What do polyphenols, such as those found in berries and red wine, do to your gut bacteria?

Gut bacteria convert polyphenols into smaller breakdown products, and it is precisely those breakdown products that appear to be largely responsible for the health effects. Polyphenols themselves are poorly absorbed in the small intestine, but the bacteria in the large intestine then break them down into active compounds that do end up in the bloodstream. This effectively makes the gut a 'factory floor' that turns an indigestible substance into something useful.

People who eat a lot of polyphenols, as in a Mediterranean diet rich in berries, olive oil, vegetables and the occasional glass of red wine, tend to have a greater diversity of bacterial species in the gut. Greater diversity is seen as a sign of a resilient gut. The contrast with a typical Western diet, low in polyphenols and fibre, is striking: that pattern is associated with a gut that becomes somewhat too 'leaky', allowing bacterial waste products to seep into the bloodstream and fuel low-grade inflammation.

In people with metabolic syndrome (a combination of abdominal fat, high blood sugar and poor blood lipids), polyphenols appear to partially restore the disrupted balance of the gut microbiome. This is promising, but most studies are still not large enough to make firm statements about which specific polyphenol has the greatest effect at which dose.

Finally, there are early, cautious indications that a polyphenol-rich diet promotes the production of substances that gut bacteria use to influence mood and sleep, such as serotonin and melatonin. However, this is based on limited research and is still a long way from practical advice.