A handful of walnuts a day for eight weeks, and young adults said their mood lifted. Their urine told a more complicated story

Nuts have a long run of good press for the heart. Whether they do anything for how people feel is a much thinner file, and that's the question a team at the University of Barcelona's Nutrition and Food Safety Research Institute set out to test with walnuts. Their trial was published in the journal Food & Function on 29 September 2026.

They recruited 31 healthy young adults, average age 24, and 28 of them were women. Each person did two eight-week stretches in random order: one eating 40 g of walnuts a day, roughly a small handful, and one eating no nuts at all, with a two-week break in between. Because everyone served as their own comparison, the trial could get by with fewer people than one with two separate groups. Along the way they filled in standard questionnaires on mood, stress, wellbeing and food cravings, and gave urine samples. The lab measured cortisol, a stress hormone; 5-HIAA, the substance the body makes when it breaks down serotonin; and urolithins, compounds gut bacteria make from the tannins in walnuts, which show whether people actually ate them.

On walnuts, the combined score for mood disturbance fell by 4.85 points compared with the nut-free weeks, a difference that was statistically significant but only just (p = 0.030). Urinary 5-HIAA went up, cortisol didn't move, and urolithins rose as you'd expect. The people whose 5-HIAA rose most tended to report the biggest drops in fatigue and in overall mood disturbance, and 5-HIAA climbed in step with urolithins. The authors suggest walnuts may act on serotonin through the gut, and call it a case for whole foods in nutrition approaches to mental health.

There are good reasons to hold that loosely. Walnuts are one of a handful of foods, with bananas, pineapple and plums, that patients are told to avoid for three days before a urine 5-HIAA test, because they push the result up. So more 5-HIAA after two months of daily walnuts may say more about what was in the walnuts than about serotonin in anyone's brain; the authors themselves say the rise could be dietary, microbial or metabolic. The trial was open-label, so people knew when they were eating walnuts and rated their own mood, which leaves room for expectation. Thirty-one people, nearly all young women, is a small and narrow group, and the summary doesn't say how big a 4.85-point change is on the scale used. Among the funders listed with the paper is the California Walnut Commission, alongside Spanish government and university grants. We could read only the summary and the paper's funding record, so the details come from those. A pleasant, promising pilot, then, and a reason for a larger, longer trial in a wider group, not proof that walnuts treat low mood.