Sea buckthorn berries are yellow fruits that contain high levels of vitamin C, tocopherols, carotenoids and flavonoids. Evidence suggests that sea buckthorn may possess antioxidant properties, and this may be responsible for its reported health effects against various parameters of cardiovascular disease. The effects of sea buckthorn berries include decreased platelet aggregation, lowered blood viscosity and enhanced cardiac function. Sea buckthorn is a complex mixture of phytochemical antioxidants and it is likely that these work synergistically to provide the antioxidant effect. However, flavonoids in particular have been researched for their benefits to the cardiovascular system, and are known antioxidants in vivo. Of the flavonoids present in sea buckthorn, the most abundant are the flavonols isorhamnetin, quercetin and kaempferol. These flavonols are contained within the berry in a mixture of aglycones and as glycosides including isorhamnetin-3-O-sophoroside-7-O-rhamnoside, isorhamnetin-3-O-rutinoside, isorhamnetin-3-O-glucoside, quercetin-3-O-rutinoside and quercetin-3-O-glucoside.
Researchers1 have investigated the absorption of sea buckthorn flavonols from a meal of 185g of oat meal with added sea buckthorn flavonol extract. Twenty two health male subjects received two doses of the flavonol extract (78mg or 39 mg) in separate meals (with and without 3g of flavonol free sea buckthorn seed and berry oil). The 78 mg extract contained 54.1mg isorhamnetin, 20.2mg quercetin and 3.4mg kaempferol. Plasma samples were taken at regular intervals and the flavonols ester bonds hydrolysed to produce free quercetin, kaempferol and isorhamnetin. The results showed that the flavonols were present in plasma mainly as sulfates and glucuronides. Increasing the dose of sea buckthorn extract increased the absorption of kaempferol and isorhamnetin. For quercetin, significantly higher absorption occurred after the meal containing sea buckthorn oil, suggesting that the fat content, or some other component may have increased absorption.
These reports support other absorption studies that show that the main flavonoids present in plasma are in the form of sulfates and glucuronides. Presumably, these are formed in the enterocytes of the small intestine during absorption as well as in the liver during first pass metabolism. Isorhamnetin (3’methylquercetin) is a 3’-O-methylated form of quercetin and so it is possible that some of the isorhamnetin detected in plasma originated from quercetin that was subsequently methylated during or after absorption. In addition, the authors explained that tamarixetin (4’methylquercetin) could not be differentiated from isorhamnetin and so co-elution may have been present during HPLC analysis. Fat has been previously been shown to increase the absorption of quercetin, but little is known about the interaction of lipids and flavonoids. Fat may increase absorption because it slows digestion somewhat, or it may increase the diffusion of the lipophilic aglycone forms of flavonoids into enterocytes.
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