By Batya Swift Yasgur, MA, LSW

Walnuts have been associated with better cognitive development and psychological maturation in teens, new research shows.

Adolescents who consumed walnuts for at least 100 days showed improved sustained attention and fluid intelligence as well as a reduction in symptoms of attention deficit hyperactivity disorder (ADHD) compared with matched controls who did not consume the nuts. However, there were no statistically significant changes between the groups in other parameters, such as working memory and executive function.

Clinicians should advise adolescents "to eat a handful of walnuts three times a week for the rest of their lives. They may have a healthier brain with better cognitive function," senior investigator Jordi Julvez, PhD, group leader at the Institute of Health Research Pere Virgili and associated researcher at the Barcelona Institute for Global Health, told Medscape Medical News.

The study was published online April 6 in Lancet Discovery Science (eClinicalMedicine).

Rich Source of Omega-3s

Adolescence is "a period of refinement of brain connectivity and complex behaviors," the investigators note.  

Previous research suggests polyunsaturated fatty acids (PUFAs) are key in central nervous system architecture and function during times of neural development, with three specific PUFAs playing an "essential developmental role."

Two omega-3 fatty acids — docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) — are PUFAs that must be obtained through diet, mainly from seafood. Walnuts are "among the richest sources" of plant-derived omega-3 fatty acids, particularly alpha-linolenic acid (ALA), a precursor for longer-chain EPA and DHA.

ALA independently "has positive effects on brain function and plasticity," the authors write. In addition, walnut constituents — particularly polyphenols and other bioactive compounds — "may act synergistically with ALA to foster brain health."

Earlier small studies have found positive associations between walnut consumption and cognitive function in children, adolescents, and young adults; but to date, no randomized controlled trial has focused on the effect of walnut consumption on adolescent neuropsychological function.

The researchers studied 771 healthy adolescents (aged 11-16 years, mean age 14) drawn from 12 Spanish high schools. Participants were instructed to follow healthy eating recommendations and were randomly assigned 1:1 to the intervention (n = 386) or the control group (n = 385).

At baseline and after 6 months, they completed neuropsychological tests and behavioral rating scales. The Attention Network Test (ANT) assessed attention and the N-back test was used to assess working memory. The Tests of Primary Mental Abilities (PMA-R) assessed fluid intelligence. Risky decision-making was tested using the Roulettes Task.

Fruit and Nuts

Participants also completed the Strengths and Difficulties Questionnaire, which provided a total score of problem behavior. Teachers filled out the ADHD DSM-IV form list to provide additional information about ADHD behaviors.

The intervention group received 30 grams/day of raw California walnut kernels to incorporate into their daily diet. It is estimated that this walnut contains about 9 g of ALA per 100 g.

All participants received a seasonal fruit calendar and were asked to eat at least one piece of seasonal fruit daily.

Parents reported their child’s daily walnut consumption, with adherence defined as 100 or more days of eating walnuts during the 6-month period.

All main analyses were based on an intention-to-treat (ITT) method (participants were analyzed according to their original group assignment, regardless of their adherence to the intervention).

The researchers also conducted a secondary per-protocol (PP) analysis, comparing the intervention and control groups to estimate the effect if all participants had adhered to their assigned intervention. They censored data for participants who reported eating walnuts for less than 100 days during the 6-month trial period.

Secondary outcomes included changes in height, weight, waist circumference and BMI, as well as red blood cell (RBC) proportions of omega-3 fatty acids (DHA, EPA, and ALA) at baseline and after 6 months.

Adherence Counts

Most participants had "medium" or "high" levels of adherence to the Mediterranean diet, with "no meaningful differences" at baseline between the intervention and control groups in lifestyle characteristics or mean scores in all primary endpoints.

In the ITT analysis, there were no statistically significant differences in primary outcomes between the groups following the intervention. As for secondary outcomes, the RBC ALA significantly increased in the walnuts group but not the control group (coefficient, 0.04%; 95% CI, 0.03% - 0.06%; P < .0001).

However, there were differences in primary outcomes between the groups in the per-protocol analysis: the adherence-adjusted effect on improvement in attention score was −11.26 ms; 95% CI, −19.92 to −2.60; P = .011) for the intervention vs the control group.

The per-protocol analysis showed other differences: an improvement in fluid intelligence score (1.78; 95% CI, 0.90 - 2.67; P < .0001) and a reduction in ADHD symptom score (−2.18; 95% CI, −3.70 to −0.67; P = .0050).

"Overall, no significant differences were found in the intervention group in relation to the control group," Julvez said in a news release. "But if the adherence factor is considered, then positive results are observed, since participants who most closely followed the guidelines — in terms of the recommended dose of walnuts and the number of days of consumption — did show improvements in the neuropsychological functions evaluated."

Adolescence "is a time of great biological changes. Hormonal transformation occurs, which in turn is responsible for stimulating the synaptic growth of the frontal lobe," he continued, adding that this brain region "enables neuropsychological maturation of more complex emotional and cognitive functions."

"Neurons that are well nourished with these types of fatty acids will be able to grow and form new, stronger synapses," he said.

Food as Medicine

Commenting for Medscape Medical News, Uma Naidoo, MD, director of nutritional and lifestyle psychiatry at Massachusetts General Hospital, Boston, "commends" the researchers for conducting an RCT with a "robust" sample size and said she is "excited to see research like this furthering functional nutrition for mental health," as she believes that "food is medicine."

Naidoo, a professional chef, nutritional biologist, and author of the book This Is Your Brain on Food, said the findings "align" with her own approach to nutritional psychiatry and are also "in line" with her clinical practice.

However, although these results are "promising," more research is needed across more diverse populations to "make sure these results are truly generalizable," said Naidoo, a faculty member at Harvard Medical School who was not involved with the study.

She "envisions a future where the research is so advanced that we can 'dose' these healthy whole foods for specific psychiatric symptoms and conditions."

This study was supported by Instituto de Salud Carlos III (co-funded by European Union Regional Development Fund "A way to make Europe"). The California Walnut Commission (CWC) has given support by supplying the walnuts for free for the Walnuts Smart Snack Dietary Intervention Trial. Julvez holds a Miguel Servet-II contract awarded by the Instituto de Salud Carlos III (co-funded by European Union Social Fund). The other authors' disclosures are listed in the original article. Naidoo reports no relevant financial relationships.

Lancet Discovery Science (eClinicalMedicine). Published online April 6, 2023. Full text

Batya Swift Yasgur, MA, LSW is a freelance writer with a counseling practice in Teaneck, NJ. She is a regular contributor to numerous medical publications, including Medscape and WebMD, and is the author of several consumer-oriented health books as well as Behind the Burqa: Our Lives in Afghanistan and How We Escaped to Freedom (the memoir of two brave Afghan sisters who told her their story).