Eat a walnut and your gut bacteria turn its ellagitannins into urolithins, compounds that may be tied to how well your brain ages. In a 2-year MRI sub-study of the WAHA trial, bigger rises in urolithin A, urolithin C, and urolithin C glucuronide were associated with preserved cortical thickness among walnut eaters, mostly in right-hemisphere regions. The pattern was stronger in people with metabotype B gut profiles. But this is an association inside a randomized trial, not proof of cause, and it says nothing direct about urolithin A supplements. Here's what the data show, who might care, and who should stay skeptical.
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