Introduction
Phenolic compounds, in particular both the condensed and hydrolysable vegetable tannins, have an astringency which contributes to the "mouth-feel" factor when foods containing them are eaten. Astringency, although often described as a taste is the feel of the changed salivary composition of the surfaces of the mouth and tongue. Sometimes astringency is perceived as a sophisticated, positive hedonic effect associated with the consumption of certain foods and drinks, e.g. red wines. At other times it is considered to be a negative organoleptic property, causing foods to be rejected, especially by the young, whose palates find the "bitterness/roughness" objectionable. Since it is inevitable that a percentage of saliva becomes intimately associated with the food/drink entering the gastrointestinal tract, its composition, especially its antioxidant capacity in the presence of astringent natural antioxidants, will be an important factor in the uptake of absorbable components from the gut. Recent scientific studies suggest that since saliva has a significant antioxidant capacity per se, of which around 85% is due to uric acid, and the remaining percentage to other important antioxidants such as vitamin C, glutathione, etc, an understanding of the changes in its composition may throw some light on the biochemistry leading to the therapeutic benefits of antioxidant-rich diets.
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Background
Pioneering colleagues, E.C. Bate-Smith, E. Haslam and G. Harborne contributed to the elucidation of the physiology, chemistry and biochemistry of the polyphenols at a time when they were classified as secondary plant metabolites (explaining the paucity of the information about their purpose). However, these workers, et al. refused to accept that plant polyphenols were merely waste products and, largely because of their astringency, viewed them at least, as plant protection chemicals if not also, protection for browsing animals, and ultimately, human consumers. The delicate balance between flora and fauna is maintained in part by the astringency of plant polyphenols. Leaf eating insects are repelled by the build up of indigestible plant polyphenols rapidly mustered at the site of attack. Similarly, browsing animals take only small amounts of plant material high in polyphenols, thus preserving the viability of the plant and avoiding toxic stress for themselves.
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Human Nutrition
Bearing in mind the above provenance for the polyphenols, things only get worse when the facts of human nutrition are added, such as the fact that the non-essential polyphenol nutrients occlude essential minerals, viamins and other important nutrients, rendering them inaccessible for digestion/absorption. So, why are these astringent polyphenols so popular as food-based antioxidants? Truth is we do not know, exactly. A recent study of certain condensed tannin-containing food extracts found a positive relationship between the sensation of astringency index (SAI), the human salivary protein precipitation capacity (HSP) and their total antioxidant activity (TAA).
Reference: R. Amarowicz et al., J. Food Lipids. 15 , 28-41, 2008.
These observations set the scene for a possible new area for debate. Is it acceptable to tolerate the negative effect of astringent antioxidants actually occluding useful, possibly non-astringent antioxidants e.g. vitamins, selenium?
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Salivary composition and its new role
Saliva is a complex physiological fluid which contains, inter alia proteins, electrolytes, lubricants, enzymes, serum albumin and low molecular weight antioxidants, of which uric acid forms around 85%, and lipid-soluble alpha tocopherol and beta carotene.
Foods may or may not contain antioxidants, but after ingestion the food bolus entering the gastroeosophagal tract always contains salivary antioxidants. The role of these physiological salivary antioxidants is being studied from the point of view of periodontal disease.
Reference: D.V. Sculley and S.C. Langley-Evans, Proc. Nut. Soc., 61, 137-143, 2002.
One or two papers in recent years have looked at the role of salivary antioxidants in the acid environment of the stomach. Although salivary antioxidants alone are only partially effective against lipid peroxidation, in the presence of e.g. red wine polyphenols in the in vitro stomach medium, total inhibition of lipid peroxidation was observed.
Reference: S. Gorelik et al., Arch. Biochem. Biophys. 458 (2), 236-243, 2007. Good news again for the red wine drinkers. In this paper, the general view is that salivary antioxidants help protect the stomach against deleterious events, and are aided significantly by the presence of food-based antioxidants. Thus supporting the notion that food-based antioxidants are necessary with every meal. Another paper endorses the protective powers of salivary antioxidants, especially uric acid, at the pH of the stomach.
Reference: D. Pietraforte et al., Free Rad. Biol. Med. 41 1753-1763, 2006.
Significantly, these authors also emphasise the similar role for dietary antioxidants.
Reference: S. Liskmann et al., Clin. Oral Implants Res. 18 (1), 27-33, 2007.
The total antioxidant capacity of saliva changes from a high "resting" value to a lower "stimulated" value, suggesting the consumption (approx. 30%) of the total antioxidants in the process of mastication.
In a paper comparing the antioxidant levels of plasma and saliva in a patient with recurrent aphthous stomatitis, salivary levels of superoxide dismutase and catalase were high in saliva and low in plasma, prompting the authors to suggest that "the organism might mobilise the antioxidant to the site where they are needed. This result warrants for Antioxidant bioavailability
Reference: Y. Karincaoglu, et al., J. Oral Pathol. Med. 34 (1), 7-12, 2005.
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Finally
It should be remembered that saliva is constantly bathing the oesophagal tract, and continuously entering the alien environment of the stomach, neutralising ROS generated in the digestion process.