Thesis (Ph.D)

Determination of phytochemicals in vegetable juices and their effects on postprandial glycaemia


High fruit and vegetable intake correlates well with positive health outcomes and reduced rates of chronic diseases such as cardiovascular diseases, type 2 diabetes, cancers, neurological decline and metabolic diseases. Bioactive phytochemicals such as polyphenols, carotenoids, vitamins, minerals and others present in fruit and vegetables may be at least partly responsible for this effect. The precise mechanisms of action for several groups of compounds, and their potential impacts upon each sphere of health have not yet been fully elucidated. This work provides novel analysis of the total antioxidant capacity and total polyphenol content of 23 commercially available vegetable juices which are available on the UK market, utilising 6 biochemical assays, before and after in vitro digestion. Beetroot juice had the highest total antioxidant capacity and total polyphenol content regardless of the method of analysis, and both measures either increased or remained stable following in vitro digestion. A commercially available beetroot juice shot was selected as a viable method to increase bioactive phytochemical intake in the general population and its carbohydrate and phytochemical profiles were obtained by HPLC and GCMS analysis. The impact of consuming beetroot juice (70 mL) as part of a mixed meal or consuming beetroot juice alone (225 mL) on postprandial glucose and insulin responses was then assessed to investigate a potential role for these phytochemicals in the control of diseases featuring insulin resistance as a primary symptom, such as type 2 diabetes and metabolic syndrome. Consumption of 70 mL of beetroot juice as part of a mixed meal containing a total of 50g available carbohydrate resulted in a significant (P<0.05) reduction in postprandial insulin concentration at 15 minutes compared to a matched control meal. Consumption of 50g available carbohydrate as 225 mL beetroot juice resulted in a significant (P<0.05) lowering of blood glucose in the 0-30 minute segment of the glucose response and a significant (P<0.05) lowering of the insulin response in the corresponding 0-60 minute segment, compared to a matched control beverage. Insulin sensitivity was estimated using a mathematical model and non-significantly increased with the dose of beetroot juice. Phytochemicals in beetroot juice, namely betanin and its degradation products, alone or in combination with polyphenolic compounds, may improve the postprandial glycaemic state with relevance to diseases characterised by insulin resistance in a similar pattern to other investigated foods such as berries and cinnamon. Further research should aim to further quantify the effects of beetroot juice phytochemicals on postprandial insulinaemia using larger cohorts and diseased populations. Phytochemicals such as neobetanin should also be given in isolation to clarify the compounds responsible for the observed effects. The potential role of phytochemicals in potentiating endogenous nitrate conversion is also worthy of further investigation.

Attached files


Wootton-Beard, P

Oxford Brookes departments

Faculty of Health and Life Sciences
Department of Biological and Medical Sciences


Year: 2012

© Wootton-Beard, P
Published by Oxford Brookes University
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