Does my gut microbiome change as I get older, and does it matter?

Your gut microbiome changes throughout your entire life, without any clear tipping point. It is a slow, continuous process influenced by your diet, medications, living environment, and ageing itself. This is well supported, including by a systematic review of 27 studies in humans¹.

What actually changes is more nuanced than you might expect. Older people, and certainly the very oldest, sometimes have greater bacterial diversity than younger people. At the same time, the share of normally dominant bacterial families, such as Faecalibacterium and Lachnospiraceae, declines. There is no uniform 'old-gut-microbiome profile': composition varies considerably from person to person.

Notably, in people aged 105 and older, bacteria typically associated with good health, such as Akkermansia and Bifidobacterium, are relatively abundant. Whether those bacteria contribute to their advanced age or are simply a by-product of their good health has not been proven. This is an association, not a demonstrated cause.

Whether these changes are harmful has no simple yes or no answer. A diverse microbiome containing bacteria that produce anti-inflammatory fatty acids from dietary fibre is associated with healthier ageing in multiple global studies. However, cause and effect are difficult to disentangle: illness, poor diet and medications can also degrade the microbiome. Mouse studies suggest that a deteriorating gut microbiome may contribute to low-grade inflammation and premature death, but this causal link has not yet been firmly established in humans.

There are early indications of associations with muscle loss and diseases such as Alzheimer's. Regarding muscle loss, mouse studies showed that gut contents influenced the muscle profile of the recipient, but across ten clinical trials using probiotic and fibre supplements the effects on muscle mass were inconsistent. The link with Alzheimer's is still at an early stage. The gut microbiome of older people appears, incidentally, to be more responsive to dietary adjustments than that of younger people. The most consistently supported lever remains a fibre-rich diet, although the long-term effects on hard health outcomes are still under investigation.