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Polyphenols in Fruits and Vegetables: The Impact on Digestive Health
Andrew L. Waterhouse, PhD
University of California, Davis
Polyphenols are well known food constituents occurring at high levels in fruits, some vegetables, chocolate, tea and coffee. At least 5000 individual polyphenols have been reported in the scientific literature. Some of the better-known compounds are catechins in tea; genistein in soy; resveratrol in grapes; and procyanidins in berries. These substances are similar in some respects; for example, all polyphenols have an antioxidant effect, though the magnitude of it can vary quite a bit. On the other hand, their specific effects on enzyme systems, receptor sites and cell signaling can be very diverse.
Antioxidant Effects of Polyphenols
One study in laboratory rats found that caffeic acids (a type of polyphenol) protected rat colon DNA against oxidative damage and inflammation, suggesting a possible anticarcinogenic effect. Since colon cells are stressed by the normal digestive process and normally show some amount of oxidation, scientists conducted additional experiments to determine whether phenols in the diet can reduce that oxidation. When they fed rats a diet containing added phenols and measured oxidized DNA bases as well as DNA strand breaks in colon cells, they found that oxidation was reduced by about 60%. This study used condensed wine tannins as the phenolic compound; the investigators concluded that condensed wine tannins in the diet could significantly reduce DNA oxidation in colon cells. Another experiment used hydroxy acids and found similar results.
Inflammation is related to oxidation and has been implicated in both cardiovascular disease and cancer; thus, an anti-inflammatory effect of polyphenols could have important implications for health. In an animal study of chemically induced intestinal inflammation, treatment with 4-coumaric acid showed a strong anti-inflammatory effect through its ability to increase superoxide dismutase, a powerful antioxidant, and to inhibit the activity of COX2, an inflammatory compound.
Finally, hydroxycinnates and other polyphenolic extracts were shown to reduce the induction of colon cancer in experimental animals. However, the whole food mixture was more effective than any of the extracts tested at suppressing the formation of colorectal adenomas.
Absorption and Metabolism
Most phenols are not absorbed, but are retained and degraded in the GI tract. They appear to have an impact on the bacterial populations and alter the gut microflora. Additionally, some of their metabolites are absorbed. Good digestive health may depend upon these transformations, but more information is needed.
Conclusions
In conclusion, it is clear that polyphenolic substances have an important impact on digestive health. They can mitigate much of the intestinal oxidative damage caused by environmental insults and provide a protective effect against chronic diseases caused by oxidation. As strong antioxidants they decrease damage to DNA, decrease inflammation and reduce carcinogenic activity. Their presence alters the gut microflora, and some of their metabolites are absorbed.
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