Polyphenols, also called phytochemicals, are bitter compounds found in plants and fruits that evolved to defend against the grazing of herbivores. In this sense, polyphenols are considered anti-nutrients because they interfere with absorption of nutrients and deter most animals from consuming them. Polyphenols are generally divided into tannins, lignins and flavonoids, which are the best studied group and present in many fruits. The antioxidant properties of polyphenols, especially flavonoids, are considered beneficial to people, although perhaps not in large quantities.
Tannins and Lignins
Tannins are especially astringent and bitter plant polyphenols. The taste of tannins is what causes the dry and puckered sensation in the mouth following consumption of unripe fruit or red wine. Tannin-rich fruits include pomegranates, persimmons and most berries, although citrus fruits are devoid of them, according to the "American Dietetic Association Complete Food and Nutrition Guide." Lignins are compounds commonly derived from wood and are found in the cell walls of many plants, including fruits.
Flavonoids
The best studied polyphenols are the flavonoids, which include several thousand compounds. Types of flavonoids include flavonols, flavones, flavanones, anthocyanidins and isoflavonoids. Flavonoids are found in the skins and seeds of legumes, vegetables and fruits, according to "Advanced Nutrition: Macronutrients, Micronutrients and Metabolism." Fruits rich in flavonoids include apples, blackberries, blueberries, pomegranates, cantaloupe, cherries, cranberries, pears, plums, raspberries, strawberries and grapes. Red wines have been touted for years as being rich in antioxidants, primarily due to the flavonoid resveratrol, which resides within the skins of darker-colored grapes. Ingesting flavonoids for health benefits in supplemental form, instead of via fruits and vegetables, is widely studied and debated.
Evidence of Benefits
Polyphenols display strong antioxidant behavior. Antioxidants scavenge free radical oxygen products that are created through cellular metabolism. Free radicals are thought to contribute to inflammatory responses, cellular damage, tissue deterioration and general aging. According to a review article published in a 2005 edition of the "American Journal of Clinical Nutrition," polyphenols have shown antimicrobial, antiviral, antimutagenic, anticarcinogenic, anti-inflammatory and vasodilatory actions in test tube studies and animal trials. ScienceDaily.com reported in July of 2010 that polyphenols found in red wine and green tea were found to halt progression of prostate cancer. Specifically, resveratrol was discovered to inhibit occurrence and growth of prostate tumors in mice.
Cautions
Although initial studies suggested that polyphenols exhibited antioxidant properties in test tubes and animal trials, large clinical trials later conducted on people did not replicate all of the benefits, and some suggested that excess supplementation may be harmful, according to "Human Biochemistry and Disease." After all, polyphenols are anti-nutrients and disrupt digestion and absorption in most animals, and human beings have not developed specialized biochemistry or tissues to overcome these difficulties. Polyphenols seem to provide health benefits to people, but at lower dosages and from natural fruit sources.
References
- "American Dietetic Association Complete Food and Nutrition Guide"; American Dietetic Association; 2006
- "Advanced Nutrition: Macronutrients, Micronutrients, and Metabolism"; Carolyn D. Berdanier; 2009
- American Journal of Clinical Nutrition: Polyphenols: Antioxidants and Beyond; January 2005
- ScienceDaily.com: Polyphenols in Red Wine and Green Tea Halt Prostate Cancer Growth, Study Suggests
- "Human Biochemistry and Disease"; Gerald Litwack; 2008
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