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Just a little

发布时间:2019-03-08 10:10:02来源:未知点击:

By Jonathan Knight BETA-CAROTENE, a yellow pigment in some fruits and vegetables, was thought to help prevent tumours—until smokers who took it were found to get lung cancer more often. Now scientists in Boston have shown that excessive doses of the substance combined with cigarette smoke cause the tumours, and that in normal doses beta-carotene may be safe after all. In a 1996 study, smokers taking 30 milligrams of beta-carotene a day—ten times the amount in an average diet—developed tumours 28 per cent more often than those taking a placebo (This Week, 27 January 1996, p 4). Xiang-Dong Wang of Tufts University in Boston suspected a case of too much of a good thing. He and colleagues fed similar amounts of beta-carotene, adjusted for body weight, to ferrets, which metabolise beta-carotene in the same way as humans. Some animals also inhaled the equivalent of 30 cigarettes a day for six months. Lung tissue from the smoking ferrets had unusually high levels of the protein AP-1, which is known to be active in tumours (Journal of the National Cancer Institute, vol 91, p 60). But non-smoking ferrets and smoking ferrets that did not get beta-carotene had normal levels of the protein. Wang says the high levels of free radicals in smokers may oxidise beta-carotene into a substance that stimulates AP-1 production. “I think this partially explains what happened in the earlier study,” he says. But he is not certain that the elevated AP-1 would have led to tumours. Wang adds that preliminary results from unpublished studies suggest that normal levels of beta-carotene appear to be safe, even for smokers. But Reuben Lotan, a cancer specialist at the University of Texas in Houston, cautions that people should not revert to the belief that beta-carotene prevents cancer. “Maybe some people were thinking you could smoke a cigarette with one hand and eat a carrot with the other and be OK,