The 18-week clinical trial, led by Dr Maria Izquierdo-Pulido, Professor of Nutrition and Food Science at the university's Torribera Campus, involved 31 adults with a mean age of 24. The study was published in Food & Function here and supported by the California Walnut Commission
Participants consumed 40g of walnuts, roughly an ounce and a half, each day for eight weeks. Researchers then compared the results with a control period.
They found that regular walnut consumption was associated with a significant reduction in overall mood disturbance, alongside decreases in reported anger and fatigue. Some measures of emotional eating and food-related guilt also improved.
The researchers believe the effects could be linked to changes in the gut microbiota – the community of microorganisms living in the digestive system – and the way the body processes serotonin, a chemical messenger involved in regulating mood.
During the walnut intervention, participants showed increased urinary levels of urolithins, compounds produced when gut bacteria break down polyphenols found in walnuts. They also recorded higher levels of 5-HIAA, the main breakdown product of serotonin.
The relationship between the two biomarkers suggests that walnut consumption could influence serotonin-related pathways through the gut. However, the researchers stress that further work is needed to establish whether this is directly responsible for the changes in mood.
"What is particularly interesting about this study is that we not only observed an improvement in mood but also changes in biomarkers that help us explore the biological mechanisms that may underlie this effect," said Izquierdo-Pulido.
The findings build on research published by the same team in 2025, which found that daily walnut consumption increased levels of a biomarker associated with melatonin, a hormone involved in sleep regulation.
Together, the studies add to growing interest in the potential role of diet and the gut-brain axis in mental and physical wellbeing.
However, the latest study has limitations. The sample was small and predominantly female, while the intervention lasted only eight weeks. Further research will be needed to determine whether similar effects occur in larger and more diverse populations, including people experiencing mood disorders.
Walnuts are already recognised as a nutrient-dense food, providing protein, fibre, omega-3 fatty acids, magnesium and polyphenols. The researchers suggest that incorporating a handful into the daily diet could be a simple way of increasing intake of these nutrients, while potentially supporting wider aspects of wellbeing.
