Can you see how fast you are ageing from your stool?

The gut microbiome changes measurably with age. A systematic review of 27 studies shows that the composition of gut bacteria differs between young adults, younger older adults, and the very oldest elderly (80+). Some bacterial species increase at older ages, while others decline. All studies were snapshots in time, so cause and effect have not been established.

Based on 55 age-related bacterial species found in stool, researchers calculated a so-called 'microbiome age': a number indicating how 'old' your gut flora looks compared with your calendar age. This model was validated in more than 4,400 people. In metabolically unhealthy people aged 60 and over, such as those with obesity or high blood sugar, a higher microbiome age was linked to an up to 117% greater risk of cardiovascular disease. However, this is an association: there is as yet no evidence that actively adjusting your microbiome also reduces the risk.

There is also mechanistic research showing how gut bacteria influence ageing. A particular gut bacterium (Clostridium sp.) produces increasing amounts of phenylacetic acid as we age, a substance that accelerates vascular disease. In mouse studies, this sped up the ageing of blood vessels. At the same time, acetate, a protective substance produced by other gut bacteria, decreases with age. Whether these insights can be translated into treatments in humans remains unknown.

Stool analysis also has diagnostic potential. A meta-analysis of 8 studies identified 29 bacterial species that are consistently elevated in colorectal cancer; the researchers involved have commercial interests, which should be taken into account. In addition, research in nearly 800 cancer patients shows that microbiome patterns can predict who will benefit from immunotherapy. Abnormal patterns have also been found in Parkinson's disease, but those findings partly contradict one another and it is unclear what is cause and what is effect.

In short: stool analysis is not a mature ageing test you can do at home. There are promising associations, but clinically validated tests based on the microbiome do not yet exist for everyday use.