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  3. Several classes of bitter citrus compound have looked promising as anticancer agents in laboratory tests. A new study indicates

Several classes of bitter citrus compound have looked promising as anticancer agents in laboratory tests. A new study indicates

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问题     Several classes of bitter citrus compound have looked promising as anticancer agents in laboratory tests. A new study indicates that long-term consumption of orange juice, a source of such chemicals cuts cancer risk in rats.
    In test-tube studies, one class of the bitter compounds—flavonoids—has inhibited the growth of breast cancer cells. Related studies showed that bitter citrus limonoids similarly ward off cancer in animals. Mulling over such data, Maurice R Bennink of Michigan State University in East Lansing wondered whether drinking orange juice would have a beneficial effect.
    His team injected 60 young rats with a chemical that causes colon cancer and then raised half of the animals on a normal diet. The others received orange juice instead of drinking water—and less sugar in their food to compensate for sugars in the juice.
    At an American Institute for Cancer Research meeting last week in Washington D. C. , Bennink reported that after 7 months, 22 of the animals receiving a normal diet had developed colon cancers. Only 17 of the rats on the orange-juice diet showed tumors. That’ s 77 percent of the control group’ s incidence.
    Concludes Bennink, whose work was supported by orange-juice producer Tropicana products of Brandenton, Fla. , " These data show orange juice helps protect against cancer. " He says that the study might also apply to breast, prostate, and lung cancers.
    Bandaru S. Reddy of the American Health Foundation in Valhalla, N. Y. , was not surprised by Bennink’ s finding of an orange juice benefit. However, he calls the reported risk reduction unimpressive. His own data show that citrus limonoids protect against chemically induced colon cancer in lab animals.
    Luke K. T. Lam of LKT Laboratories in St. Paul, Minn. , finds Bennink’ s data " quite interesting ," although he describes as " borderline" the suppression of cancer incidence observed by Bennink. Lam has inhibited tumors in the lung, skin and forestomach of mice with limonoinds.
The scientists don’ t know what compounds in orange juice underlie its effect. The juice is rich in one limonoid—a sugar-containing version of limonin, which suppressed tumors in Lam’ s experiments. It’ s possible, Lam speculates, that rats convert the juice’ s limonoid into limonin.
    Indeed, argues Gary D Manners of the Agricultural Research Service in Albany, Calif. , " there is no doubt that these [ anticancer ] citrus compounds are bioavailable in animals to the site of a cancer. The question remains whether they are similarly available in people". To find out, his team will soon begin measuring the human body’ s uptake of limonoids from orange juice.
From the passage we can learn that scientists are still in the dark about______.
选项 A、the substance that suppresses tumors
B、the existence of bioavailability in the human body
C、the uptake of limonoids from orange juice in people
D、the bioavailability of citrus compounds in the human body
答案D
解析 末段提到，毫无疑问，these[anticancer]citrus compounds are bioavailable in animals tothe site of a cancer，但是，问题是这些抗癌物质是否similarly available in people，这是科学家需要证明的，故D项正确。
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