This slight genetic change in a single tissue had dramatic effects. By three months of age, Kahn's modified mice had up to 70 per cent less body fat than normal control mice, despite the fact that they ate 55 per cent 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."
41 Ronald Kahn and his colleagues can make mice live longer by
A offering them less food.
B giving them a balanced diet.
C disrupting the specific genes in their fat cells.
D preventing them growing larger.
42 According to the passage, we do not know whether humans will benefit from taking in fewer calories partly because
A humans, worms and rodents are different.
B most people are not willing to be put on a strict diet.
C the effect is not known.
D genetic changes in tissues can not be performed on humans.
43 What does the last sentence in the third paragraph imply?
A People like to lose weight, but they do not like to eat less.
B People want to go to heaven, but they do not want to die.
C Mice will go to heaven if they lose
