Twenty-five years ago, a landmark study by Richard Doll and Richard Peto concluded that 75–80% of cancers diagnosed in the United States in 1970 might theoretically have been prevented by altering environmental factors such as smoking, alcohol consumption and diet (R. Doll & R. Peto J. Natl Cancer Inst. 66, 1191–1308; 1981). More recent work has added obesity and physical inactivity to the list of factors that increase cancer risk. Today, tobacco use accounts for 30% of cancers in the United States, obesity accounts for 15% and poor diet for up to 25%. Clearly, while avidly pursuing promising avenues of therapy (see page 735), the biomedical community and policy-makers need to tackle cancer prevention with just as much zeal.

The success of campaigns to combat melanoma and lung cancer are testaments to the impact that prevention can make.

The success of campaigns to combat melanoma and lung cancer are testaments to the impact that prevention can make. For example, the Australian government's exhortation to “slip on a shirt, slop on a sunscreen, slap on a hat” has led to melanoma having less of a health impact in Australia than in cloudier countries such as Britain. The incidence of smoking-related cancers has dropped sharply since the 1990s — the delayed effects of anti-smoking campaigns started in the 1960s and 1970s.

But much still needs to be done. Tobacco remains a problem, especially in the developing world, where consumption is climbing. The next biggest threat facing the developed world is the growing epidemic of obesity. In the United States and Europe, for example, around two-thirds of the population is either overweight or obese. Research into how people form dietary and exercise habits will help to inform intervention campaigns, and research on nutrition will help to determine why a vegetable-rich, low-saturated-fat diet seems to provide protection against cancer. Studies on genetic susceptibility could one day help us all to tailor our lifestyles to suit our risk profiles.

But such research is likely to have little effect unless policy-makers are prepared to go further than just educating the public. Avoiding the Sun is straightforward and cheap. The same cannot be said of factors that affect obesity. Once associated with wealth and excess, obesity now disproportionately affects the poorer sections of society, because high-calorie, low-nutrient, processed food is often much more easily accessible than healthy alternatives. Governments need to help ensure that eating the recommended five portions of fruit and vegetables a day is a realistic aim for everyone.

Physical activity needs to be tackled too. Encouraging children and adults to take up sport is a start, but policies that encourage people to build exercise into their daily lives — town planning that makes walking to the shops or cycling to work easier and safer, for example — are likely to have a longer-lasting impact.

Our modern lifestyles are increasingly at odds with the environment in which our physiology originally evolved. Finding a truce between the two would go a long way to reducing the burden of diseases, such as cancer, that result.