Diet, Microbiota and Metabolic Health

Diabetes is one of the largest global health crises, affecting some 415 million adults worldwide, with type 2 diabetes mellitus (T2DM) accounting for at least 90% of all cases of diabetes. In line with international statistics, New Zealand has a high prevalence of diabetes with around 240,000 New Zealanders living with diabetes. However, therapeutic options for T2DM have limited success in restoring normal physiology. The human colon is colonised by some 20 trillion bacterial cells, collectively referred to as the gut microbiota. Recent research implicates the gut microbiota in energy balance and metabolism, and differences in the gut microbiota have been demonstrated between healthy individuals and those with T2DM. However, studies to date have not reported consistent changes in the gut microbiota composition across the glycaemic spectrum. The aim of this thesis was to further investigate the relationship between the gut microbiota, diet and glucose tolerance across the glycaemic spectrum. As part of the dietary analysis, associations between dietary vitamin C intakes, plasma vitamin C status and glycaemic control were also investigated. Findings from baseline data of the Canterbury Health, Ageing and Lifecourse (CHALICE) study were used to characterise the metabolic profile and self-reported dietary intake of 49-51 year-olds from Canterbury, New Zealand. Results from this study showed the mean plasma vitamin C concentration to decrease with deterioration in blood glucose, despite no significant differences in self-reported dietary vitamin C intake. A cross-sectional observational pilot study involving well characterised participants was undertaken to investigate associations between the gut microbiota, glycaemic control, self-reported dietary intake, and other indicators of metabolic health. Multiple regression analysis showed the class Actinobacteria was independently positively associated with fasting glucose and negatively associated with plasma vitamin C concentrations. Congruent with the CHALICE results, this study also showed plasma vitamin C concentrations to be significantly lower in individuals with T2DM compared to those with normal glucose tolerance (NGT). In the search for a novel therapy for T2DM, a logical extension from the observational study was to investigate if the gut microbiota could be manipulated by dietary change in people with prediabetes. SunGold kiwifruit is both high in vitamin C and is known to alter microbial ecology and metabolism in vitro, and therefore, could potentially impact human health. A pilot exploratory study was carried out where participants were asked to consume two SunGold kiwifruit daily as part of their usual diet for 12 weeks. Results showed a significant increase in the relative abundance (RA) of the class Coriobacteriia with no deterioration in HbA1c. It is hypothesised that the polyphenols in the SunGold kiwifruit act as an energy source for this class of bacteria. Supplementation with two SunGold kiwifruit per day also significantly increased participants’ plasma vitamin C concentrations. This thesis posits that the class Actinobacteria is associated with glycaemic control and the class Coriobacteriia can be manipulated by SunGold kiwifruit. However, as the study cohorts in this thesis were small, findings need to be replicated in larger populations. Furthermore, the implications of these results need to be investigated