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Daily Walnuts Lower Mood Disturbance Scores In Young Adults

A small crossover trial found daily walnuts eased mood disturbance and raised a serotonin breakdown marker in urine, hinting at a gut-brain link.

By Dr. Axe Editorial Team · October 23, 2026 · 1 source · Watch

Photo: Pexels

Eating 40 grams of walnuts a day for 8 weeks lowered total mood disturbance scores by 4.85 points compared with a nut-free control period, according to a study published October 19, 2026 in Food & Function.

4.85pt lower mood score

40gwalnuts daily

8-weekintervention

31adults studied

In this 18-week open-label crossover trial, 31 healthy young adults (average age 24, 90.3% female) ate 40 grams of walnuts daily for 8 weeks and spent another 8 weeks on a nut-free diet, separated by a 2-week washout. Compared with the nut-free period, the walnut period lowered total mood disturbance scores by 4.85 points. Walnut intake also raised urinary 5-hydroxyindoleacetic acid (5-HIAA), a breakdown product of serotonin measured in urine, with no change in urinary cortisol, a stress hormone. Urinary urolithins, compounds the gut microbiota makes from walnut polyphenols, rose during the walnut period and tracked closely with the 5-HIAA increase.

Within each person, bigger rises in urinary 5-HIAA were linked to less fatigue and lower mood disturbance scores. The authors say these simultaneous urinary changes may reflect dietary, microbial or metabolic responses to walnuts, though they say the pathways still need investigation. Serotonin is largely produced and processed in the gut, so a gut-bacteria byproduct lining up with a serotonin marker is a reasonable biological thread, though this study cannot pin down the exact chain of events.

Mood Disturbance Score Drop With 8 Weeks of Walnuts

Food & Function, 2026, Zerón-Rugerio et al.

This was an open-label trial with no blinding, so participants knew which period they were in, and the sample was small and overwhelmingly female. The urinary metabolite associations are correlations within individuals, not evidence that walnuts or urolithins directly caused the mood change.

Dr. Axe’s Take

A 4.85-point drop in mood disturbance from a handful of walnuts a day is a meaningful signal, especially paired with a rise in a serotonin-related marker that tracked with less fatigue. I see this as more support for treating the gut as part of the mental health picture, not just a digestion organ. Walnuts bring omega-3 fats and polyphenols that gut bacteria turn into urolithins, compounds that may support brain signaling. This week, I would add a small daily serving of raw walnuts to breakfast or a snack rather than reaching for coffee when energy and mood dip midafternoon.

— Dr. Axe

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Study summary

Who: 31 healthy young adults, average age 24, 90.3% female
Design: 18-week open-label randomized crossover trial with 2-week washout
Dose: 40 g walnuts daily for 8 weeks vs. nut-free control for 8 weeks
Primary result: Total mood disturbance score 4.85 points lower during walnut period vs. control
Secondary: Urinary 5-HIAA rose during walnut period with no change in cortisol; urolithin increases tracked with 5-HIAA increases; higher 5-HIAA linked to less fatigue and less mood disturbance
Funding / conflicts: Funders listed in the journal record: Ministerio de Ciencia, Innovación y Universidades; Agencia Estatal de Investigación; Universitat de Barcelona; California Walnut Commission. Conflicts not reported in the abstract.
Date: Oct 19, 2026