While our bodies need calories and a variety of nutrients, as well as insoluble fibre to provide bulk, the microbiome’s “food” is largely soluble fibre (see below) along with some complex carbohydrates which make it through to the large intestine. It’s what’s sometimes called “prebiotics”, as opposed to “probiotics” which are live yeasts and good bacteria found in foods like live yogurt, sauerkraut and kimchi.

Some sweeteners, such as inulin, claim to be prebiotic fibre, but Dr Nikpay is wary: “If they say it has a fibre base and is sweetening, I would say that it’s almost not a fibre any more.” To create a sweetener, the manufacturers “use enzymes or heat methods or acid to actually break open molecular bonds between the different sugar units in a fibre or in a complex carb… and once you get to one or two units, those are sugar from a fibre-health perspective.”

The simplest solution, you might think, would be to pop some fibre supplements – easy to find next to the vitamin pills in any health shop. But, says Dr Nikpay, these are too standardised, too simplified. He compares gut bacteria to the population of London – a great, diverse mass which needs a varied diet. He’s concerned by the idea that “you can supplement yourself out of trouble. No,” he argues, “you had better stick with normal food.” 

Sprinkling bran on your cereal in the morning is great, though, or do as Dr Nikpay does and add cut-up fruit with a mix of seeds, eating it with milk. “Yogurt is probably better for you, but I don’t like it,” he admits. Yogurt does indeed have gut-healthy probiotic cultures in it – but as he makes clear in the book, any change in diet has to be sustainable. “If I don’t enjoy it I’m not going to stick with it.”

So how else can we get fibre into our diet? Wholegrains, fruit and vegetables are good sources, as well as, surprisingly, herbs – Middle Eastern-style herb-rich salads like tabbouleh turn out to be terrifically gut-friendly. Baking is trickier, so I asked Atherton for advice. “A wholewheat cake would be very dense and probably not very nice. But if you just swap five per cent or 10 per cent of the white flour for wholewheat flour, you will still get the really nice light texture you get with white flour, but you will be adding fibre.”

He also suggests boiling whole grains such as buckwheat and spelt until they’re soft, and then keeping them in a pot in the fridge. “Then if I’m having an omelette, or a pasta salad, I’ll just throw some of these grains in. They actually add a nice texture and crunch.”

Is it possible to consume too much fibre? “Where we stand right now, the answer is no,” says Dr Nikpay. “I really would like to have that problem.” The worst that can happen is a bit of wind. Although, truth be told, that’s pretty distressing – and embarrassing. “Avoid anything that is a fructo oligo saccharide,” he advises – the indigestible compounds notorious for causing wind, found in Jerusalem artichokes and inulin. 

But, he stresses, keep in mind that “you and I will have different bacteria. So we might react slightly differently to the same thing. My sister, for example, can’t eat chickpeas. She will just bloat.”

Introducing foods slowly should help you habituate, but (as I know all too well) this doesn’t always work. “It could be that you have a slight intolerance to some things, so just avoid them,” says Dr Nikpay pragmatically. “Nature is full of fibres, you don’t have to have chickpeas.”

What is fibre?

Dietary fibre is the indigestible part of the plants that we eat: there’s no fibre in meat or dairy. It falls broadly into two types, soluble and insoluble fibre, and both kinds provide bulk which tends to make us feel fuller, so it can help with maintaining a healthy weight.

Insoluble fibre (for example wheat bran) is made mostly of cellulose, and is not broken down in the body at all, passing straight through the gut. It speeds up the gut processing time, which is protective against bowel cancer and diverticulitis.

Soluble fibre (for example oat bran) will dissolve in water to make a viscous mix – in the case of oats, to make porridge. Once eaten, it passes through the first, small intestine (where most of the nutrients from food are absorbed) intact. But once it reaches the second large intestine, much of it is fermented and provides food for the all-important bacteria (sometimes called the microbiome) there.

The best sources of fibre, according to Dr Nikpay
  1. Seeds: Flaxseed (aka linseed, which contain 27g fibre per 100g), sesame seeds (12g per 100g), poppy seeds (20g per 100g) and sunflower seeds (9g per 100g) are all a great source of fibre. Chia seeds, meanwhile, pack a whopping 34g fibre per 100g
  2. Grains: Cooked brown rice has 1.8g fibre per 100g, while popcorn has 13g per 100g. Wholewheat flour delivers 11g per 100g, but be aware that 100 per cent wholemeal bread is fairly dense, so “wholemeal” loaves, even the beautiful artisan sourdoughs, are generally made with a mixture of white and wholemeal flour; sometimes that’s as little as 15 per cent of wholemeal
  3. Berries: Strawberries may have just 1.8g fibre in a 100g serving, but raspberries have 6.5g and blackberries have 5.3g. Wild blackberries may be even higher, so head for the hedgerows
  4. Cooked lentils: Fibre content varies from brand to brand, but a teacupful of home-cooked green lentils (ie 200g, or about 100g uncooked weight) may provide you with half your daily fibre needs or even more. Tinned lentils tend to have less fibre
  5. Herbs: Many contain around 10g fibre per 100g, so add them to everything
  6. Nuts: Walnuts have 6.7g fibre per 100g; almonds and hazelnuts 9.7g per 100g. Roasted peanuts – technically a legume – pack 10g fibre per 100g
  7. Legumes: Half a tin (120g) of haricot beans has 9g fibre, while half a tin of chickpeas has 5g. A generous serving of peas delivers 6g per 100g
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