Walnuts, Long-Chain Polyunsaturated Fatty Acids and Adolescent Brain Development: A Dietary Intervention

Adolescence, an important window of opportunity and vulnerability for brain development Although less well studied, adolescence is also a critical period of brain development. The brain undergoes important organizational changes, with the re-emergence of gonadotropin-releasing hormone triggering a cascade of hormone dependent processes. Other biological processes involve epigenetic factors, which are highly sensitive to the environment and may therefore make this period of growth more vulnerable to external insults (Morrison, 2013). Moreover, the prefrontal cortex, which carries out important functions such as internally guided behaviors (control of emotion), logical thinking, working memory and organizing skills (executive function), is the last region of the brain to mature (early-twenties). The synaptic plasticity of the prefrontal cortex is accentuated during adolescence, a process that involves loss of grey matter density, and an increase in white matter volume, cerebral blood flow and synaptic pruning. Adolescence is also a time of refinement of brain connectivity and complex behaviors (Selemon, 2013). It is widely recognized that the synaptic plasticity of the brain decreases with age (Morrison, 2013; Selemon, 2013), but this pattern does not seem to follow a linear trend, and adolescence is an important period during which brain development can be enhanced and protected from environmental hazards with long-term consequences (everything from air pollution to social stress). There is a need for greater understanding of the biological pathways through which the environment affects human brain development, particularly during adolescence, when the most complex behaviors are refined to adulthood sophistication.

Long-chain polyunsaturated fatty acids, walnuts and brain development Nutrition during pregnancy, lactation, childhood and adolescence has a fundamental influence on overall development. Essential fatty acids are long chain polyunsaturated acids (LCPUFAs) that the body cannot synthesize by itself, and must be obtained through the diet (mainly from oily fish, seeds and nuts). Three of these fatty acids play an essential role in brain development: omega-3 (docosahexaenoic (DHA) and eicosapentaenoic (EPA) acids) and omega-6 (arachidonic acid (AA)) (McCann, 2005). LCPUFAs comprise approximately 15 to 30% of the brain's dry weight (Hallahan, 2005), and are involved in the function and architecture of the central nervous system throughout various stages of life. Studies show that DHA regulates neurotransmission systems such as serotonergic, dopaminergic, norepinephrinergic, and acetylcholinergic systems (Fontani, 2005). Of all edible plants, walnuts have the highest level of the omega-3 fatty acid, alpha-linolenic acid (ALA). ALA is the precursor for DHA and EPA, the two most essential fatty acids for the brain (Carey, 2013). Walnuts are also rich in fiber, vitamins, minerals, and other bioactive substances, such as phenolic antioxidants and phytosterols (Bao, 2013). Several studies have confirmed an increment in blood n-3 PUFA levels and improvements in lipid profile after a short-term intervention of a few walnuts per day (Marangoni, 2007; Maranhao, 2011). While most human studies have shown the cardiovascular benefits of walnut intake (Poulose, 2014), there is a need to investigate the potential beneficial effects on brain development, some incipient research on adults show positive results in cognition, these results are further described in Background section (Pribis, 2012; Sánchez-Villegas, 2011). These findings in adults provide a proof of concept for the potential brain development benefits of walnuts, which never has been studied among young teenagers.

This question could be investigated by using a population-based randomized controlled trial to assess whether walnut intake shows beneficial effects on teenage brain function (using neuropsychological assessments). It is ethical and feasible to recommend and facilitate walnut intake (30 g/day; Marangoni, 2007; Maranhao, 2011) over a long period of time (e.g. 12 months; de Ruyter, 2012). A Californian walnut contains ~0.40g of n-3 PUFAs (USDA National Nutrient Database for Standard Reference, 2012), such that a few walnuts (kernel 30g) per day would meet recommendations to consume at least 1g of n-3 PUFAs per day (Food and Nutritional Board, Institute of Medicine of the National Academies, US, 2014). Young Spanish adults (16-25 years) eat about 10g of nuts per day on average (Salas, 2013). Sea food also contains high n-3 PUFA concentrations, but sea food interventions are contra-indicated due to their methylmercury content, an important neurotoxic environmental pollutant (Grandjean, 2014). A limitation of using walnuts is that intake is less consistent than with a daily pill supplement of 1g of DHA, although this specific food recommendation is socially cost-effective and would be complied with by a wider range of people in the global health context.

Neuropsychological and behavioral assessment, using repeated measurements Recent experience in incorporating neuropsychological assessment into our cohort studies from INMA (Environment and Childhood, Guxens, 2012) has resulted in fruitful exchanges of knowledge between neuropsychologists and epidemiologists. Computerized neuropsychological tests performed in repeated exams are the most sensitive tool for studying high-order brain function and development in the general population (Julvez, 2010). Moreover, these tests have been shown in population-based studies to be very sensitive to the effects of environmental and nutritional determinants (Julvez, 2013). The investigators will build on this experience to efficiently select the appropriate range of tests to cover the complex neurofunctions that rapidly develop during adolescence, such as computer tests of working memory, executive function, motor speed, visual reasoning, selective and sustained attention and inhibitory functions (Selemon, 2012). Furthermore, the neuropsychological tests will be complemented by behavioral rating scales aimed at covering the socio-emotional endpoints that are so important during early adolescence (Selemon, 2012; Morrison, 2013). The fact to evaluate other behavioral areas (i.e., social competence and ADHD symptoms) may enrich the interpretation of the findings, and, possibly it contributes to further understand the intrinsic mechanisms. One limitation is the risk of losing outcome consistency because of selecting much dispersed behavioral areas without taking into account the link between them. The best option is a careful selection of neuropsychological and behavioral tools that efficiently embrace several areas of teenage brain development. Only a few trials of n-3 PUFA among adolescents over 11 years have used computer-based neuropsychological tests (Janssen, 2014). A recent large randomized controlled trial among healthy young adults using computerized cognitive tests found that reaction time latencies and working memory were improved after 6 months of DHA supplementation (Stonehouse, 2013). A double-blind, placebo-controlled trial among children with ADHD (7-12 years) found no treatment associations, but in the observational data, a positive association between DHA levels and computerized test scores of divided attention (Milte, 2012) was reported. Large trials measuring neuropsychological functioning using repeated computerized tests within the same subjects have not yet been carried out among healthy young adolescents, and this is required to better understand 'normal' brain development.

The specific hypotheses are: