Consumption of plants is associated health. The exact reason for this is not known, but one hypothesis suggests that this may result from the phytonutrient chemicals that plants contain. In particular, the polyphenol class of chemicals has been identified as possessing possible health benefits. A number of large scale epidemiological studies have investigated the associations between the polyphenols and human disease and although controversial, evidence does support a role for polyphenols in disease prevention. The pharmaceutical model of food, which has infested nutritional research, suggests that for polyphenols to be protective of disease they must be bioavailable. That is to say they must be absorbed and enter the circulation where they can have biological effects within cells. Studies have measured the absorption of polyphenols and it is known that they are metabolised by enterocytes in the gut prior to absorption, and that blood levels of these chemicals increase postprandially, indicating they are bioavailable compounds.
However, nutrition and pharmacology should not be confused. While it is true that the vast majority of compounds must be absorbed to be of use to the body, certain nutrients can provide health benefits without absorption. Food contained within the intestine is not considered to be part of the body as it has not passed the absorptive surface of the gut (an inside out skin), but this does not stop indigestible plant material such as fibre having beneficial health effects. Polyphenols too may not require absorption in order to show health benefits, and this relates to their ability to act as antioxidants within the gut. Of particular interest is the ability of polyphenols to inhibit oxidation of fatty acids, thus reducing or inhibiting the presence of potentially damaging and genotoxic lipid peroxidation compounds. For example the fish oils eicosapentaenoic acid (EPA, C20:5 (n-3)) and docosahexaenoic acid (DHA, C22:6 (n-3)) may be susceptible to oxidation during transit through the gut, and polyphenols may inhibit this process.
Malonaldehyde is a possible product of lipid peroxidation that may cause genetic damage if absorbed. Consumption of rancid oxidised oils increases the risk of absorption of malondialdehyde and other lipid peroxide derived compounds such as lipid hydroperoxides. Such compounds if consumed regularly may increase the risk of cardiovascular disease and cancer. Lipid hydroperoxides may form during the digestion process, when these fatty acids can be exposed to oxidising compounds. For example the low pH of the stomach can increase the formation of lipid hydroperoxides formed in the presence of haemoproteins, that may be present in fish along with the long chain fatty acids EPA and DHA. The consumption of polyphenols in wine concomitantly with food may explain some of the health benefits of traditional diets where this practice is common. Similarly, the catechins found in high concentrations in green tea when consumed with food, may have health effects because of the ability to prevent lipid peroxidation in the gut.
The ability of polyphenols to prevent lipid peroxidcation in the intestines has been studied in models. For example, in one study researchers used a model of the gut, to assess the potential effects of polyphenols on the production of oxidised fatty acids1. Most of the oxidation of fatty acids occurred in the stomach, and these oxidation products then became available for absorption in the jejunum and ileum. However, this oxidation was inhibited by addition of a polyphenol rich grape seed extract (GSE) and this reduced the amount of oxidised fatty acids present in the small intestinal compartment. Use of a Caco-2 cell culture model showed that oxidation of fatty acids reduced their absorption to about 10 % of non-oxidised polyunsaturated fatty acid absorption. The grape seed extract inhibited the oxidation of these fatty acids in Caco-2 cell culture media and significantly increased the fatty acid absorption. Therefore models of the human intestine show that polyphenols may benefit health by decreasing lipid peroxidation in the gut.
The effects of polyphenols on the absorption of oxidised fatty acids has also been tested in human subjects. For example in one study2, researchers used a randomised controlled trial to assess the impact of red wine polyphenols on malondialdehyde in plasma and urine in human subjects. Healthy participants were fed turkey meals containing 250 grams of turkey as cutlets. The turkey was either served with no treatment, soaked in red wine after cooking, or soaked in red wine before cooking. Plasma malondialdehyde levels rose from baseline with consumption of the plain turkey. Consumption of the turkey soaked in red wine after cooking, caused a 75 % reduction in plasma malondialdehyde. However, elevations in plasma malondialdehyde were completely eliminated after the turkey soaked in red wine prior to cooking. The urine concentrations of malondialdehyde mirrored those of the plasma. Therefore polyphenols may not only inhibit fatty acid oxidation in the gut, they may be beneficial during cooking.
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