How much does your gut bacteria actually change if you switch to a vegan diet?

Your gut bacteria can shift pretty noticeably within a few weeks of going vegan - mostly because you're suddenly eating a lot more fiber and plant-based compounds that feed different bacterial populations than meat and dairy do. The exact changes depend on what you were eating before and what your new diet looks like, but studies generally show increased diversity and growth of bacteria that break down fiber. It's not a total overhaul of your microbiome or anything, but it's a real and measurable change that your gut adapts to fairly quickly.