Dietary polyphenol intake is not associated with changes in overall gut microbial diversity or community composition in healthy adults, despite responses in specific functional genes.

When people eat polyphenol-rich foods, the undigested compounds reach the gut and are broken down by specific bacterial enzymes. This doesn't change which bacteria are present, but it makes the ones already there better at processing these plant compounds. Different polyphenols activate different enzymes, so a varied diet increases the range of enzymes the gut bacteria can use. The bacteria that already have these enzymes grow more active or become more common, but no new types of bacteria take over.