Title: Dietary Phenols And Tannins
Key words: Phytochemical, polyphenol, tannins, antioxidants, free radical, phytoestrogens, flavonoids
Date: Feb 1999
Category: 4. Food Data
Type: Article
Author: Dr van Rhijn
Dietary Phenols And Tannins
Introduction
There are numerous bioactive compounds derived from foods of plant origin, other than the conventionally known micronutrients, generally referred to as phytochemicals, or possibly ‘non-nutrient antioxidants’ as currently discussed at the US National Academy of Sciences. Numerous groups have been identified. Due to their complexity, these are currently under review. This paper will discuss our present understanding of phenols, polyphenols, flavonoids and tannins.
Phytochemicals Identified
Phenols occur naturally in grains (cereals), legumes (pulses/soy), fruits, vegetables, alcoholic and non-alcoholic beverages providing the characteristic colour, taste, odour and flavour. Simple phenols have an aromatic ring, bearing a hydroxyl group usually with another substituent (glucosides, benzoic acids, cinnamic acids). Polyphenols, are phenols with more than one phenyl ring. Tannins are large polyphenols (teaflavin, tannic acid). Most flavonoids occur as glycosides.
Biochemical Effects
They are considered to act primarily as dietary antioxidants (free radical scavengers1), and are therefore defined as possessing the ability to decrease the adverse effects of reactive oxygen species (ROS) and/or reactive nitrogen species (RNS) in human physiology. They are also involved in modulating (activation & inhibition) of numerous enzyme systems. Other properties suggested include the suppression of radical-generating enzymes. In addition, they are anticarcinogenic2, antimicrobial, anti-inflammatory, anti-coagulatory, encourage glutathione conjugation of free radicals, protect vitamin E, bind to receptors (opiate & oestrogen) and binding metals3 (Fe3+, Zn2+ & Cu2+). There is also an oestrogenic effect, relieving menopausal symptoms due to phytoestrogens4.
Some phenols are known to be toxic, but this discussion is outside the remit of this paper.
Postulated Health Benefits of Phenols and Tannins
Various health benefits have been postulated to be secondary to a combination of phytochemicals such as; cancer, atherosclerosis (CHD)5, anticoagulant, diabetes, rheumatoid arthritis, neuro-degenerative, respiratory, inflammatory bowel and age-related diseases.
Epidemiological associations in cardiovascular disease gave rise to the French paradox 6, a concept linking the consumption of red wine to low mortality rates of CHD in some French cities, despite a very high fat consumption. Results from other studies supported the protective association from flavonoids in CHD7. Cancer studies found protective associations for wine against oesophageal and gastric cancers, coffee against colon and rectal cancer, green tea against colon, rectum, pancreas, and stomach cancer, and black tea against rectal cancer.
Volunteer in vitro studies found that consumption of red wine and tea (green & black), reduces susceptibility of HDL to oxidation. Animal studies found that black tea protects against skin cancer and colon damage and has a cholesterol lowering effect8. Red wine (tannins) also protects against liver damage.
Difficulties with the Phytochemicals
There are thousands of phenols in the plant kingdom, hundreds that are consumed in our diets9, and their complexity poses enormous problems regarding identification and quantification, let alone proper understanding of all their properties (alone or synergistic), their significance or even what dietary suggestions to give.
Conclusion
We do not have enough information regarding the beneficial effects of tannins and phenols. We require more reliable research, with single isolated compounds to establish strength of association and evidence of a dose-response relationship.
References