What are Polyphenols?

Polyphenols are a diverse class of molecules containing multiple phenol rings. They are synthesized in large amounts by plants, certain fungi and a few animals, and serve many purposes, including defense against predators/infections, defense against sunlight damage and chemical oxidation, and coloration. The color of many fruits and vegetables, such as blueberries, eggplants, red potatoes and apples comes from polyphenols. Some familiar classes of polyphenols in the diet-health literature are flavonoids, isoflavonoids, anthocyanidins, and lignins.

The Case Against Polyphenols

Mainstream diet-health authorities seem pretty well convinced that dietary polyphenols are an important part of good health, due to their supposed antioxidant properties. In the past, I’ve been critical of the hypothesis. There are several reasons for it:

  1. Polyphenols are often, but not always, defensive compounds that interfere with digestive processes, which is why they often taste bitter and/or astringent. Plant-eating animals including humans have evolved defensive strategies against polyphenol-rich foods, such as polyphenol-binding proteins in saliva 1.
  2. Ingested polyphenols are poorly absorbed 2. The concentration in blood is low, and the concentration inside cells is probably considerably lower*. In contrast, essential antioxidant nutrients such as vitamins E and C are efficiently absorbed rather than excluded from the circulation.
  3. Polyphenols that manage to cross the gut barrier are rapidly degraded by the liver, just like a variety of other foreign molecules, again suggesting that the body doesn’t want them hanging around 2.
  4. The most visible hypothesis of how polyphenols influence health is the idea that they are antioxidants, protecting against the ravages of reactive oxygen species. While many polyphenols are effective antioxidants at high concentrations in a test tube, I don’t find it very plausible that the low and transient blood concentration of polyphenols achieved by eating polyphenol-rich foods makes a meaningful contribution to that person’s overall antioxidant status, when compared to the relatively high concentrations of other antioxidants in blood uric acid; vitamins C and particularly inside cells SOD1/2.
  5. There are a number of studies showing that the antioxidant capacity of the blood increases after eating polyphenol-rich foods. These are often confounded by the fact that fructose in fruit and some vegetables and caffeine in tea and coffee can increase the blood level of uric acid, the blood’s main water-soluble antioxidant. Drinking sugar water has the same effect 2.
  6. Rodent studies showing that polyphenols improve health typically use massive doses that exceed what a person could consume eating food, and do not account for the possibility that the rodents may have been calorie restricted because their food tastes horrible.

The main point is that the body does not seem to “want” polyphenols in the circulation at any appreciable level, and therefore it gets rid of them pronto. Why? I think it’s because the diversity and chemical structure of polyphenols makes them potentially bioactive– they have a high probability of altering signaling pathways and enzyme activity, in the same manner as pharmaceutical drugs. It would not be a very smart evolutionary strategy to let plants that often don’t want you eating them take the reins on your enzyme activity and signaling pathways. Also, at high enough concentrations polyphenols can be pro-oxidants, promoting excess production of free radicals, although the biological relevance of that may be questionable due to the concentrations required.