Inhibition of iron absorption by polyphenols as an anti-cancer mechanism.

It is well known that cancer might be considered in part to be a preventable disease, highly susceptible to modulation by dietary factors. Polyphenol compounds, which are abundant in fruits, vegetables, seeds and drinks, have been shown to play a potential role as chemopreventive agents. The mechanisms underlying this beneficial effect are not fully understood. Assessing the real impact of such constituents on human health is difficult, as in many cases the exact composition of foods and the bioavailability of active molecules are not known. However, there is evidence that polyphenol compounds can prevent the DNA damage caused by free radicals or carcinogenic agents. The mechanisms can be direct radical scavenging, chelating divalent cations involved in Fenton reaction and modulating enzymes related to oxidative stress. We suggest that some of the involved mechanisms might be related to inhibition of iron absorption by polyphenols present in the diet. Iron is an essential nutrient in humans and both iron deficiency and iron excess can result in deviation from the optimal health. Although the absolute amount of iron extracted from the diet is small, its absorption is highly regulated in humans since there is no physiologic pathway for active iron excretion. Haem and non-haem iron enter intestinal mucosal cells by two independent pathways, haem iron being more efficiently absorbed (25–30%) from the diet than non-haem iron (5–15%). Although non-haem iron is not absorbed as well as haem iron, it is an important source because haem iron comprises only about 15% of total iron in the typical western diet. Beyond its chemical form, the availability of iron for absorption is determined by the presence of enhancers and inhibitors in the meal. Among the latter, polyphenols represent important compounds. The phenolic compounds are released from food or beverages during digestion, and can combine with iron in the intestinal lumen making it unavailable for absorption. The polyphenols are such powerful inhibitors of non-haem iron that substantial changes in the amount of iron absorbed are more likely to occur if the timing of consumption is altered, rather than the quantity. For example, a serving of yod kratin (leaves of the lead tree, Leucaena glauca, a vegetable with a high content of polyphenols and widely consumed in Thailand) reduced iron absorption from a composite meal of rice, fish and vegetables by almost 90%. Among elderly participants in the Framingham Heart Study, each cup of coffee (236ml) consumed in a week was associated with 1% lower serum ferritin, a good measurement of body iron stores. Interestingly, all major types of food polyphenols can strongly inhibit dietary non-haem iron absorption, and a dose-dependent inhibitory effect of polyphenol compounds on iron absorption has been demonstrated. In particular, it has been reported that any beverage providing 20–50mg total polyphenols reduce iron absorption from a bread meal by 50–70%, whereas beverages containing 100–400mg total polyphenols reduce iron absorption by 60–90%. It was noticed that a single serving of instant coffee contains 120mg of polyphenols.