In a recent study published in Food & Function, researchers evaluated whether 8 weeks of walnut consumption affects mood states in young adults and whether these effects are associated with urinary 5-hydroxyindoleacetic acid, or 5-HIAA, cortisol, and urolithin levels.
The 18-week open-label randomized crossover trial found that consuming 40g of walnuts daily for 8 weeks was associated with lower total mood disturbance and higher urinary 5-HIAA in healthy young adults. However, researchers caution that the underlying pathways remain uncertain.
Why Walnuts May Influence Mood
Mental health disorders affect 13.9% of the global population, with the burden expected to rise among young people. Diet has emerged as a lifestyle factor relevant to mental health due to the brain’s high energy and nutrient requirements.
Serotonin is a neurotransmitter involved in mood regulation and is produced from the dietary amino acid tryptophan. Its main metabolite, 5-HIAA, can be measured in urine as an indicator of serotonin metabolism.
Walnuts are rich in omega-3 fatty acids, tryptophan, fiber, and polyphenols, including ellagitannins that gut microbiota convert into urolithins. These metabolites may indicate walnut intake and biological responses. However, further research is needed to clarify how walnut consumption relates to mood and metabolic pathways.
Putting Daily Walnut Consumption to the Test
The study used an 18-week open-label randomized crossover trial involving adults aged 20–35 years. Researchers recruited participants from December 2021 to February 2022 at the University of Barcelona. To be eligible, candidates had to have a body mass index of 19 to 26 kg/m² and maintain moderate physical activity. Exclusion criteria included nut allergies, acute or chronic illnesses, smoking, use of drugs or dietary supplements, restrictive diets, and shift work. Of 84 screened individuals, 36 were enrolled; 5 withdrew, leaving 31 participants who completed both arms.
The University of Barcelona Ethics Committee approved the study, and it was registered at ClinicalTrials.gov. Participants were randomly assigned to consume 40g of walnuts per day with dinner or to refrain from nuts and nut products for 8 weeks. After a 2-week washout period, participants crossed over to the alternate condition.
A nutritionist provided standardized guidance to follow a Mediterranean-style diet in both conditions. Mood was assessed with the Profile of Mood States Questionnaire, or POMS, while perceived stress, well-being, and trait food cravings were assessed with validated Spanish questionnaires.
First-morning urine samples were obtained for 5-HIAA and cortisol analysis using ELISA, normalized to creatinine, and for urolithins using HPLC-QTOF-MS. Researchers used linear mixed-effects models and repeated-measures correlations.
Key Findings: Walnut Consumption Reduced Total Mood Disturbance
The 31 participants had a mean age of 24 years, 90.3% of whom were women. At baseline, mean BMI was 21.6 kg/m² and body fat percentage was 25.7%.
During the walnut condition, total mood disturbance was 4.85 points lower than during the control condition. Anger and fatigue also decreased, but these differences did not remain statistically significant after correction for multiple comparisons. Furthermore, no significant differences occurred for friendliness, vigor, perceived stress, or well-being.
Two trait food-craving domains showed initial reductions: anticipation of relief from negative states and feelings after eating and guilt associated with cravings. However, neither finding remained significant after multiple-testing correction. In repeated-measures analyses, higher scores in both domains were associated with increases in anger, fatigue, or total mood disturbance.
Urinary 5-HIAA increased significantly during walnut consumption, while urinary cortisol did not differ significantly between conditions. Higher urinary 5-HIAA was associated with lower total mood disturbance. The paper also reports an association with lower fatigue, although the reported significance is internally inconsistent.
Urinary urolithin A and urolithin B concentrations were also significantly higher during walnut consumption, and increases in both metabolites were associated with concurrent increases in urinary 5-HIAA. Meanwhile, BMI, body fat percentage, and physical activity did not differ significantly. Diet quality was modestly but significantly higher during the walnut condition.
Analysis: What the Findings Mean for Mood and Serotonin Metabolism
1. Potential link, not proven mechanism: Together, these findings suggest a potential association between walnut-derived microbial metabolites and whole-body serotonin metabolism. Urinary 5-HIAA reflects whole-body serotonin metabolism and can be influenced by peripheral serotonin production, diet, intestinal metabolism, and renal excretion.
2. Brain serotonin not measured: The study did not measure brain serotonin or central serotonergic function. Therefore, these observed associations cannot establish a mechanism linking walnuts with changes in brain serotonin activity.
3. Cortisol and well-being unchanged: The study found no significant differences in urinary cortisol levels, perceived stress, well-being, or positive mood, suggesting effects were specific to mood disturbance.
4. Important limitations: The small sample of 31, predominance of women, and open-label design may have introduced expectancy bias. Researchers did not conduct another assessment immediately before the second intervention period, so they could not directly confirm return to comparable baseline states after washout. Participants also avoided all nuts during control, which may have differed from usual diets, and the young population limits generalizability.
Q&A
Q: What did the walnut study test?
A: Whether 40g of walnuts daily for 8 weeks affects mood states and urinary 5-HIAA, cortisol, and urolithins in healthy adults aged 20-35 in a randomized crossover trial published in Food & Function.
Q: What was the main result on mood?
A: Total mood disturbance on the POMS questionnaire was 4.85 points lower during the walnut condition versus control. Anger and fatigue reductions did not remain significant after correction.
Q: How did serotonin-related markers change?
A: Urinary 5-HIAA, the main metabolite of serotonin, increased significantly during walnut intake and was associated with lower mood disturbance. Urolithin A and B also increased and correlated with 5-HIAA changes.
FAQ
1. How many walnuts are 40 grams?
About a small handful, roughly 7-8 whole walnut halves per 10g, so 40g is about 28-32 halves, typically eaten with dinner in this study.
2. Did walnuts reduce stress or improve well-being?
No significant differences were found for perceived stress, well-being, friendliness, vigor, or urinary cortisol between walnut and control conditions.
3. What are urolithins?
Urolithins are gut microbiota metabolites produced from ellagitannins in walnuts. They may indicate walnut intake and biological responses.
4. Can walnuts increase brain serotonin?
This study did not measure brain serotonin. Urinary 5-HIAA reflects whole-body metabolism, not central brain activity, so brain effects cannot be confirmed from this data.
5. Who should not try this?
People with nut allergies, chronic illnesses, or restrictive diets were excluded from the trial. The findings apply to healthy young adults and need larger, more diverse studies.
