资料题：The passage mainly talks about（  ）。

A third of Britons are overweight, states a report published in January by the Royal Collegeof Physicians, the result of an 18- month-long study.About five per cent of children weigh too much, and are likely to stay that way for life; in the mid-twenties age group the proportion of fat people rises to a third, and of the middle-aged population half are overweight.Fat people risk severe heath problems, says the report, including high blood pressure, breathlessness and various forms of heart disease.Smoking is particularly risky for overweight people.The safest way to lose weight is to eat cereals, bread, fruit and vegetables, and cut down on fatty meats, butter and sweet foods.Fad (目前流行的) diets do far more harm than good;slimming machines that vibrate muscles have not been proved useful; saunas (蒸汽浴)merely remove a little body water, and health farms, says the report, serve as expensive holidays.Exercise is most important to health, the report emphasizes; though it doesn't necessarily reduce weight; it maintains the correct proportion of body fat to body muscle.And it isn't only for the young; from middle age a minimum of 20 minutes of gentle physical jerks should be practiced three times a week.The report advocates several public health measures to combat the high prevalence of overweight in this county.They include an increase of tax on alcohol to reduce its increasing, and dangerously fattening, consumption; and the provision of more sports facilities by local authorities.Britain's doctors, the report concludes, must learn to be more sympathetic and specific in their advice to the overweight, encouraging a change in eating habits on a long-tern basis, and taking into account the many often complex reasons why fat people are fat.