Fortunately, following the second World War, interest in the epidemiology of cancer was reawakened. The most striking discovery, by Doll and Hill, was that smoking causes lung cancer.(9) Further population studies subsequently linked many other types of cancer to cigarettes so that smoking is now held responsible for 30% of cancer deaths. Put into human terms, this represents 150,000 American lives.
Doll and Hill's success paved the way for more epidemiological discoveries and today we know that 80 - 90% of fatal cancers are potentially preventable. The culprits include diet, smoking, alcohol, radiation, pollution and occupational hazards such as asbestos. Viral infection is associated with about 5% of Western cancer deaths. According to Doll, the knowledge gained through epidemiology 'has led ... to nearly all the steps that have been taken to reduce the incidence of cancer in practice'.(10)
And as with heart disease, there were early clues that a diet rich in fruit and vegetables could be beneficial: in 1933 epidemiological research by London University indicated that people consuming larger amounts of fruit and vegetables were less likely to develop cancer. Although this received little attention at the time, later research confirmed the findings: by 1992 128 epidemiological studies had shown the protective effect of fruit and vegetables.(11)
Stroke is the third commonest cause of death in Western society, also resulting in much disability. Every year in the US alone, there are some 500,000 new cases with 150,000 deaths. Yet epidemiological studies have shown that stroke is preventable.(12) The research has identified high blood pressure as the chief cause, but other public health measures identified by epidemiology include avoiding smoking, heavy alcohol consumption and fatty diets.
Epidemiology has also shown that reducing salt intake can effectively combat high blood pressure. Careful comparative studies revealed that some cultures, such as the Alaskan Eskimoes, consume very little salt and do not suffer from high blood pressure. In fact, evidence showing that dietary salt restriction might reduce blood pressure dates back to the ancient Greeks! But the dangers of salt were finally confirmed by researchers at London's St Bartholomew Hospital who analysed 78 previous trials involving 47,000 people. They found that a daily individual reduction of 3 grams of salt would be more effective than drugs in lowering high blood pressure. The findings indicated that individual action, together with reduced levels of salt in processed food, would cut Western stroke by a massive 39% (and heart disease by 30%). This means preventing an estimated 56,000 stroke deaths every year in the US, together with much disability. As the researchers point out, 'Few measures in preventative medicine are as simple and economical and yet can achieve so much.'(13)
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