A handful of walnuts a day eased mood disturbance slightly, though depression scores didn’t budge
STUDY: Zerón-Rugerio MF et al, Food & Function 2026
STUDY TYPE: Open-label randomized crossover trial
FUNDING: California Walnut Commission University of Barcelona
Background
Walnuts contain omega-3 fats, tryptophan, and polyphenols that gut bacteria convert into urolithins, compounds that may affect brain signaling.
The Study
- 36 healthy, normal-weight Spanish adults (mean age 24, 90% women) were randomized; 31 completed both arms.
- Participants ate 40 g of walnuts (about a handful) with dinner for 8 weeks and avoided all nuts for 8 weeks, in random order, with a 2-week washout.
- Primary outcome: total mood disturbance on the Mood scale (POMS). Urine was tested for 5-HIAA, a serotonin breakdown product, and urolithins.
Results
Total mood disturbance was about 5 points lower while on walnuts (21 vs 26 on a 15 to 75 scale).
Fatigue and anger dropped slightly, but those findings didn’t hold up after correcting for multiple comparisons. Depression, weight, perceived stress, and well-being didn’t change.
Urinary 5-HIAA rose on walnuts, and higher levels correlated with lower fatigue and mood disturbance.
Side Effects
None.
Limitations
- Small sample of healthy young women, none with a mood disorder.
- Unblinded. Funded by the walnut industry.
- Diet quality was higher during the walnut phase.
Practice Implications
- This study joins other small ones showing gains in mood and sleep with a daily serving of nuts.
- Patients with unhealthy diets are likely to benefit most.
- Aim for low salt, but start with any nuts to get the habit going. More on diet for mood.
—Chris Aiken, MD
Director, Psych Partners
Editor in Chief, Carlat Psychiatry Report
