used to provide sweetness with little or no added sugar or calories. They include substances such as aspartame, sucralose, acesulfame-K and steviol glycosides.
Interest in their effects on the gut microbiome has increased following studies suggesting that some non-sugar sweeteners may alter the composition or function of gut bacteria in certain individuals. However, the findings are inconsistent, and individual responses may vary. Some of the most interesting research has found that the effects of sweeteners on the microbiome may differ substantially between people.
The World Health Organization currently advises against using non-sugar sweeteners as a strategy for long-term weight control and reducing non-communicable disease, based on evidence that they do not appear to provide sustained benefits for reducing body fat and may be associated with some adverse health outcomes. However this recommendation is not specifically a recommendation to avoid sweeteners due to their effect on the gut.
When it comes to gut health, the evidence is still emerging. There is currently not enough evidence to say that all non-sugar sweeteners are harmful to the gut, but if your diet is high in sweetened drinks and highly processed foods, reducing them may be a sensible way to make room for more nutritious, fibre-rich foods.
Food additives include anything that enhances the taste, texture, appearance or life-span of foods. They commonly include emulsifiers which ensure food has a consistent texture and artificial food colourants (AFCs) to alter appearance.
Some food additives, particularly certain emulsifiers, have been shown in laboratory and animal studies to influence the gut microbiome, intestinal barrier and inflammatory pathways. However, human evidence is currently more limited, and it is not possible to conclude that all food additives have harmful effects on gut health.
The evidence around emulsifiers and artificial food colourants is an area of active research. Some findings are concerning, but much of the evidence comes from animal models or relatively small human studies. We therefore need larger and longer-term clinical trials before we can understand whether these effects translate into meaningful differences in human health.
Research suggests that polyphenol-rich foods may influence the composition and activity of the gut microbiome and increase the production of beneficial microbial metabolites. This is another reason to aim for variety and colour when choosing plant foods.
Some of the most beneficial foods for gut health include:
A Mediterranean-style eating pattern naturally incorporates many of these foods and is consistently associated with better gut health.
Research suggests that changes in the gut microbiome can begin within days of changing your diet. However, lasting improvements in gut health are more likely to come from consistent eating habits maintained over weeks to months. Rather than looking for a quick fix, focus on building sustainable habits such as eating more fibre, increasing plant diversity and reducing reliance on ultra-processed foods.
