What do polyphenols, such as those found in berries and red wine, do to your gut bacteria?
Polyphenols from berries and red wine have a positive influence on your gut bacteria, but the human studies are still small; eating a varied and colourful diet of fruit is a practical and safe starting point, without expecting sweeping health claims to hold up.
Polyphenols from berries and red wine largely reach the large intestine undigested. There they act as food for certain bacteria. Two small randomised crossover studies (ten participants each) show that 20 to 30 days of red wine polyphenols significantly increased the numbers of protective bacteria such as Bifidobacterium, Lactobacillus and butyrate producers. In participants with metabolic syndrome, bacteria that produce pro-inflammatory substances simultaneously declined.
Alongside that shift in bacterial composition, researchers in one study also observed a drop in blood pressure, blood lipids and an inflammatory marker (CRP). The increase in Bifidobacterium was associated with the decrease in cholesterol and CRP, but this is an association, not a proven cause-and-effect relationship. That nuance matters: the studies were small and the magnitude of the effects remains uncertain.
With anthocyanins, the pigments that give berries and purple-red fruit their colour, the picture is similar but less well studied in humans. Laboratory and animal research consistently shows that they stimulate Bifidobacterium and improve microbiome diversity. Direct evidence in humans is still limited. How anthocyanins suppress inflammation via the microbiome has so far been demonstrated mainly in the laboratory.
Another striking point: what polyphenols do in the body varies considerably from person to person. Polyphenols from soy, flaxseed or pomegranate are converted by certain gut bacteria into substances that are better absorbed and more potent than the original compound. Whether you obtain that benefit depends on which bacteria you have in your gut. Not everyone possesses the right bacteria for that conversion.
A systematic review that assessed seven human studies for quality confirmed that gut bacteria convert polyphenols into measurable products in blood and stool. However, direct measurements of microbiome composition were absent in six of those seven studies. The researchers concluded that firm health claims are premature. Larger and longer studies in humans are needed to establish how large and durable the effects truly are.
Two small randomised crossover studies (each n=10), one systematic review of seven human studies, and multiple reviews. The human randomised studies are limited in size; animal and laboratory studies support the mechanisms but are not directly transferable to humans.
What does alcohol do to your gut microbiome?
Chronic heavy drinking demonstrably disrupts the gut microbiome, with consequences for the heart and liver. Moderate drinking was not examined in these studies, but the mechanisms identified provide a clear picture of how harm arises.
What does eating red meat do to your gut in the long term?
Eating a lot of red and processed meat measurably increases your risk of colorectal cancer, although for most people the absolute risk remains limited. Cutting back on processed meat is the most concrete step you can take yourself.
What does drinking coffee every day do to your gut bacteria?
Drinking coffee in moderation every day (up to 4 cups) is associated with a more favourable composition and greater diversity of gut bacteria, but the evidence is observational and not always consistent. Above 5 cups per day risks begin to appear, so more is not better.
Does eating a varied diet help your gut bacteria?
Fermented products have the strongest evidence base for increasing gut bacterial diversity. Eating broadly varied meals also helps, but above all avoid heavily processed products containing emulsifiers such as CMC.
What does eating fibre do to your gut microbiome and your long-term health?
Fibre demonstrably feeds your gut bacteria, improves stool consistency and protects the gut wall. But how your microbiome and immune system respond varies considerably from person to person.
What does smoking do to your gut and gut microbiome?
Smoking disrupts your gut microbiome and increases the risk of intestinal inflammation, gastrointestinal complaints and certain forms of cancer. Quitting smoking is the most direct way to reduce these risks.