• The drift away from the bacteria most people share begins between 40 and 50 years of age.
• Studies of people who reach 100 in different countries find different bacteria.
• They differ because those groups eat entirely different food.
• Different bacteria can do the same jobs, so chasing a named species makes little sense.
• Your bacteria make compounds that reach your own genes and change which ones are read.
• A richer community has more species covering each job, so it copes better when one is lost.
• Fermented foods moved microbial diversity in a trial where extra fibre alone did not.
• No supplement gives you microbial diversity. It builds from eating fermented food regularly and including a wide range of plants.

People who reach 100 in good health are worth being curious about. Their gut bacteria have been studied closely, and the findings often appear as a list of species to copy. But, the studies behind those lists name different bacteria from each other. The reason they differ changes what you would sensibly do about your own gut.

This post looks at what happens to your gut bacteria as you age. It covers why the research on very old people looks contradictory. And, it covers what those studies do agree on, which is not a list of species.

Researchers looked at more than 9,000 people aged 18 to 101, across three separate groups. They found that gut bacteria become more individual with age. The bacteria most people share, particularly a group called Bacteroides, gradually decline. What is left is a community more distinctive to you.

That drift begins between 40 and 50 years of age. So if you are in your forties or fifties, this is the window the research describes. It is also why the changes in your gut after 40 are worth understanding rather than ignoring.

Among the oldest people studied, the drift carried on in those who were healthy. It stopped in those who were less healthy. In a group of older men followed for four years, holding on to a high level of Bacteroides went with worse survival. That survival finding came from men, so it may not read across directly to women.

Put the main studies side by side and they point in different directions.

Japanese people aged 100 and over carry bacteria that make an unusual bile acid. Italian people aged 105 to 109 show a shrinking shared core, with Akkermansia, Bifidobacterium and Christensenellaceae rising.

Sardinian centenarians were low in Faecalibacterium prausnitzii, which other papers treat as a marker of a good gut.

And in a Chinese study of 1,575 people aged 20 to 117, including 297 centenarians, the pattern reversed. Those centenarians were over-represented for a Bacteroides dominated gut, which the authors described as youth associated. That is the opposite direction to the finding above.

So there is no single centenarian microbiome to copy. What these groups share is not a set of species but a set of functions.

These groups eat entirely different food. The bacteria differ because the food differs.

The clearest single difference between industrialised and non-industrialised guts is the ratio of two bacteria, Prevotella and Bacteroides. Prevotella is higher where people eat more fibre, and it has been found across rural populations from Burkina Faso to Tanzania to Venezuela.

The most telling evidence comes from people who migrated to a different country. In those who migrated from Southeast Asia to the United States, Prevotella strains were displaced by Bacteroides within months of arrival. The people had not changed, and neither had their genes, but the food had.

Different bacteria can do the same work. Researchers call this functional redundancy.

One study compared people eating omnivorous, pescatarian, vegetarian and vegan diets. The metabolic jobs their bacteria performed were much the same across all four groups. The species doing those jobs were different. So what matters is the work the bacteria do, not which species are doing it.

Across ageing studies, the jobs that keep recurring are much the same. Making short-chain fatty acids, transforming bile acids, and processing the amino acid tryptophan.

Three things, and they build on each other.

Your gut wall becomes more permeable with age

Researchers have shown this in people, not only in animals. More microbial material then crosses into the circulation, which keeps the immune system permanently occupied. That was associated with fewer new T cells being made, and more worn-out ones accumulating. In germ-free mice, which carry no gut bacteria at all, the age-related rise in permeability did not happen. So the ageing gut wall appears to take an active part in ageing the immune system, which connects it to low-grade inflammation.

Your bacteria make compounds that keep troublemakers down

The Japanese centenarians carried bacteria producing an unusual bile acid, and its production route had not been described before that study. It strongly inhibits several drug-resistant bacteria, including one that causes serious bowel infections. That was shown in the laboratory and in mice, rather than in people.

And your bacteria talk to your genes

When they ferment fibre they make short-chain fatty acids, one of which is butyrate. Butyrate blocks a family of enzymes whose job is to keep genes switched off. Blocking them lets those genes be read. So a compound made by bacteria in your bowel changes which of your own genes are active. This has been shown for genes involved in immune regulation, inflammation and cholesterol handling.

So the compounds your bacteria make reach your own cells and change how your genes work, regardless of which species are making them.

If several species can do a job, losing one does not stop the job being done. Researchers found that this redundancy was reduced in communities low in richness and dominated by Bacteroides. They suggested such communities may be less resilient to disturbance.

A depleted, Bacteroides dominated gut is a different thing from a Bacteroides associated pattern in a community that is otherwise rich and even. The bacterium has the same name in both cases, but the community around it is completely different.

In a study lasting ten weeks, 36 healthy adults were split into two groups. Their average age was 52, and most of them were women.

One group raised their fibre from about 21 g a day to about 45 g. Their microbial diversity stayed flat. The other group raised their fermented foods from almost none to about six servings a day. Their diversity rose steadily, and 19 inflammatory proteins in their blood fell. Larger servings had stronger effects.

The fibre group did see changes. It increased the enzymes their bacteria use to break down plant carbohydrates. And, the researchers noted more undigested carbohydrate in stool, which suggests the bacteria could not keep up with the increase.

The trial included 36 people over ten weeks, with no control group. The drop in inflammatory proteins was a secondary finding, not the outcome the researchers designed the trial to measure.

In clinic, I have seen far more come from changing what someone eats every day than from adding a probiotic on top of a diet that stays the same.

No supplement gives you microbial diversity. The variety of bacteria in your gut is built by what you eat every day.

That means something fermented most days, in the kind of quantities that made a difference in the trial above, around six servings a day. It means a range of plants across the week rather than a large amount of one fibre. And it means enough fibre to give your gut bacteria the raw material they need to produce short-chain fatty acids.

If your gut has changed through menopause, paying attention to what you eat becomes more important.

The drift in your gut bacteria begins between 40 and 50, which makes this a useful time to pay attention to it. Not because any single bacterium is the answer, but because what you eat every day is what your gut bacteria live on.