根据以下内容回答题：
A third of Britains are overweight，states a report published in January by the RoyalCollgeg of Physicians，the resuh of an l8-month-long study．About five percent of children weigh too much，and are likely to stay that way for life；in the mid-twenties age group the pro-portion of fat people rises to a third，and of the middle-aged population half are overweight． Fat people risk severe health problems，says the report，including high blood pressure，breathlessness．and various forms of heart disease．Smoking is particularly risky for overweightpeople． The safest way to lose weight is to eat cereals，bread，fruit and vegetables，and cut down on fatty meats，butter and sweet foods，Bad diets do far more harm than good slimming ma-chines that vibrate muscles have not been proved useful．Saunas（桑拿浴）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 general physical jerks should be practiced three times a week． The report advocates several public health measures to combat the high prevalence of over-weight in this country．They include an increase of tax on alcohol to reduce its increasing，and erously fattening，consumption；and the provision of more sports facilities by local authori-ties．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-terra basis，and taking into account the many often complex reasons why fat people are fat．
The passage mainly talks about__________ ．