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Quercetin is a typical flavonoid present mostly as glycosides in plant foods; it has attracted much attention for its potential beneficial effects in disease prevention. In this study, we examined human volunteers after the short-term ingestion of onion, a vegetable rich in quercetin glucosides. The subjects were served diets containing onion slices (quercetin equivalent: 67.6-93.6 mg/day) with meals for 1 wk. Quercetin was only found in glucuronidase-sulfatase-treated plasma, and its concentration after 10 h of fasting increased from 0.04 +/- 0.04 microM before the trial to 0.63 +/- 0.72 microM after the 1-wk trial. The quercetin content in low-density lipoprotein (LDL) after glucuronidase-sulfatase treatment corresponded to <1% of the alpha-tocopherol content. Human LDL isolated from the plasma after the trial showed little improvement of its resistance to copper ion-induced oxidation. It is therefore concluded that conjugated metabolites of quercetin accumulate exclusively in human blood plasma in the concentration range of 10(-7) approximately 10(-6) M after the short-term ingestion of vegetables rich in quercetin glucosides, although these metabolites are hardly incorporated into plasma LDL.
The pharmacokinetics of dietary capsanthin was measured in four male volunteers to assess the bioavailability of oxygenated carotenoids (xanthophylls). Capsanthin was used because this carotenoid was not detected in the men's plasma before ingestion of paprika juice. Supplementing capsanthin-rich paprika juice for 1 wk (equivalent to three doses of 5.4 micromol capsanthin/d; 16.2 micromol/d), the level of capsanthin reached a plateau (0.10-0.12 micromol/L) between d 2 and 7 and was not detectable by d 16. Capsanthin was distributed in the plasma lipoproteins (VLDL, 13 +/- 3%; LDL , 44 +/- 3%; HDL, 43 +/- 3%) at the end of the experiment. In a separate experiment involving the single ingestion of paprika juice (equivalent to 34.2 micromol capsanthin) in the same men, the plasma concentration of capsanthin ranged from 0.10 to 0.29 micromol/L at 8 h after ingestion. In contrast, the elevation of the plasma concentration of an acyclic hydrocarbon carotenoid, lycopene, by a single ingestion of tomato soup (equivalent to 186.3 micromol lycopene) in the same subjects was minimal (0.02-0.06 micromol/L). The areas under the curves (AUC) for capsanthin between 0 and 74 h and for lycopene between 0 and 72 h were 4.68 +/- 1.22 and 0.81 +/- 0.17(micromol.h)/L, respectively. The half-lives (t1/2) were calculated to be 20.1 +/- 1.3 and 222 +/- 15 h for capsanthin and lycopene, respectively. We conclude that the clearance of capsanthin is much faster than that of lycopene, although capsanthin is transported into plasma lipoproteins in larger amounts. This polar carotenoid may be metabolized in the human body more rapidly than lycopene. These data justify further research on the physiological functions of capsanthin and other xanthophylls.
To study the role of carotenoids in the antioxidant defense against
oxidative modification of low-density lipoprotein (LDL) particles, human LDL rich in β-carotene and
lycopene was prepared from
a healthy volunteer following long-term supplementation with tomato
juice. This carotenoid-supplemented LDL accumulated cholesteryl ester hydroperoxides (CE-OOH)
more slowly than the
LDL prepared before supplementation when the suspensions containing
these LDL were subjected
to a singlet oxygen-generating system. However, there was no
significant difference in the rate of
CE-OOH accumulation between the two suspensions when they were exposed
to a water-soluble
radical generator. Therefore, it is strongly suggested that
supplementation of LDL with carotenoids
mainly improves the antioxidant defense against the attack of singlet
oxygen.
Keywords: Carotenoids; singlet oxygen; low-density lipoprotein;
atherosclerosis
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