, 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 attension deficit hyperactivity disorder, 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,” said senior investigator Jordi Julvez, PhD, group leader at the Institute of Health Research Pere Virgili, Barcelona, and associated researcher at the Barcelona Institute for Global Health.
The study was published online in eClinicalMedicine.
Rich source of omega-3s
Adolescence is “a period of refinement of brain connectivity and complex behaviors,” the investigators noted.
Previous research suggests polyunsaturated fatty acids 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 and eicosapentaenoic acid – 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 wrote. 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 assessed attention, and the N-back test was used to assess working memory. The Tests of Primary Mental Abilities 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 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 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 proportions of omega-3 fatty acids (DHA, EPA, and ALA) at baseline and after 6 months.