The role of polyphenols in modern nutrition

Abstract

Polyphenols are found in plant-based foods and beverages, notably apples, berries, citrus fruit, plums, broccoli, cocoa, tea and coffee and many others. There is substantial epidemiological evidence that a diet high in polyphenol-rich fruit, vegetables, cocoa and beverages protects against developing cardiovascular disease and type 2 diabetes. The absorption and metabolism of these compounds have been well described and, for many, the gut microbiota play a critical role in absorption; taking into consideration the parent compound and the metabolites from colon bacteria catabolism, more than 80% of a dose can be absorbed and ultimately excreted in the urine. Common polyphenols in the diet are flavanols (cocoa, tea, apples, broad beans), flavanones (hesperidin in citrus fruit), hydroxycinnamates (coffee, many fruits), flavonols (quercetin in onions, apples and tea) and anthocyanins (berries). Many intervention studies, mechanistic in vitro data and epidemiological studies support a role for polyphenols against the development of chronic diseases. For example, flavanols decrease endothelial dysfunction, lower blood pressure and cholesterol, and modulate energy metabolism. Coffee and tea both reduce the risk of developing type 2 diabetes, through action of their constituent polyphenols. Despite extensive research, the exact mechanisms of action of polyphenols in the human body have not been decisively proven, but there is strong evidence that some targets such as nitric oxide metabolism, carbohydrate digestion and oxidative enzymes are important for health benefits. Consumption of polyphenols as healthy dietary components is consistent with the advice to eat five or more portions of fruit and vegetables per day, but it is currently difficult to recommend what 'doses' of specific polyphenols should be consumed to derive maximum benefit.

Keywords: cardiovascular disease; coffee; flavonoids; fruit; tea; type 2 diabetes.

Figure 1

Highly simplified metabolic pathways involved in the absorption and metabolism of polyphenols. Phenolics = hydroxycinnamic acids. Chlorogenic acids and hesperidin are poorly absorbed intact, and so their attached organic acids and sugars, respectively, are efficiently removed by the gut microbiota. Deglycosylation is catalysed by brush border enzymes, especially lactase phlorizin hydrolase, and is most efficient when glucose is attached. Conjugates circulating post‐prandially in blood may be with methyl, glucuronic acid and/or sulphate groups.

Figure 2

Relative risk reduction of developing type 2 diabetes with tea and coffee consumption. Plotted values for tea are from Yang et al. (2014b) and for coffee are from Ding et al. (2014). Reduction in risk calculated directly from [100‐relative risk (%)], and error bars indicate the upper and lower confidence intervals as reported in the cited papers. Data on coffee are consistent with studies by Bhupathiraju et al. (2013), who also showed that the reduction in risk was similar or even better (depending on cohort) for consumption of decaffeinated coffee compared to regular. Data on tea are supported by the meta‐analysis reported by Yang et al. (2014a). [Colour figure can be viewed at wileyonlinelibrary.com]