Doctors in Britain are warning of all obesity time bomb, when children who are already overweight grow up. So, what should we do? Exercise more? Eat less? Or both? The government feels it has to take responsibility for this expanding problem. The cheerful Mr. Pickwick, the hero of the novel by Charles Dickens, is seen in illustrations as someone who is plump (胖乎乎的) and happy. In 18th century paintings beauty is equated with rounded bodies and soft curves. But nowadays being overweight is seen as indicating neither a cheerful character nor beauty but an increased risk of heart disease and stroke. So what do you do? Diet? Not according to England's chief medical officer, Sir Liam Donaldson. He says that physical activity is the key for reducing the risks of obesity, cancer and heart disease. And the Health Secretary John Reid even said that being inactive is as serious a risk factor in heart disease as smoking. So, having bought some cross trainers, how much exercise should you do? According to Sir Liam Donaldson, at least 30 minutes of moderate activity five days a week. Is going to the gym the answer? Luckily for those who find treadmills (跑步机) tedious, the Health Development Agency believes that physical activity that fits into people's lives may be more effective. They suggest taking the stairs rather than the lift, walking up escalators, playing active games with your children, dancing or gardening. And according to a sports psychologist, Professor Biddle,gyms are not making the nation fit', and may even cause harm. There's new scientific evidence that too much exercise may actually be bad for you.Scientists at the University of Ulster have found that unaccustomed exercise releases dangerous free radicals that can adversely affect normal function in unfit people. The only people who should push their bodies to that level of exercise on a regular basis are trained athletes. So, should we forget about gyms and follow some experts' advice to increase exercise in our daily life? After all, getting off the bus a stop early and walking the rest of the way can't do any harm! One final thought. How come past generations lacked gym facilities but were leaner and fitter than people today? This passage is mainly about ______.