Multiple human studies confirm that the gut microbiome changes gradually with age, but the direction of that change is not automatically harmful. A diverse microbiome with fatty-acid-producing bacteria is associated with healthier ageing, although cause and effect have not yet been proven. The evidence is largely observational and the effects of targeted interventions on hard health outcomes remain inconsistent.
The gut microbiome changes throughout life, without any specific tipping point. It is a slow, continuous process influenced by what you eat, which medications you take, where you live, and by ageing itself. This is solidly supported by multiple human studies, including a systematic review of 27 studies.
What exactly changes is more nuanced than you might expect. Older people, and certainly the very oldest, sometimes turn out to have greater diversity in bacterial species than younger people. At the same time, the presence of bacterial families that are normally dominant, such as Faecalibacterium, Bacteroidaceae and Lachnospiraceae, declines. Species composition varies considerably from person to person, so there is no uniform 'old gut microbiome profile'.
Notably, in people who reach extremely old age, such as those aged 105 to 109, relatively large numbers of bacteria that are generally associated with good health are present: Akkermansia, Bifidobacterium and Christensenellaceae. Whether those bacteria are the key to their advanced age, or are instead a by-product of their good health, has not yet been proven. This is an association, not a demonstrated cause.
The question of whether changes in the gut microbiome are harmful has no simple yes or no answer. A diverse microbiome with bacteria that produce short-chain fatty acids (substances that gut bacteria produce from dietary fibre and that suppress inflammation) is associated in multiple global studies with healthier ageing. But cause and effect are difficult to disentangle here: illness, poor diet and medications can also degrade the microbiome. Animal studies suggest that a deteriorating gut microbiome may contribute to a 'leaky gut', low-grade inflammation and premature death, but this causal link has not yet been firmly established in humans.
There are also early indications of links between gut microbiome disruptions and muscle loss (sarcopenia) and neurodegenerative diseases such as Alzheimer's. In the case of sarcopenia, animal experiments showed that gut contents themselves influenced the muscle profile of the recipient, but across the ten clinical studies that tested interventions such as probiotics and prebiotics, the effects on muscle mass and strength were inconsistent. The link with Alzheimer's is still at an early stage of research in humans. These associations are interesting but not yet strong enough to draw firm conclusions from.
The gut microbiome of older people appears to be more responsive to modification through diet, probiotics and prebiotics than that of younger people. This offers potential avenues for intervention in principle, but which specific intervention produces which health benefit has not yet been established conclusively. The most consistently supported approach is a fibre-rich diet that favours bacteria producing short-chain fatty acids, although even here the long-term effects on health outcomes are still under investigation.
The claims are based on multiple human observational studies and a systematic review of 27 studies (PMID 33297486), supplemented by animal experimental and mechanistic research. No large randomised controlled trials (RCTs) with hard health outcomes are available on this topic. All associations are observational unless stated otherwise.