Bitter Is Better : The Glucoregulatory Effects Of Bitter Compounds And The Bitterness Of Australian Diets

Abstract

Previous research has indicated the potential health benefits associated with the bitter taste of foods and beverages (F&Bs), mediated by bitter compounds. Although clinical studies are limited, preliminary evidence suggests that administering bitter compounds in the gastrointestinal tract can regulate glycemia. However, further research is needed to establish the effectiveness of using bitter compounds as a strategy for glycaemic management. Moreover, clinical and observational studies have shown positive health outcomes, including glycaemia regulation, from consuming bitter-enriched F&Bs such as tea, coffee, and brassica vegetables. These studies, however, have primarily focused on the relationship between individual preferences and the consumption of bitter F&Bs. The broader concept around the bitterness of diets remains relatively unexplored, warranting a comprehensive investigation. This thesis aimed to uncover the potential glucoregulatory effects and underlying mechanisms of bitter compounds in humans and explore the bitterness of population diets and how we might increase consumption of bitter foods. The first study (a systematic review and meta-analysis) reviewed randomised controlled trials involving post-oral bitter compounds to determine their pooled effects on postprandial glycaemic control. The potential mechanisms underlying the glucoregulatory effects of bitter compounds in the gastrointestinal tract were investigated in the second study (a randomised controlled trial). The third study (a secondary analysis- part one) examined the bitterness of diets among a nationally representative sample of Australian adults and explored factors influencing the bitterness of diets. The nutritional quality of more bitter diets among Australian adults were examined in the fourth study (a secondary analysis- part two). The final study (a cross-sectional, online survey) investigated the facilitators, barriers, and strategies for promoting the consumption of bitter F&Bs within the diet. The results from the meta-analysis suggested that chronic interventions involving post-oral bitter compound administration, especially from the quinine family, may hold therapeutic promise for individuals with glycaemic dysregulation; reduced postprandial plasma glucose at t= 2 h (n= 21 studies, weighted mean differences= -0.35 mmol/L, 95% confidence intervals= -0.58, -0.11). The results from the randomised controlled trial indicated that, compared to placebo, both low and high doses of intraduodenal intervention of quinine (300 and 600 mg) can effectively improve postprandial glycaemia in healthy women (n= 10) by slowing gastric emptying (p <0.05). The two secondary analyses used data from a nationally representative sample of Australian adults (n= 9,118). The first part quantified the overall bitterness of diets. While the bitterness varied by several sociodemographic, health and dietary related factors, age was the strongest predictor of the bitterness of diet (β= 0.26). The second part of the analysis revealed that individuals with more bitter diets tended to have healthier food intake patterns, consuming more serves of vegetables (30.2%), fruits (33.9%), whole grain/high fibre cereals (36.2%) and low-fat dairy (57.2%), and lower serves of refined grain/low fibre cereals (-18.0%), compared with those with less bitter diets (p < 0.001 for all). Lastly, the cross-sectional survey of 879 Australian adults identified the bitter foods, particularly vegetables, as the items that can more effectively enhance the bitterness of diets. There was a great willingness among respondents to increase their intake of each bitter F&B (e.g., between 59% and 94% of low-consumers). The facilitators and strategies varied by individuals’ familiarity with the F&Bs and for foods versus beverages, which could possibly help to tailor approaches used in future dietary interventions in order to increase the bitterness of diets. Taken together, the findings from this thesis provides a detailed understanding of the health implications related to bitter taste and highlights potential strategies to increase consumption of bitter F&Bs. These insights could be used to inform for leveraging bitter taste into dietary recommendations. Figure 1 presents a graphical abstract for this thesis.