More than half the cells in your body are not yours

You have probably read that your body holds ten bacteria for every human cell. The figure came from an estimate in the 1970s, repeated in books and lectures until it passed for a fact.

In 2016, three researchers redid the arithmetic with real data. The ratio is close to one to one: about 38 trillion bacteria to 30 trillion human cells 1. By cell count, you are half human. The other half works for you.

This page is written in that spirit: counted facts, sources you can open, and plain words.

We were taught to see bacteria as enemies. Most of those living in you are residents: they form a dense, organised community, like a forest, where every species has its place and its job.

When researchers sequenced the microbiome of hundreds of healthy people, they found that two people in great shape can host very different bacteria doing the same jobs 2.

So there is no “normal” microbiome to copy. There is yours, and its health shows in its diversity and balance, the way a forest shows in the variety of what grows there.

Your own enzymes can do nothing with the fibre in lentils, whole grains, leeks or leafy greens. Your bacteria can. They ferment it and make short-chain fatty acids, including butyrate, the favourite fuel of the cells lining your colon 3.

So fibre is not just a broom. It is food for your bacteria, which feed you in return.

These same bacteria train your defences: the gut is where your immune system learns to tell friend from foe 3.

The microbiome differs from one country to another and from one age to another, from early childhood 4. And genes play a small part: in over a thousand people of different origins, heredity explained almost nothing. Unrelated housemates resembled each other more than relatives living apart. What mattered: diet, medication and lifestyle 5.

That is good news: what shapes your microbiome is largely what you do. And it means a programme designed for “everyone” is addressed to no one in particular.

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Left in peace, your microbiome is remarkably faithful: most of your bacteria stay with you for years, and you share strains with your family 6.

But it responds fast. In volunteers switched to an entirely plant-based or entirely animal-based diet, the community changed within days 7.

It also responds to antibiotics: within three or four days diversity collapses, and the return to the starting point is often incomplete 8. A necessary antibiotic is taken without hesitation. An unnecessary one has an invisible cost.

Researchers call dysbiosis the shift from a rich, stable community to a depleted one. It is associated with a long list of diseases of modern life: metabolic disorders, inflammatory and allergic diseases, some cancers, some mood and behavioural disorders 9 10 11. Chronic fatigue has also been linked to a microbiome that produces less butyrate 12.

In 2022, Douglas Hanahan, author of the reference “hallmarks of cancer” framework, added microbiome polymorphism to the characteristics that make cancer possible, alongside inflammation 13. In mice, transferring a microbiome transfers part of what it does to the body 14.

Linked does not mean guilty: in humans, research is still establishing what causes what. What this work already says is that the diversity of your microbiome is worth looking after, and that you have the means to do it.

Eat a wide variety of minimally processed plants, and add some fermented foods: that is what has the most evidence behind it. Simple, cheap, and it holds.

The fibres your bacteria prefer are the ones your digestion lets through. Deprived of them, mice carrying a human microbiome lose diversity, and part of it is lost for good across generations 15. In humans, inulin-type fibres at three to twenty grams a day measurably raise bifidobacteria, according to fifty trials in over two thousand five hundred people 16.

Sleep, stress and physical activity matter too. Your starting questionnaire takes them into account.

In eight hundred people whose blood sugar was monitored continuously, the same meal did not produce the same response from one person to the next, and the microbiome helped predict it 17.

Even a probiotic does not settle the same way in everyone: in healthy volunteers, its colonisation depended on the person and their starting microbiome 18.

That is why Rhealiz does not hand you a formula once and for all. Your answers are re-read every quarter, and your programme follows.