Naringenin (4’, 5, 7-trihydroxyflavanone) and hesperetin (3′, 5, 7-Trihydroxy-4′-methoxy flavanone) are flavanones that belong to the flavonoid group of phytochemicals. Flavanones possess antioxidant activity like all flavonoids, and in addition are phytoestrogens that may exert anti-oestrogenic effects. Naringenin has also been researched for its ability to inhibit the phase I detoxification enzyme cytochrome P450. As with most flavonoids, flavanones appear in nature mainly in their glycoside forms linked to a sugar moiety, with citrus fruit being a particularly rich source. Hesperidin (hesperetin-7-rutinoside) and narirutin (naringenin-7-rutinoside) are the main flavanone glycosides present in oranges, and naringin (naringenin-7-neohesperidinoside) and narirutin are the main flavanones glycosides in grapefruit. Unusually, the main flavanone in tomato skin, naringenin, is not glycosylated but in its free aglycone form. Glycosylation may affect the absorption of flavonoids and so comparisons between tomato and citrus fruit absorption are of interest nutritionally.
The absorption of naringenin and hesperetin glycosides has been researched1 in 8 healthy subjects following consumption of 8 mL/kg bodyweight orange juice or grapefruit juice. The plasma peak concentration for total naringenin from orange juice was 0.6 µmol/L at 4.8 hours and from grapefruit juice was 6.0 µmol/L at 5.5 hours. The peak plasma concentration for total hesperetin from orange juice was 2.2 µmol/L at 6 hours. Research2 has also determined the bioavailability of naringenin from 150g cooked tomato paste in five healthy subjects. The peak plasma concentration of naringenin was 0.12 µmol/L, 2 hours after the test meal was consumed. No free unconjugated naringenin was detected following consumption of the tomato paste, instead all naringenin was in its conjugated form. Rutin (quercetin-3-O-rutinoside) and chlorogenic acid were both detected in the tomato paste, but neither was detected in plasma.
These results suggest that the aglycone flavanone naringenin in tomato is absorbed more quickly to the plasma than the glycoside flavanones in citrus fruit. This is consistent with the hypothesis that the absorption of the glycoside forms of flavonoids takes place in the distal small intestine. Here the necessary β-glucosidase enzyme (lactase) is present and this is able to hydrolyse the glycosidic bond, which allows subsequent diffusion of the aglycone naringenin to the intestinal enterocytes. In contrast the aglycone form of flavonoids is probably absorbed in the proximal small intestine via passive diffusion. The researchers studying the bioavailability of citrus flavanones did not measure conjugates of flavanones in the plasma but instead hydrolysed the samples to form the aglycone before analysis. Had they measured concentrations of conjugates in plasma, they would likely have found almost exclusively conjugated forms of the parent flavonoids, as with the tomato paste.
RdB
