How does diet affect gut health?

The intestines are responsible not only for the final stages of digestion, but also for the absorption of nutrients, water balance and the body’s exposure to a vast number of microorganisms. In 1681, Antonie van Leeuwenhoek, using a microscope he had built himself, described the microorganisms present in a sample of his own stool, which is regarded as one of the earliest stages of research into the human microbiota. Today, we know that the composition of the gut microbiota can change, amongst other things, as a result of diet.

1. Functions of the intestines
2. The impact of diet on the intestines
3. Diets for the gut
4. FAQ – frequently asked questions about diet and gut health

Functions of the intestines

The small intestine is the main site where food is broken down into simpler components and then absorbed into the body. Among other things, simple sugars, amino acids, fatty acids, vitamins, minerals and water pass into the blood or lymph there. The large intestine then reabsorbs some of the remaining water and electrolytes and plays a part in the formation of stools. It is home to important microorganisms that make up the gut microbiota. Some of these utilise food components that humans cannot digest on their own, particularly certain types of fibre.

The intestinal wall forms a barrier separating the interior of the digestive tract from the body’s tissues. It is made up of tightly packed epithelial cells, a layer of mucus and components of the immune system. This barrier allows the absorption of necessary nutrients whilst limiting the passage of microorganisms and other undesirable particles. The intestines are also propelled by muscles located within their walls. These movements mix the intestinal contents and move them towards the rectum.

The impact of diet on the intestines

The European Food Safety Authority (EFSA) considers 25 g of fibre per day to be an adequate amount for healthy bowel function in adults. The diet of a healthy person should therefore be based on plant-based foods that are sources of fibre: vegetables, fruit, wholegrain cereals, pulses, nuts and seeds. It is worth drawing on a variety of sources, as different types of fibre vary in their solubility and susceptibility to fermentation by the gut microbiota. If your diet has been low in fibre up to now, it is best to increase your intake gradually, as a sudden increase in consumption may lead to increased flatulence and a feeling of fullness.

What is good for the gut?

Good choices include oat flakes, wholemeal bread, groats, brown rice, beans, lentils, chickpeas, apples, berries, carrots, broccoli, onions, leeks, walnuts, almonds and seeds. The diet can also include yoghurt, kefir, sauerkraut and other fermented products, which contain bacteria that may influence the gut microbiota.

What is harmful to the gut?

A diet based mainly on highly processed foods, with a low intake of fibre and antioxidants, is definitely detrimental to the gut. It is worth limiting frequent consumption of sweets, salty snacks, fast food, processed meat and products that provide a lot of energy but little dietary fibre. Alcohol, cigarettes and excessive use of medication (including painkillers from the non-steroidal anti-inflammatory drug group) are also known to disrupt the gut balance.

Estimated fibre content in typical portions of fibre-rich foods

Diets for the gut

Most of the evidence regarding a beneficial dietary pattern for the microbiota and the functioning of the gastrointestinal tract relates to the Mediterranean diet. In interventional studies, this dietary pattern altered the metabolic activity of the microbiota and, in some studies, increased its diversity, although individual responses varied. The Nordic diet shares similar characteristics, with whole grains, vegetables, pulses, berries, nuts and fish playing an important role.

The diet low in fermentable oligosaccharides, disaccharides, monosaccharides and polyols (low FODMAP), which is popular amongst people with gastrointestinal disorders, is not a routine diet for healthy individuals. It is mainly used on a temporary basis for some patients with irritable bowel syndrome, with the diet subsequently being expanded.

"Gut health does not depend on a single product or supplement, but on one’s overall diet. A varied diet rich in fibre, plant-based foods and an adequate intake of fluids creates the right conditions for the digestive tract to function properly and supports the gut microbiota." Anna Zięba – Dietitian

FAQ – frequently asked questions about diet and gut health

What should you eat to support gut health?

Your diet should be based on vegetables, fruit, whole grains, pulses, nuts and seeds. These provide fibre and various nutrients utilised by the gut microbiota. It’s worth focusing on variety, rather than just one ‘gut-friendly’ food.

How much fibre does an adult need each day?

The EFSA considers around 25 g of fibre a day to be the appropriate amount for healthy bowel function in adults. If your diet has previously been low in fibre, it is best to increase your intake gradually to minimise the risk of bloating and excessive gas production.

What is the most harmful aspect of a diet for the gut?

A diet based mainly on highly processed foods, combined with low intake of vegetables, fruit, whole grains and other sources of fibre, is generally the least beneficial. It is the overall dietary pattern that matters, rather than a single food eaten from time to time.

Is the Mediterranean diet good for the gut?

It is one of the best-researched dietary patterns in this context. It is rich in plant-based foods, fibre, pulses, nuts, olive oil and fish. Research suggests that it can influence the composition and metabolic activity of the gut microbiota, although the response is not the same for everyone.

Is the low-FODMAP diet suitable for everyone?

No. The low-FODMAP diet – which is low in fermentable oligosaccharides, disaccharides, monosaccharides and polyols – is mainly used on a temporary basis by some people with irritable bowel syndrome. It is not a standard preventive diet for healthy people.

Does sudden bloating after increasing my vegetable intake mean that vegetables are bad for me?

Not necessarily. If your diet was previously low in fibre, a sudden increase in fibre intake may temporarily lead to increased fermentation, gas and a feeling of fullness. It is often better to increase your intake of plant-based foods gradually.

Can stress also affect the gut?

Yes. The digestive tract and the nervous system are in constant communication with one another, which is why stress can, in some people, alter bowel motility and exacerbate the urge to go, abdominal pain or other symptoms. However, this does not mean that chronic symptoms should automatically be attributed to stress.

When should bowel problems be referred to a doctor?

It is particularly advisable to see a doctor if there is blood in the stools, unexplained weight loss, severe or worsening pain, a fever, persistent diarrhoea, prolonged constipation or a marked change in bowel habits.

Sources:

• Zhang, L., Tuoliken, H., Li, J., & Gao, H. (2024). Diet, gut microbiota, and health: a review. Food science and biotechnology, 34(10), 2087–2099. https://doi.org/10.1007/s10068-024-01759-x
• So, D., Whelan, K., Rossi, M., Morrison, M., Holtmann, G., Kelly, J. T., Shanahan, E. R., Staudacher, H. M., & Campbell, K. L. (2018). Dietary fiber intervention on gut microbiota composition in healthy adults: a systematic review and meta-analysis. The American journal of clinical nutrition, 107(6), 965–983. https://doi.org/10.1093/ajcn/nqy041
• Kimble, R., Gouinguenet, P., Ashor, A., Stewart, C., Deighton, K., Matu, J., Griffiths, A., Malcomson, F. C., Joel, A., Houghton, D., Stevenson, E., Minihane, A. M., Siervo, M., Shannon, O. M., & Mathers, J. C. (2023). Effects of a mediterranean diet on the gut microbiota and microbial metabolites: A systematic review of randomized controlled trials and observational studies. Critical reviews in food science and nutrition, 63(27), 8698–8719. https://doi.org/10.1080/10408398.2022.2057416