单选题
Clever genetic detective work may have found out the reason why a near starvation diet prolongs the life of many animals.
Ronald Kahn at Harvard Medical School in Boston, U.S., and his colleagues have been able to extend the lifespan of mice by 18 per cent by blocking the rodent's increase of fat in specific cells. This suggests that thinness and hot necessarily diet promotes long life in "calorie restricted" animals.
"It's very cool work." says aging researcher Cynthia Kenyon of the University of California, San Francisco. "These mice eat all they want, lose weight and live longer. It's like heaven."
Calorie restriction dramatically extends the lifespan of organisms as different as worms and rodents. Whether this works in humans is still unknown, partly because few people are willing to submit to such a strict diet.
But many researchers hope they will be able to trigger the same effect with a drug once they understand how less food leads to a longer life. One theory is that eating less reduces the increase of harmful things that can damage cells. But Kahn's team wondered whether the animals simply benefit by becoming thin.
To find out they used biology tricks to disrupt the insulin (胰岛素) receptor (受体) gene in lab mice but only in their fat cells. "Since insulin is needed to help fat cells store fat, these animals were protected against becoming fat," explains Kahn.
This slight genetic change in a single tissue had dramatic effects. By three months of age, Kahn's modified mice had up to 70 percent less body fat than normal control mice, despite the fact that they ate 55 percent more food per gram of body weight.
In addition, their lifespan increased. The average control mouse lived 753 days, while the thin rodents averaged a lifespan of 887 days. After three years, all the control mice had died, but one-quarter of the modified rodents were still alive.
"That they get these effects by just manipulating the fat cells is controversial," says Leonard Guarente of the Massachusetts Institute of Technology, who studies calorie restriction and aging.
But Guarente says Kahn has yet to prove that the same effect is responsible for increased lifespan in calorie restricted animals. "It might be the same effect or there might be two routes to long life," he points out, "and that would be very interesting./

单选题 Ronald Kahn and his colleagues can make mice live longer by ______.

单选题 According to the passage, we do not know whether humans will benefit from taking in fewer calories partly because ______.

单选题 What does the last sentence in the third paragraph imply?

单选题 The average modified mouse lived ______.

单选题 What can be inferred from the passage about the route to long life?