Diet-derived phenols in plasma and tissues and their implications for health

Abstract

This paper seeks to catalyse a reappraisal of the nature, fate and biological significance in humans of phenols, polyphenols and tannins (PPT) consumed in normal diets, and in particular questions the primacy of PPT radical-scavenging mechanisms for the supposed health benefits of diets rich in fruits and vegetables. PPT are classified by structure and function. Arguments are presented to show that cinnamates and derived polyphenols make significantly larger contributions to the total PPT intake than the flavonols and flavones upon which the vast majority of attention has been focussed previously. Daily intakes of total PPT may range from less than 100 mg to in excess of 2 g, and the critical importance of coffee and black tea as the major dietary sources is shown. Only some 5% of the dietary PPT is absorbed in the duodenum, and of this only some 5%, mainly flavanols, reaches the plasma unchanged, the balance being mammalian conjugates. Over 95% of the intake passes to the colon and is fermented by the gut microflora. A fraction of the resulting microbial metabolites is absorbed and appears in the plasma primarily as mammalian conjugates. Even following high intakes of PPT, the plasma metabolites collectively make a very small (less than 5%) and transient contribution to the total concentration of redox active substances in plasma. This explains the failure of most studies that sought to detect an increase in plasma antioxidant power after consuming a PPT-rich meal or supplement. The powerfully antioxidant PPT aglycones, much used in in vitro studies, do not reach the plasma. The redox potential of those unchanged PPT and PPT metabolites that reach the plasma enables them to scavenge damaging radicals, but the endogenous plasma antioxidants, especially ascorbate, are required for disposal of the resultant phenoxyl radicals. Black tea and coffee, the major sources of PPT, are poor sources of ascorbate. It is suggested that if diets rich in fruits and vegetables are health-promoting, and if these effects are due to PPT, then alternatives to radical-scavenging mechanisms must be sought. Evidence is presented to show that some mammalian metabolites of PPT may indeed be able to protect the vascular endothelium and that diets rich in PPT may in humans at normal dietary levels have the ability to protect against Type II diabetes and the metabolic syndrome through effects on glucose absorption and associated hormones. Such effects are recommended for further investigation.