A randomised crossover trial published in Food & Function found that eating 40g of walnuts daily for 8 weeks reduced total mood disturbance in healthy young adults compared to avoiding nuts altogether. The walnut condition also significantly raised urinary 5-HIAA, a marker of serotonin metabolism, along with urolithin A and B, metabolites produced when gut bacteria break down walnut polyphenols, and increases in these urolithins tracked alongside the 5-HIAA changes. No significant differences emerged in cortisol, perceived stress, well-being or positive mood. Since the study only measured urinary markers, not brain serotonin directly, researchers caution it cannot confirm how walnuts might influence central nervous system activity, and the small, mostly female sample and open-label design further limit how broadly the findings apply.
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