Food & Function: Daily Consumption of 40 Grams of Walnuts Linked to Improved Overall Mood

Daily consumption of 40 grams of walnuts over an eight-week period was found to be associated with an improvement in overall mood state among young adults. Researchers also identified changes in serotonin turnover markers and the concentration of metabolites produced when gut bacteria process compounds found in walnuts. The findings were published in the journal Food & Function.

The study involved 31 healthy individuals aged 20 to 35. The mean age of the participants was 24 years, with approximately 90% being women.

For eight weeks, participants either consumed 40 grams of walnuts daily with dinner or completely excluded walnuts and walnut-derived products. Following a two-week washout period, the groups crossed over. In both phases, participants were advised to follow a Mediterranean diet.

Following the walnut consumption period, total mood disturbance scores were, on average, 4.85 points lower than during the control period. Researchers also observed reductions in irritability and fatigue scores; however, these differences did not maintain statistical significance after adjustments.

Concurrently, urinary concentrations of 5-hydroxyindoleacetic acid (5-HIAA)—the primary breakdown product of serotonin—increased in participants. Higher levels of this metabolite correlated with lower overall mood disturbance. Meanwhile, cortisol levels, perceived stress, and overall sense of well-being did not change significantly.

Participants also showed increased concentrations of urolithins A and B—substances synthesized by gut microbiota when processing the polyphenols present in walnuts. Simultaneous increases in both metabolites were associated with elevated 5-HIAA levels.

The authors emphasize that the results do not prove that walnuts directly boost brain serotonin levels. Urinary 5-HIAA reflects systemic serotonin turnover, which is influenced by peripheral processes outside the brain. The study did not directly measure serotonergic activity within the central nervous system.

The researchers also noted several limitations of the study: it involved a small cohort of young adults—predominantly women—and used an open-label design, meaning participant expectations could have influenced the self-reported outcomes.

The authors conclude that these findings suggest a potential link between walnut consumption, gut microbiota metabolites, and serotonin metabolism. Larger trials involving diverse age groups and longer follow-up periods will be necessary to confirm these observations.