Flavonoids are a heterogenous group of polyphenolic molecules that may confer health benefits to animals and humans. Flavonoids are secondary metabolites of plants are accumulate in particular plant tissues, usually the outer layers such as peels. Different plants have different types and concentrations of flavonoids in their tussies, and so consuming a range of plant foods provide a range of flavonoids. When consumed flavonoids are absorbed in the small intestine and almost immediately undergo phase 2 metabolism within gut cells. A large proportion of the products of this metabolism are then excreted to the gut before passing to the large intestine, where bacteria degrade the metabolites further, before absorption. Those flavonoids that are absorbed from the small intestine, pass to the liver and undergo another round of metabolism before entering the blood. Flavonoids present in the blood are in a highly metabolised form, and it is unlikely that flavonoids survive in their original plant form before being absorbed.
Flavonoids are responsible for many of the bright colours in plants such as in petals, leaves and fruits. Animals and humans consume plants and this results in the ingestion of flavonoids. However, these precursor forms of flavonoids, often present as glycosides in plants, are not present in high concentrations in animal and human tissues. Instead, highly metabolised forms are the norm, as flavonoids are metabolised in the small intestine, the large intestine and the liver, before entering the blood. Many flavonoids are broken down from their original multiple ring structure to simpler single phenolic ring structured molecules, and this group of molecules may confer some of the health effects seen after consumption of flavonoids. However, flavonoids may also have protective effects in the gut, in which case the parent precursor forms of the flavonoids may be more important that were previously thought.
Because evidence suggest that flavonoids are present in human tissues in highly metabolised forms, researchers have been interested in comparing the cellular effects of unmetabolised flavonoids with the metabolised forms in cell culture models. While cell culture models have significant limitations and results must be taken in context, they can provide important mechanistic explanations of observed phenomenon in animals and humans. One study for example, compared the cellular effects of phenolic metabolites of flavonoids, as might be produced in the large intestine, with their unmetabolised precursor flavonoids. The results of the study showed that vascular cellular adhesion molecule (VCAM)-1 mRNA expression was stimulated significantly more by the phenolic metabolites of the flavonoids compared to their parent precursor forms. As VCAM-1 may have beneficial effects on blood vessels, this study suggests metabolites of flavonoids may be more bioactive that precursor forms.
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