Fiber intake is the strongest dietary predictor of microbiome diversity

Fiber is NOT the strongest predictor of microbiome diversity: This large re-analysis of 21 fiber intervention studies found that who you are (your individual biology) accounts for 82% of microbiome composition, while fiber intake accounts for only 1.5%

Fiber actually decreased diversity: Contrary to popular belief, fiber interventions consistently reduced overall microbiome diversity — likely because fiber selectively feeds specific bacterial groups rather than broadly enriching the whole microbiome

Key limitations: Results come mostly from Western populations, used many different fiber types pooled together, and lacked full control for factors like age, sex, and baseline diet

Bottom line: Across 12 independent human studies, fiber explains a tiny fraction of microbiome variation — the claim that it is the "strongest" dietary predictor is not supported by this evidence