The foods that feed your gut bacteria, and the foods that starve them

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What happens when they run out of food

Your gut bacteria run almost entirely on fibre. It's the one thing you eat that you can't digest yourself, so it travels intact to the colon where they're waiting for it.

When you give them plenty, they ferment it and produce short-chain fatty acids, particularly one called butyrate. Butyrate is the preferred fuel of the cells lining your colon. It keeps the gut barrier strong, calms inflammation, and influences the hormones that tell your brain you've had enough to eat.

Now, what happens when the fibre stops arriving?

Researchers tested this directly. When they put mice on a fibre-deprived diet, the bacteria didn't politely die off. They turned to the next available food source: the mucus layer lining the gut wall.

That mucus layer is your barrier. It's what sits between a hundred trillion microbes and the rest of you. As it thinned, the bacteria moved closer to the gut wall, and the animals became markedly more vulnerable to inflammation and infection.

It's mouse research, and I won't overstate it. But the principle holds and it's worth sitting with. A low-fibre diet doesn't just fail to feed your microbes. It gives them a reason to start eating you.

Most people in the West eat roughly half the fibre they should. Think about what that means is happening, quietly, every single day.

The foods that feed them

Beans and lentils. The best fibre source there is, and the type they like most. One tin of chickpeas does more for your gut than any supplement on the shelf.

Oats and barley. They contain beta-glucan, a soluble fibre that forms a gel and ferments beautifully.

Onions, garlic and leeks. These carry inulin, a prebiotic fibre that specifically feeds the beneficial strains. Which is a nice coincidence, because they're also the things that make vegetables taste like something worth eating.

Cooled potatoes and rice. Here's one most people have never heard. When you cook starch and then cool it, some of it converts into resistant starch, which behaves like fibre instead of sugar. Yesterday's rice, used for a fried rice tonight, is genuinely better for your gut than it was fresh. Same with potato salad made from cold boiled potatoes.

Colourful plants, in variety. One of the largest gut microbiome studies found that the strongest predictor of a diverse, healthy microbiome wasn't vegetarian versus omnivore. It was the number of different plants eaten in a week. People eating thirty or more plant types had noticeably more diverse gut bacteria than people eating ten or fewer. Herbs, spices, nuts, seeds and wholegrains all count.

Fermented foods. Yogurt, kefir, sauerkraut, kimchi, miso. A Stanford trial found that people who increased fermented foods for ten weeks showed greater microbial diversity and lower markers of inflammation. Notably, a high-fibre group in the same study didn't show the same effect in that timeframe, which is a useful reminder that this field is still full of surprises.

Extra virgin olive oil and dark-coloured plants. The polyphenols in them act as food for certain beneficial strains too.

The foods that starve them

Ultra-processed food. This is the main offender, and not only because of what it contains. It's because of what it displaces. Every meal built from refined flour, sugar and oil is a meal where the fibre simply never arrives. Around 60% of calories eaten by adults in the US now come from ultra-processed products, which gives you a sense of the scale of the problem.

Refined grains. White bread, white pasta, white rice. The refining process removes the bran, which is where almost all the fibre lived.

Emulsifiers and additives. Many processed foods contain emulsifiers to keep texture smooth. Animal research has linked some of them to a thinner mucus layer and more gut inflammation. The human picture isn't settled, but it's another reason the packet is worse than the plate.

Artificial sweeteners. The evidence here is genuinely mixed and keeps shifting, with some studies finding changes to the microbiome and others finding little. I wouldn't panic about one diet drink. I also wouldn't build a daily habit around them while the science is still arguing.

Alcohol, in quantity. Regular heavy drinking is reliably hard on both the gut lining and the balance of bacteria.

A badly done low-carb diet. This one catches people out. Cut carbs without deliberately replacing the fibre, and your microbes are the collateral damage. Plenty of people feel worse on low-carb for exactly this reason and never connect the two.

What about probiotic supplements?

Worth a straight answer, since the aisle is enormous and growing.

Most probiotic supplements contain a handful of strains. Your gut hosts hundreds. And most of what you swallow doesn't take up permanent residence; it passes through, does whatever it does, and leaves.

They have real uses, after antibiotics for instance. But for general gut health, a tub of yogurt and a bowl of lentils will do more for your microbiome than a capsule, for a fraction of the price.

Feeding the bacteria you already have beats trying to import new ones.

This week

Forget supplements. Do the plant count.

Write down every different plant food you eat this week. Vegetables, fruit, beans, nuts, seeds, wholegrains, herbs and spices all count, and each one counts once.

Most people are surprised, usually unpleasantly. If you land under fifteen, that's your answer, and the fix is simple: tinned beans into things, a handful of frozen mixed vegetables instead of one, nuts and seeds on top of what you already eat, herbs by the fistful rather than the pinch.

You're not removing anything. You're widening the menu for the thirty-nine trillion other diners you've been quietly neglecting.

The way of eating that does this automatically

Look back at the "feed them" list. Beans, lentils, onions, garlic, olive oil, wholegrains, yogurt, herbs, a wide range of plants.

You've just described Mediterranean cooking. Not as a diet, but as what it actually is: a cuisine that happens to feed gut bacteria beautifully, invented long before anyone knew they existed.

That's why I put together The World's Healthiest Diet in 30 Minutes. 60 Mediterranean recipes built on exactly these foods, every one on the table in half an hour or less, plus 6 bonus guides so you always know what to cook next. Hitting thirty plants a week stops being a project and just becomes what dinner looks like.

👉 Grab The World's Healthiest Diet here 
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Talk soon, The Healthy Mail 🫒

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P.S. There's something clarifying about realising that every meal is a decision about two bodies, yours and the civilisation living inside you. Feed them well and they repay you with a stronger gut barrier, calmer inflammation and better signals to your brain about when you've had enough. Starve them and they go looking for something else to eat. The World's Healthiest Diet makes feeding them properly the easiest option in your kitchen, 60 dinners, half an hour each. It's 50% off with code 2026 right now.👉 Grab it here

P.P.S. We're live on YouTube! If you're trying to lose fat, don't make the mistake of thinking you need another complicated diet. I just broke down the 7 simple things that actually move the needle — protein, calories, lifting, walking, sleep, and the habits that make all of it sustainable. 👉 Watch here