Reishi mushroom (Ganoderma lucidum) spore powder reversed signs of kidney aging in mice by reshaping their gut microbiota. The supplement boosted beneficial bacteria from the Lachnospiraceae family, which produced a metabolite called nicotinamide riboside (NR), a known NAD+ precursor that improved kidney function by supporting steroid metabolism. This research shows how mushroom spore powder can target the gut-kidney axis and may offer a natural strategy to delay age-related renal decline.