Dietary fibre and its associations with inflammation and depression in adolescence

Dietary fibre and its associations with inflammation and depression in adolescence Dietary fibre, defined as non‚ÄövÑv™digestible carbohydrates obtained from plant foods, is a key part of a healthy diet. It has a wide range of health benefits; however, dietary fibre intakes are often inadequate in both adults and adolescents. There is evidence to suggest that improvement in mental health may be one of the benefits of dietary fibre. Despite the high incidence of depression in adolescence, associations of dietary fibre intake with depression in this age group are insufficiently studied. A mechanism of action for dietary fibre may include reducing systemic inflammation through alterations to the gut microbiota. Although research supports a connection between dietary fibre and inflammation in adults, studies in adolescents are lacking. An understanding of the relationships between dietary fibre intake and depression in adolescence, along with the potential pathways through which an effect may occur, could help improve quality of life for adolescents with depression. The aim of this thesis was to determine dietary fibre intake in a population of Australian adolescents and to investigate the associations between dietary fibre intake, inflammation, and depressive symptoms. Data for this thesis came from the 14-and 17‚ÄövÑv™year follow‚ÄövÑv™ups of the Raine Study, a Western Australian pregnancy cohort study. The study had 2868 infants enrolled at birth who have been followed up regularly throughout childhood, adolescence, and young adulthood. In total, 1864 adolescents participated in the 14‚ÄövÑv™year follow‚ÄövÑv™up and 1726 in the 17‚ÄövÑv™year follow‚ÄövÑv™up. Dietary data were collected using the Commonwealth Scientific and Industrial Research Organisation (CSIRO) semi‚ÄövÑv™quantitative food frequency questionnaire (FFQ). Inflammatory markers(C‚ÄövÑv™reactive protein, leptin, and adiponectin) were measured from fasting blood tests while depressive symptoms were assessed using the Beck Depression Inventory for Youth. The analysis of dietary fibre intakes in adolescents in this thesis found that mean intakes of dietary fibre did not meet Australian dietary guidelines other than for females aged 14. In all adolescents, the mean dietary fibre intake per day and per megajoule was lower at 17 years (23g/day (SD 10g/day) and 2.5g/MJ (SD 0.9g/MJ)) than at 14 years (24g/day (SD 9g/day) and 2.6g/MJ (SD 0.7g/MJ)). When food group sources of dietary fibre were examined, the greatest source was cereals and grains, followed by fruits, then vegetables. In multivariable mixed model analysis, female sex, Caucasian race, being aged 14, good family functioning, high level of parental education, and high energy intake were independently associated with higher dietary fibre intake. Regarding the relationship between dietary fibre intake and inflammation, associations between overall fibre or fruit and vegetable fibre with the inflammatory markers C‚ÄövÑv™reactive protein, leptin, or adiponectin were negligible after adjustment. However, there was a small significant association between higher cereal and grain fibre intake and lower levels of leptin (˜í‚â§=‚ÄövÑv™0.06, 95% CI ‚ÄövÑv™0.10, ‚ÄövÑv™0.01). Raine Study adolescents with the highest dietary fibre intakes had the lowest odds of having moderate/extreme (clinically relevant) depressive symptoms, adjusting for sex, age, energy intake, adiposity, and family and lifestyle factors (OR=0.273, 95% CI 0.09, 0.81). This association was unaffected by adjustment for C‚ÄövÑv™reactive protein concentrations. Conversely, further adjustment for dietary patterns attenuated the results, suggesting that the impact of dietary fibre does not occur independent of overall diet. The findings from this thesis show that adolescents do not eat adequate quantities of fibre as defined by Australian guidelines. Although there was not strong evidence of an effect of dietary fibre intake on inflammation, the thesis provides observational evidence to suggest that a good quality diet which is high in fibre may have potential to improve symptoms of depression in adolescents. Further research is required to confirm whether a high fibre diet may play a beneficial role in the prevention or management of depression.