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  3. Caffeine Caffeine is probably the most widely used drug in the world. Humans have been consuming caffeine for hundreds of yea

Caffeine Caffeine is probably the most widely used drug in the world. Humans have been consuming caffeine for hundreds of yea

admin2012-12-09  116
问题 Caffeine
   Caffeine is probably the most widely used drug in the world. Humans have been consuming caffeine for hundreds of years, primarily in the form of coffee, tea, and cocoa. Habitual coffee and tea drinkers had long been observed to have a lower incidence of non-melanoma(黑色素瘤) skin cancers, although no one knew why. A recent study found that caffeine affects skin cells damaged by ultraviolet radiation, a main cause of skin cancer. Caffeine interferes with a protein that cancerous cells need to survive, leaving the damaged cells to die before they become cancerous. Drinking caffeinated coffee has also been associated with a decreased incidence of endometrial(予宫内膜的) cancer - that is, cancer of the cells lining the uterus - The strongest effect appears to be in overweight women, who are at greatest risk for the disease. Researchers believe blood sugar, fat cells, and estrogen(雌性激素) may play a role. Although the mechanism remains unknown, people who drink more than two cups of coffee or tea a day reportedly have about half the risk of developing chronic liver disease as those who drink less than one cup of coffee daily; caffeinated coffee has also been associated with lowered risk of cirrhosis(肝硬化) and liver cancer.
   While many of caffeine’s undesirable effects, such as elevated heart rate and blood pressure, are brief, some short-term benefits, including pain relief, increased alertness, and increased physical endurance, have also been attributed to caffeine. As a component of numerous over-the-counter diet pills and pain relievers, caffeine increases their effectiveness and helps the body absorb them more quickly. By constricting(收缩) blood vessels in the brain, it can alleviate headaches -even migraines(偏头痛)–and can help counter the drowsiness(眩晕)caused by antihistamines(抗组胺药).
   Caffeine does not alter the need for sleep, but it does offer a temporary solution to fatigue for people who need to stay alert. Research has shown that sleep-deprived individuals who consumed caffeine had improved memory and reasoning abilities, at least in the short term. Studies of runners and cyclists have shown that caffeine can improve their stamina-hence its addition to energy-boosting sports drinks.
   People who consume a lot of caffeine regularly may develop temporary withdrawal symptoms, headache being the most common, if they quit or cut back on it abruptly. Fortunately, these symptoms last only a day or two in most cases. Individuals who are more sensitive to the stimulatory side effects of caffeine may want to avoid it, but most doctors agree that the equivalent of three cups of coffee a day does not harm healthy people. There is no medical basis to give up daily caffeine and many reasons to include a moderate amount in one’s diet.
Some athletes use caffeine to
选项 A、improve their speed.
B、increase their endurance.
C、maintain their alertness.
D、relax their muscles.
答案B
解析
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