Intestinal tract absorption flavonoids

The information about absorption, metabolism and excretion of individual flavonoids in humans is scarce (Cook and Samman, 1996). The flavonoids are absorbed from the intestinal tracts of humans and animals and are excreted either unchanged or as flavonoid metabolites in the urine or feces. Most studies have dealt with the absorption of flavonoids after oral administration of pharmacological doses of individual flavonoids rather than dietary levels of the flavonoids. Furthermore, most experiments have been conducted using flavonoids without glycosidic substitution rather than the glycosylated flavonoids, which predominate in plants. 

The real availability of fruit and vegetable phenolics ready to be absorbed in the intestinal tract should also be studied. Analysis of the flavonoid and phenolic content of fruit and vegetables does not provide a real picture of the compounds which may be released from plant tissues after ingestion and which may be available for absorption. Studies concerning the real availability of food phenolics should be carried out. 

Considering flavonoids, food matrix is a very important issue because it can influence the availability of the compounds to be absorbed in several ways first, the flavonoid must be liberated from the food matrix where it is inserted, and the difficulty of this process is dependent on the type of matrix second, if the food matrix has a more lipophilic environment, it can facilitate flavonoids solubiUzation and absorption. Ethanol seems to exert crucial effects on anthocyanin intestinal bioavailability, favoring its transport across intestinal epithelia. Also, interaction between different compounds may occur and interfere with absorption as well, competition or interactions for specialized transport systems are likely to occur due to the variety of molecules present in GI tract after a meal. Another important concept should be the frequency of the consumption. For anthocyanins, it is known that intestinal epithelial cells chronically exposed to anthocyanins are more prone to their own transport [33]. This is an important finding, justifying dietary recommendations, highlighting chronic consumptions of fruits and vegetables as healthy food habits. 

Curcumin possesses strong antioxidant capacities, which may explain its effects against degenerative diseases in which oxidative stress plays a major role. As previously described for flavonoids, it is unlikely that curcumin acts as a direct antioxidant outside the digestive tract since its concentration in peripheral blood and organs is very low (near or below 1 pM, even after acute or long-term supplementation). Indeed, it has been shown that the intestinal epithelium limits its entry into the body, as reflected by absorption studies in various models (portal blood perfusion, everted bags). ... 

Not all the dietary flavonoids are absorbed in the digestive tract. The absorption also depends on the flavonoid source for example, the quercetin from onions is better absorbed than that supplied by apples (Scalbert and WiUiamson, 2000). The nature of conjugates may influence the bioavailabihty of flavonoids, although both nonconjugated and conjugated aglycones may be absorbed in the intestine and enter the bloodstream (Hollman and Katan, 1999). 

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