The question the reader will ask at this point is ‘Given all this epidemiological study, do we know the causes of cancer?’ Broadly the answer is ‘yes’ in many circumstances and for many cancers, and the opportunities for prevention that this understanding generates are there to be taken. We do not always know how the factors that have been identified by the epidemiological studies discussed in this chapter link up to what is being learned in the laboratories of the molecular biologists. This connection is being made rapidly and will be increasingly clear by the end of the century. Epidemiology has been very successful in discovering or confirming which features of our lives in the Western world can be now identified as causes of cancer.

Experimental studies undertaken in a number of countries suggest that dietary alteration can alter the growth of tumours, and links between a Western diet and some cancers have become apparent. However, it is not proving quite so easy to demonstrate precise links between specific elements of a diet and specific cancers. Dietary fat may be a factor in breast and bowel cancer but there may also be other explanations for some of the links between these cancers and our way of life in the Western world. Cooked-meat intake may be a factor in bowel cancer and fibre or starch may be protective against this cancer. Obesity is a risk factor for some cancers in women, particularly cancer of the body of the uterus. Whether being overweight causes cancer by altering circulating hormones is not yet dear. Green vegetables and fresh fruit seem to be associated with a lower incidence of several cancers but the link is a complicated one. Vitamins and minerals m diet are also being studied but we cannot claim to have all the answers as to how they may prevent the development of cancer.