India, July 27 -- Think about the last time you blamed a food for how you felt. Maybe it was the dahi at lunch, so you skipped it for a week. Then it was the wheat, so you switched to millet rotis. Then it was dairy altogether, so you started ordering black coffee instead of chai. Most of us have run this exact experiment, cutting one food, then another, then another, without ever getting a real answer for what was actually happening inside our gut. We've all become amateur nutritionists by trial and error, and most of us are still wrong.

That instinct isn't irrational. It's the beginning of something the science is finally catching up to: elimination diets are no longer a fad; they're a diagnostic starting point. The question the food ...