Kaempferol is a flavonoid that belongs to the sub-group the flavonols. Like most flavonoids, kaempferol has been shown to be biologically active, and likely functions in vivo as an antioxidant on account of its polyphenolic structure. In addition, kaempferol may be able to act as a regulator of gene expression, as flavonoids generally are known to increase expression of phase II detoxification enzymes and decrease expression of phase I detoxification enzymes. Their antioxidant activity may be enhanced because they can down-regulate the free radical generating enzymes cyclooxygenase, lipoxygenase and xanthine oxidase. Flavonoids are widely distributed in plant material, and kaempferol is present in a number of foods of plant origin including broccoli, beans, tea, endive apples and brussels sprouts. The ability of kaempferol to affect human physiology is dependent on its absorption from the gastrointestinal tract and delivery to tissues and cell.
The absorption of kaempferol from endive has been studied1 in 8 healthy human subjects (4 male and 4 female). Subjects were requested to refrain from eating flavonoid containing foods for 48 hours prior to being fed a test meal of 300 g of endive soup. Blood and urine sample were taken at baseline, as well as at regular intervals throughout the time in the study unit and again after 24 hours. Generally, kaempferol appeared in the plasma within an hour of eating the soup (except one subject), with maximum plasma concentrations occurring at 5.8 hours (0.10 µmol/L). Analysis of the sample showed that 40% of the plasma kaempferol, and 16% of urine kaempferol, was in the aglycone form. Kaempferol-3-glucuronide was detected in plasma and urine samples, but no other metabolites could be definitively identified, although kaempferol-sulfates were suspected based on retention times.
The researchers also analysed the kaempferol metabolite ratio of endive and found it to contain 79% kaempferol-3-glucuronide, 14% kaempferol-3-glcoside and 7% kaempferol-3-(6-malonyl)-glucoside. This provided 8.65mg of kaempferol equivalents in the endive soup. The detectable excreted kaempferol was equal to 1.9% of the ingested dose which is similar (0.9%) to previous reports. Biochemical individuality was low between subjects with consistent measurements between seven of the eight individuals. However, one of the subjects showed little absorption of any kaempferol. The peak of maximal plasma levels suggest absorption occurred primarily from the distal small intestine with bacterial hydrolysis of kaempferol-3-glucuronide occurring prior to uptake of the aglycone. An early peak in plasma may have been due to uptake of kaempferol-3-glucoside by the sodium glucose linked transporter-1 followed by hydrolysis by intracellular β-glucosidase, or hydrolysis by brush boarder lactase phlorizin hydrolase followed by passive diffusion.
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