Dieting, according to an old joke, may not actually make you live longer, but it sure feel

s that way. Nevertheless, evidence has been accumulating since the 1930s that calorie restriction-reducing an animal's energy intake below its energy consumption—extends lifespan and delays the appearance of age-related diseases in rats, dogs, fish and monkeys. Such results have inspired thousands of people to put up with constant hunger in the hope of living longer, healthier lives. They have also led to a search for drugs that imitate the effects of calorie restriction without the pain of going on an actual diet.

A study, known as CALERIE (Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy), was sponsored by America's National Institutes of Health. It took 48 men and women aged between 25 and 50 and assigned them randomly to either a control group or a calorie-restriction regime. Those in the second group were required to cut their calorie intake for six months to 75% of that needed to maintain their weight.

The CALERIE study is a landmark in the history of the field, because its subjects were either of normal weight or only slightly overweight. Previous projects have used individuals who were clinically obese(肥胖的), thus confusing the unquestionable benefits to health of reducing obesity with the possible advantages of calorie restriction to the otherwise healthy.

At a molecular level, CALERIE suggests these advantages are real. For example, those on restricted diets had showed drops in body temperature and blood-insulin(胰岛素) levels—both phenomena that have been seen in long-lived, calorie-restricted animals. Dr Rattan doubts whether calorie restriction will extend maximum human life expectancy. He argues that the concepts of ageing and longevity(长寿) must be separated. It may, indeed, be possible to reduce or eliminate particular age-related diseases, and that would increase average lifespan in the way that eliminating other diseases has done in the past. But this is not the same as slowing down aging itself, and thus increasing maximum lifespan. Longevity is a more complex trait than any individual disease, and, in his opinion, it will not be altered so easily.

Cynthia Kenyon, a researcher looking into anti-aging drugs, believes that some molecules are likely to be approved in the next five to ten years, for guarding against age-related diseases. People then will start taking them, and a huge natural experiment will get under way. If Dr Rattan is wrong, maximum lifespan as well as average lifespan will increase. If he is right, at least people will enjoy a healthier old age.

It can be inferred from the first paragraph that ______.

A．an effective calorie restriction involves taking little food while doing much exercise

B．calorie restriction is proved to be effective in preventing illness in some animals

C．some drugs are taken by people to live longer without suffering from starvation

D．before the 1930s,people did not believe in the positive effect in extending lifespan