The ultimate guide to your microbiome and its links to mental health from Dr Norman Swan, author of So You Want To Live Younger Longer?

Bugs, bowels and hormones

Our colonic clock

It looks as though you can tell the biological age of someone with reasonable accuracy by analysing their microbiome. We probably acquire our initial microbiome during pregnancy from the amniotic fluid and placenta and at birth from vaginal delivery. A baby’s microbiome changes a lot in the first three years and is influenced by being breastfed versus formula, how many brothers and sisters you have, whether there are pets, exposure to fermented foods (Metchnikoff wasn’t all wrong), and geography, travel, hospital stays and antibiotic prescriptions. After the age of around 20, the microbiome is fairly stable until it starts to deteriorate later in life or due to poor diet, disease and medication use. Young and healthy people have forests of different kinds of organisms with huge diversity and fairly predictable families of bugs that do good things like produce substances called short chain fatty acids (SCFAs) which, among many functions, make the immune system more tolerant and less likely to fire up unnecessarily. They also reduce oxidative stress and probably influence the gut’s incredibly complicated nervous system, which is thought to regulate how we absorb nutrients from the diet. These microbial families – or taxa – also shore up the lining of our gut, making it less leaky and therefore less likely to absorb toxins and pass them around the body. I said biological age because healthy older people can have the microbial signature of people a fraction of their age.

Conversely, people who are becoming biologically old and frail have thinned-out forests with far less diversity. Their microbial families are biased towards damaging metabolism where irritating substances like bile acids, p-cresol and even ethanol and carbon dioxide build up.

Chicken or egg? Before I go on, what’s not been fully sorted out yet is cause and effect. I said earlier that the microbiome could be both passenger and driver of the ageing bus. The bus may be badly driven, fuelled with dirty diesel, stuffing up the passengers. Equally the driver herself could be part of the microbiome guiding the bus to happier destinations and making it less polluting. All this means that sometimes you’re heading for disappointment when trying to manipulate the microbiome. Our ignorance of how it all works in detail might end up with us inadvertently targeting the passengers rather than the drivers. May not be harmful, just frustrating.

Even so, probiotics should work against ageing, right?

Well, wrong, at least at the moment, partly because the anti-ageing effects come from taxa – massive groups of bacteria – of different kinds but which have similar metabolic profiles. Researchers are trying to see if there are, within these populations, specific bacteria which would do the job if given as a supplement but trials of such specific probiotic bacteria have not shown benefits – at least not yet. Interestingly, metformin (see Which Pill and Why?) works on the microbiome to increase SCFA production, which may be why it could have an antiageing effect. Over the next few years expect to see more and more microbiome-targeted therapies but in the meantime …

A prebiotic approach does seem to work

The reason why people’s microbiomes become less diverse and shift to a more pronounced state of inflamm-ageing is thought to be that people’s diets become less diverse, containing more fat – especially saturated fats – and more refined carbohydrates. In fact, there are groups of bacteria in the bowel which are fairly tightly linked to chronic diseases like diabetes, cognitive impairment and frailty. This is even more exaggerated if someone is admitted to residential aged care with its often notoriously bad food. Antibiotics don’t help either and can cause havoc in the microbiome. Low fat, high fibre diets, on the other hand, are associated with more diverse, healthier microbiomes. The prebiotic approach aims to maintain and increase the diversity of foods, with more fruit and vegetables, more protein from legumes and fish and less from red meat, more polyunsaturated and monounsaturated fats (olive oil) and less saturated fat. That pattern gives you a good dose of bioactives which interact with each other, enhancing their activity and profoundly influencing the microbiome.