Your gut contains trillions of bacteria, viruses, and fungi - collectively called the microbiome. These organisms aren't just passengers; they actively influence your health, from extracting nutrients from food to producing neurotransmitters that affect your mood.
A diverse, balanced microbiome is associated with better health outcomes. An imbalanced one - called dysbiosis - is linked to digestive issues, inflammation, metabolic problems, and even mental health challenges. Diet is the most powerful tool you have to shape your gut ecosystem.
What the Microbiome Does
Digests fibre - Bacteria ferment fibre into short-chain fatty acids (SCFAs) that fuel gut cells and reduce inflammation.
Produces vitamins - Certain bacteria synthesise vitamin K and B vitamins.
Trains immunity - Around 70% of your immune system resides in your gut.
Influences mood - Gut bacteria produce about 95% of your body's serotonin.
Protects against pathogens - A healthy microbiome crowds out harmful bacteria.
The Key Principle: Diversity
30 Plant Foods Per Week
Research from the American Gut Project found that people who eat 30+ different plant foods per week have significantly more diverse microbiomes than those who eat fewer than 10. This includes vegetables, fruits, whole grains, legumes, nuts, seeds, herbs, and spices - they all count. The variety matters as much as the quantity.
Different bacteria feed on different fibres. Eating the same foods repeatedly - even healthy ones - feeds only certain bacterial strains while starving others. Variety is the foundation of gut health.
Foods That Feed Good Bacteria
Prebiotic Foods (Feed Bacteria)
- Garlic, onions, leeks
- Jerusalem artichokes
- Asparagus
- Bananas (especially slightly green)
- Oats
- Legumes (beans, lentils, chickpeas)
- Flaxseeds
- Chicory root
Probiotic Foods (Add Bacteria)
- Live yoghurt
- Kefir
- Sauerkraut (unpasteurised)
- Kimchi
- Miso
- Tempeh
- Kombucha
- Some aged cheeses
Prebiotics vs Probiotics
Prebiotics are types of fibre that you can't digest but your gut bacteria can. They're food for the beneficial bacteria already living in your gut. Think of them as fertiliser for your internal garden.
Probiotics are live bacteria found in fermented foods or supplements. They add new beneficial strains to your gut ecosystem, though many don't permanently colonise - they provide benefits while passing through.
Both matter, but prebiotics may be more important for long-term gut health because they support the bacteria already adapted to your gut environment.
Foods That Harm Gut Bacteria
Limit These
- Ultra-processed foods
- Artificial sweeteners (especially saccharin)
- Excessive sugar
- Processed meats
- Refined carbohydrates
- Excessive alcohol
- Emulsifiers (common in processed foods)
Use Cautiously
- Antibiotics (only when necessary)
- NSAIDs (ibuprofen) long-term
- Proton pump inhibitors (antacids)
- Restrictive elimination diets long-term
On Supplements
Probiotic supplements are heavily marketed but the evidence is mixed. Most supplements don't contain strains that are well-researched, and many don't survive stomach acid. For general gut health, food sources are preferable. Supplements may help specific conditions (antibiotic-associated diarrhoea, IBS) - discuss with your doctor if relevant.
Fibre: The Most Important Factor
Most Britons eat around 18g of fibre daily. The recommendation is 30g. This shortfall directly affects gut health - fibre is the primary food source for beneficial bacteria.
Different fibre types feed different bacteria:
- Soluble fibre (oats, legumes, apples) - Dissolves in water, forms gel, fermented by bacteria
- Insoluble fibre (whole grains, vegetables) - Adds bulk, speeds transit time
- Resistant starch (cooled potatoes, green bananas) - Reaches the colon intact, powerfully fermented
Increase fibre gradually. Adding too much too quickly causes gas and bloating as your microbiome adapts.
The Gut-Brain Connection
The gut and brain communicate constantly via the vagus nerve and through chemical signals. This "gut-brain axis" explains why digestive issues often accompany anxiety and depression, and why stress affects digestion.
Research increasingly shows that gut bacteria influence mood, stress response, and cognition. While you can't cure mental health conditions through diet alone, supporting gut health may be one piece of the puzzle.
Practical Steps
- Add variety gradually - Try one new vegetable, grain, or legume per week
- Eat fermented foods daily - Even a small portion of live yoghurt or sauerkraut
- Prioritise whole foods - Minimise ultra-processed items
- Include prebiotic-rich foods - Onions, garlic, leeks, legumes
- Increase fibre slowly - Add 5g per week to avoid discomfort
- Drink enough water - Fibre needs water to work properly
- Cook and cool starches - This creates resistant starch (potatoes, rice)
The Bottom Line
Gut health is largely determined by diet. The key principles: eat a diversity of plant foods (aim for 30+ types per week), include both prebiotic foods (fibre-rich plants) and probiotic foods (fermented products), increase fibre gradually toward 30g daily, and minimise ultra-processed foods. Your microbiome adapts to what you feed it - changes can begin within days of dietary shifts. A diverse, well-fed gut microbiome supports digestion, immunity, mental health, and metabolic function.
References
- McDonald, D., et al. (2018). American Gut: an Open Platform for Citizen Science Microbiome Research. mSystems, 3(3). doi:10.1128/mSystems.00031-18
- Valdes, A.M., et al. (2018). Role of the gut microbiota in nutrition and health. BMJ, 361, k2179. doi:10.1136/bmj.k2179
- British Dietetic Association. (2023). Gut Health. bda.uk.com
- Cryan, J.F., et al. (2019). The Microbiota-Gut-Brain Axis. Physiological Reviews, 99(4), 1877-2013. doi:10.1152/physrev.00018.2018
