Polyphenols
A vast family of antioxidant plant compounds (over 8,000 identified) made by plants for defence, with beneficial effects on inflammation, the microbiome and ageing.
Polyphenols include flavonoids, phenolic acids, stilbenes (resveratrol) and lignans. Major sources: tea, coffee, cocoa, berries, grapes, olives, spices and red wine.
A fresh angle (bioavailability): it was long believed that polyphenols acted directly as antioxidants in the blood. In fact, they are poorly absorbed (often less than 10%)! The vast majority reach the colon, where the microbiome metabolises them into smaller, bioactive compounds.
In other words, much of the benefit of polyphenols comes from their transformation by gut bacteria. It is a two-way relationship: polyphenols feed the good microbiome (a prebiotic effect), and the microbiome « activates » the polyphenols. Without a healthy microbiome, you get less benefit from polyphenols.
A striking example: ellagitannins (pomegranate, raspberry) are converted by certain bacteria into urolithin A, a compound that stimulates mitophagy (the renewal of mitochondria) and may slow muscle ageing. But only around 40% of people have the bacteria needed for this conversion!
This explains why the same polyphenol-rich foods do not have the same effects from one person to another: it all depends on the microbiome.