In developed countries, the overall cancer mortality is more than twice as high as in developing countries. The main reasons for the greater cancer burden of affluent societies are the earlier onset of the tobacco epidemic, the earlier exposure to occupational carcinogens, and the Western nutrition and lifestyle.
In developing countries, up to 25% of malignancies are caused by infectious agents, including hepatitis B and C virus (liver cancer), human papilloma viruses (cervical cancer), and Helicobacter pylori (stomach cancer). In developed countries, cancers caused by chronic infections only amount approximately 8% of all malignancies.
In North America and some European countries, the overall cancer mortality started to decrease in the 1990s. This is mainly due to the marked decrease in the stomach cancer rate worldwide, to the advances in the early detection and treatment of some major cancer types particular cervical and breast cancer and to a reduction in smoking prevalence.
Tobacco consumption remains the most important avoidable cancer risk. In the 20th century, approximately 100 million people died world-wide from tobacco-associated diseases (cancer, chronic lung disease, cardiovascular disease and stroke). Recent epidemiological studies indicate that the adverse health effects are even greater than previously estimated. Half of regular smokers are killed by the habit. One quarter of smokers will die prematurely during middle age (35 to 69 years).
It has been long recognized that in addition to lung cancer, tobacco smoking causes tumours of the oral cavity, pharynx, larynx, oesophagus, pancreas and bladder. In a recent consensus conference at the International Agency for Research on Cancer in Lyon (World Health Organization), this list was extended to include several organs with a two- to three-fold elevated risk in smokers versus non-smokers: tumours of the kidney, stomach, uterine cervix, liver, nasal cavities, and myeloid leukemia. However, there was no evidence indicating that breast and prostate cancer and endometrial carcinoma of the uterus are caused by smoking. It was also unanimously concluded that frequent exposure to involuntary (passive) tobacco smoking is associated with a 20% increase in lung cancer risk.
There is increasing evidence that in industrialized countries, approximately one third of malignant tumours are due to the Western nutrition and lifestyle, characterized by a high caloric diet, combined with low physical activity. Tumours associated with the Western lifestyle include cancer of the breast, prostate, colon/rectum, and endometrial carcinoma of the uterus which are much less frequent in poor countries. The risk of developing some of these tumours can be reduced by frequent consumption of fruit and vegetables. In addition to an elevated cancer risk, the Western lifestyle is associated with an increased prevalence of obesity, type II diabetes (late-onset, non insulin dependendent diabetes mellitus) and cardiovascular diseases.
Given the current trend of a world-wide increase in the prevalence of obesity and the increasing acquisition of the Western lifestyle by many countries, it is unlikely that the burden of associated tumours will decrease in the foreseeable future. A reduction in mortality will, therefore, largely depend on progress in early detection and treatment.