The clinical research, led by Dr. Maria Izquierdo-Pulido, Professor of Nutrition and Food Science at the Torribera Campus of the University of Barcelona, examined the effects of daily walnut consumption on mood in healthy young adults. Published in the journal Food & Function and supported by the California Walnut Commission, the study provides new evidence on the potential relationship between walnut consumption, emotional well-being and the gut-brain connection.
During the 18-week randomized crossover clinical trial, 31 healthy young adults living in Spain, with an average age of 24 years, consumed 40 grams of walnuts daily for eight weeks. Compared with the control period, the intervention was associated with a significant reduction in overall mood disturbance, along with decreases in anger and fatigue. Researchers also observed improvements in certain aspects of emotional eating and food-related guilt, although these findings were exploratory in nature.
The improvements in mood were observed after eight weeks of walnut consumption, indicating a potential role for walnuts as part of a healthy dietary pattern.
The researchers also investigated the biological processes that could help explain the observed changes in mood. Their findings point towards a possible connection involving gut microbiota, walnut polyphenols and serotonin metabolism.
During the intervention, participants recorded significant increases in urinary levels of urolithins, which are metabolites derived from walnut polyphenols, as well as 5-HIAA, the main metabolite of serotonin.
Walnuts contain polyphenols, including ellagitannins, which are metabolised by gut microbiota after consumption. These compounds are transformed into urolithins, bioactive metabolites associated with antioxidant, anti-inflammatory and neuroprotective properties that may contribute to communication between the gut and the brain.
The study also found that increased urolithin levels were associated with higher urinary levels of 5-HIAA. This suggests a possible relationship between the metabolism of walnut polyphenols and serotonin-related pathways. However, further research is required to establish how these processes may influence 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 Dr. Maria Izquierdo-Pulido, the study's lead investigator at the University of Barcelona.
"The observed relationship between urolithins and serotonin metabolism opens an interesting avenue for further research into the connection between walnut consumption, gut microbiota, and emotional well-being," she added.
The latest findings complement research published in 2025 by the same research team, which examined the relationship between walnut consumption and sleep. That randomized crossover trial found that daily walnut consumption increased urinary levels of 6-sulfatoxymelatonin, a biomarker of melatonin, after the intervention period compared with the control period. Melatonin is a key hormone involved in sleep regulation.
Considered together, the two studies suggest that regular walnut consumption may have beneficial effects on different aspects of well-being, including mood and sleep. They also provide further research opportunities into the potential role of the gut-brain axis in these effects.
However, the researchers have highlighted limitations in the latest study. The trial involved a relatively small population of 31 participants and included a predominantly female sample. Longer-term research involving larger and more diverse populations is needed to confirm the findings and determine whether similar effects can be observed across different age groups and among people with mood disorders.
Beyond their potential relationship with mood, walnuts provide several nutrients and bioactive compounds. A serving contains approximately 4 grams of protein, 2 grams of fibre, 38 milligrams of tryptophan, 45 milligrams of magnesium and 2.5 grams of plant-based omega-3 alpha-linolenic acid (ALA).
Walnuts are also a source of polyphenols, with an antioxidant capacity measurement of 69.3 ± 16.5 µmol catechin equivalents per gram. However, antioxidant capacity measurements obtained through test-tube methods cannot be directly extrapolated to effects in humans. Clinical trials investigating the health benefits of dietary antioxidants have produced mixed results.
A 28-gram serving of walnuts provides 18 grams of total fat, including 2.5 grams of monounsaturated fat and 13 grams of polyunsaturated fat, of which 2.5 grams is plant-based omega-3 ALA. The serving also contains 1.5 grams of saturated fat.
The nutritional profile of walnuts includes protein, fibre, omega-3 fatty acids, minerals and bioactive compounds. Tryptophan, ALA and polyphenols are among the components that have been studied for their potential roles in processes associated with brain function and mood. Their inclusion in a balanced diet may therefore be of interest in broader nutritional approaches to well-being and brain health.
The researchers have also highlighted that incorporating walnuts into everyday meals does not require significant changes to existing eating habits. They can be added to cereals, yoghurt or oatmeal at breakfast, or incorporated into homemade bread. A handful can also be consumed as a snack between meals.
For main meals, walnuts can be included in salads, pasta, legumes and Mediterranean-inspired dishes.
"Walnuts are very easy to incorporate into daily eating," the researchers noted. "Our results suggest that a simple habit such as including a handful of walnuts a day could also be of interest in relation to mood, always as part of a healthy eating pattern."
The researchers and registered dietitian-nutritionists Drs. Maria Izquierdo-Pulido, Fernanda Zerón and Isabella Parilli emphasised that including a handful of walnuts in a daily diet is a simple dietary habit. They said the latest findings strengthen the interest in further investigating the relationship between walnut consumption and mood.
For recipes, usage ideas and additional information, consumers can visit californiawalnuts.in and follow @californiawalnutsindia on Instagram.
The research is supported by the California Walnut Commission (CWC), which represents more than 3,700 California walnut growers and approximately 70 handlers whose walnuts are sold worldwide. More than 99% of the walnuts grown in the United States come from California. These walnuts are cultivated primarily in multi-generational family orchards and are recognised for their quality, nutritional profile and wellness benefits.
Established in 1987, the California Walnut Commission conducts domestic and export market development activities, alongside health research, to support the sale of California walnuts across global markets.
The latest clinical findings add to growing research interest in the relationship between everyday dietary choices, gut microbiota and emotional well-being. While the results indicate a possible link between regular walnut consumption and improved mood, further clinical studies will be necessary to establish the extent and consistency of these potential benefits.
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