Associations between diet and mood disorders during youth and young-to mid-adulthood

Background: The mood disorders depression and dysthymia are highly prevalent conditions that affect an individual's capacity to function and participate in society. Dietary intake supports neurobiological functions and may influence mental health outcomes. There is consistent evidence of cross-sectional associations between better diet quality and lower prevalence of depressive symptoms, but there are few prospective studies, particularly among child or adolescent (herein referred to as youth) cohorts. Moreover, some dietary behaviours related to timing of meals (e.g., skipping breakfast) are understood to be associated with diet quality and may influence hormones and neurobiology. However, there are few longitudinal studies on associations between timing of eating occasions and mood disorders. There are also few longitudinal studies that have examined associations between overall diet or dietary behaviours and mood disorder outcomes defined by diagnostic criteria. Due to the high prevalence of mood disorders and in some cases, limited efficacy of or access to clinical and pharmaceutical treatments, it is important to consider the lifestyle factors such as diet that could help prevent or modulate mood disorders. Aims: To develop a consistently structured measure of diet quality for use among both children and adults, and examine whether diet quality and time-of-day eating patterns were associated with mood disorder outcomes that met diagnostic criteria, during youth and adulthood among an Australian cohort. Methods: The research aims were examined through four original studies using data from the Childhood Determinants of Adult Health (CDAH) study. The CDAH study comprises data from a nationwide health, fitness and dietary survey of 8,498 Australian schoolchildren aged 7-15 years in 1985, and three follow-ups in adulthood between 2004 and 2019. Key measures include: a baseline 24-hour food record in youth; food frequency and food habit questionnaires in adulthood; structured diagnostic interviews in adulthood to identify 12-month (at all three follow-ups) and lifetime prevalence (at second and third follow-up) of DSM-IV major depressive disorder and dysthymic disorder; and a range of demographic, socioeconomic, psychosocial, and physical covariates at baseline and follow-up. Results: The first study involved development and validation of a sex- and age-specific Dietary Guidelines Index (DGI) that measured adherence to the Australian Dietary Guidelines. The DGI was found to be an adequate measure of diet quality among youth and young adults and the DGI scores highlighted that, on average, diet quality among the cohort was poor. In the second study it was found that diet quality in youth was not associated longitudinally with onset of mood disorders in adulthood. Similarly, the third study found that diet quality in adulthood was not associated cross-sectionally or longitudinally with mood disorders. However, there was a non-significant association between better diet quality in youth and a lower risk of mood disorder in late adolescence/early adulthood in the second study, and significant cross-sectional associations in the minimally adjusted analyses in mid-adulthood in the third study, suggesting further research is needed. The fourth study found that in adulthood, higher adherence to a Late‚ÄövÑvp time-of-day eating pattern, characterised by delayed or skipped breakfast, was associated with a higher prevalence of mood disorder, and the relationship may be bidirectional. Conclusion: There was very limited evidence of longitudinal associations between diet and risk of mood disorders. Continuing work on public health campaigns and regulation of the food industry to support individuals to make healthy food choices, is required to improve diet quality and meal habits among the Australian population. This could help lower mood disorder risks due to improvements in physical and psychosocial health, but further prospective research is needed to determine if dietary intake is independently and causally associated with mood disorder outcomes.