Lifestyle plays bigger part than genes in cancer

The world's biggest study of cancer in twins has shown that the risk of developing the disease depends on how you live rather than who are your parents.

The world's biggest study of cancer in twins has shown that the risk of developing the disease depends on how you live rather than who are your parents.

Although genetic factors play a minor role in some cancers, including those of the breast in women and the prostate in men, the environment is the main contributor to cancer at all 28 anatomical sites of the body studied.

The finding, published in the New England Journal of Medicine, is an important corrective to the growing view that heredity plays a major part in cancer, fuelled by the explosion in genetic research which has revealed genetic mechanisms underlying the disease.

Even among identical twins, who share the same genes, if one has a cancer the chances of the other developing it are mostly less than 15 per cent.

Scientists who studied records of 90,000 identical and non-identical twins (45,000 pairs in total) living in Scandinavia found that among 10,000 who had cancer, most involved the common sites of lung, bowel, breast, prostate and stomach. In all cases except prostate cancer, environmental effects accounted for at least 65 per cent of the risk.

Paul Lichtenstein of the Karolinska Institute in Stockholm and colleagues from Denmark and Finland found that for prostate cancer, 42 per cent of the risk could be explained by hereditable factors, 35 per cent in the case of bowel cancer and 27 per cent in breast cancer.

These figures relate to the proportion of the risk attributable to genetic factors. They do not imply that individuals with a family history of the disease are at this level of risk.

"The overwhelming contributor to the causation of cancer in the population of twins that we studied was the environment. The relatively large effect of hereditability at a few sites suggests major gaps in our knowledge of the genetics of cancer," the authors say.

The importance of environmental causes of cancer has been demonstrated by studies of breast cancer in Asian- American women. Those who recently immigrated have the same low rates prevalent in their homelands. But the among third generation the rates are similar to those in white American women.

The relatively high genetic component in prostate cancer fits with evidence showing marked international variations in the disease, according to an editorial in the Journal by Robert Hoover of the US National Cancer Institute.

"Large scale studies have searched for risk factors for prostate cancer and found few," he said. But in breast, lung and stomach cancer studies, lifestyle-related, infectious, reproductive and other environmental factors have been identified.