Author Identifier

Joanna Rees

https://orcid.org/0000-0002-9165-5189

Date of Award

2022

Document Type

Thesis - ECU Access Only

Publisher

Edith Cowan University

Degree Name

Doctor of Philosophy

School

School of Medical and Health Sciences

First Supervisor

Amanda Devine

Second Supervisor

Claus Christophersen

Third Supervisor

Prof Mary Boyce

Fourth Supervisor

Josh Lewis

Fifth Supervisor

Johnny Lo

Sixth Supervisor

Armaghan Shafaei Darestani

Abstract

The prevalence and burden of obesity and mental health disorders have increased in recent years. Better understanding of their relationship is required to identify modifiable risk factors to direct future interventions for improved health outcomes. In Australia dietary patterns have shifted away from home-cooked nutritious foods, towards reliance on pre-prepared convenience meals which are typically energy-dense, nutrient-poor. Latest statistics reflect a decline in diet quality, with significant barriers to healthier eating featured in the living environment. A healthy diet is central to lowering the risks of metabolic disease and for the protection of mental health. The global effort to tackle obesity and improve knowledge around cooking and eating behaviour is gaining momentum. In parallel, tremendous advances have occurred in gut microbiome research, highlighting the intricate relationship between the gut microbiota and human health. Diet directly affects microbiome composition, and food-based microbially-derived metabolites play a pivotal role in human physiology that are correlated with multiplicitous diseases including mental illness. This study examined the links between fruit and vegetable intake, diet quality and mental health. Then followed an evaluation of effects of a community-based cooking program on cooking confidence and ability to overcome barriers to healthy eating. Finally, the associations between cooking confidence, dietary and gut biomarkers, the gut microbiome and mental health were investigated.

Part one of this PhD involved the creation of a dietary fibre, resistant starch and polyphenolic phytochemical database with which to estimate amounts of these important nutrients in everyday Australian foods. In Part two the database was applied to the pre-collected data from a large cohort of Australian adults, to explore associations between fruit and vegetable intake and mental health after 5 years. Parts three and four studied participants of a 7-week cooking program. Part three comprised evaluation of the program on cooking confidence, ability to overcome barriers to healthy eating, dietary intake and mental health post-program. Part four involved an investigation of the relationship between diet quality, mental health, dietary and gut biomarkers, including the novel inclusion of bile acids, the gut microbiome and underlying mechanisms of the gut-brain axis.

Microbiome-specific dietary components positively affected mental health outcomes. The 7-week cooking program overcame some barriers to healthy eating and increased cooking confidence and mental health but did not change diet quality or dietary and gut biomarkers associated with the gut-brain axis. Bile acids significantly contributed to microbial variance as did dietary fibre derived from fruit and cereals and grains. Diet quality and BMI were associated with gut biomarkers and microbiome composition, and phylum level microbial diversity significantly decreased for those in the lowest diet quality category. Additionally, there were iii diet-driven shifts in the relative abundance of predominant phyla that have been linked to poorer mental and physical health outcomes.

Our understanding of the diet-gut microbiome-mental health connection is still emerging but there is mounting evidence that diet quality has a profound effect and presents a key modifiable risk factor in the protection against poor health due to both physiological and neurological processes via the gut-brain axis.

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