Could something as simple as adding a handful of walnuts to dinner influence how you feel?
A new study suggests it might. Researchers found that young adults who ate walnuts every day for eight weeks experienced a modest improvement in overall mood, alongside changes in a metabolite linked to serotonin metabolism. The findings add to growing interest in how food, gut microbes and the brain communicate, but they do not mean that walnuts directly increase brain serotonin or treat depression.
The study involved 31 healthy young adults in Spain, with an average age of about 24. Participants followed an 18-week randomized crossover design. During one eight-week period, they consumed 40 grams of walnuts every day. During another, they avoided walnuts and other nuts. A two-week washout period separated the two phases. About 90% of the participants were women.
After the walnut period, participants had lower scores for total mood disturbance compared with the control period. Anger and fatigue also appeared lower, although these individual effects did not remain statistically significant after correction for multiple comparisons. They did not find significant improvements in perceived stress, well-being or vigor. The more intriguing finding came from the participants’ urine.
They measured 5-hydroxyindoleacetic acid (5-HIAA), the main breakdown product of serotonin. Urinary 5-HIAA increased significantly during the walnut period, and higher 5-HIAA levels were associated with lower total mood disturbance. Urinary 5-HIAA is a marker of whole-body serotonin metabolism. It does not tell researchers how much serotonin was present in the brain. The study did not directly measure brain serotonin activity. So the results cannot establish that walnuts made participants happier by increasing serotonin in the brain. Instead, they provide a possible biological connection worth investigating.
Walnuts contain polyphenols, including ellagitannins. These compounds can be transformed by gut microorganisms into metabolites called urolithins. They found that urinary levels of urolithin A and urolithin B increased during walnut consumption. More interestingly, increases in these metabolites were associated with increases in urinary 5-HIAA. This raises the possibility that compounds from walnuts, after being processed by gut microbes, could be involved in pathways connected with serotonin metabolism. It is still only a possibility, however. The study was not designed to prove that urolithins cause mood changes.
The findings are interesting, but the study has clear limitations. Only 31 participants completed both intervention periods, they were young and healthy, and the sample was predominantly female. The study was also open-label, meaning participants knew whether they were eating walnuts. Previous research has produced mixed results.
A 2016 randomized crossover trial involving 64 college students found no significant overall mood improvement when men and women were analyzed together, although a benefit was observed in the male subgroup. That makes the new findings better viewed as evidence of a possible relationship rather than proof that walnuts are a mood treatment. Still, the combination of mood changes, serotonin-related metabolism and gut-derived metabolites gives researchers an intriguing pathway to explore. For now, walnuts remain a nutrient-rich food that can fit easily into a healthy diet. Whether they can reliably improve mood and exactly how the gut and brain might be involved, will require larger and longer studies.
