I have been staring at the same shelf of probiotic bottles in my pantry for the better part of a decade. Thirty billion CFUs in the blue bottle. Fifty billion in the white one. A refrigerated capsule that cost me seventy dollars and promised to repopulate my gut with strains I could barely pronounce. And for most of those years, I swallowed them faithfully, trusting that I was doing something profoundly good for my body. Then my nutritionist drew me a diagram on the back of a napkin that made every one of those bottles feel like a detour. She…