Separating fact from hype in the study of cancer and obesity

The recent report by Cancer Research UK (Obesity set to be bigger cause of cancer in women than smoking, 24 September) hypes a very minor and unsurprising result – that the fraction of cancers attributable to smoking in the UK may be expected to decline modestly over the next quarter century – by emphasising a notional crossover point when female cancers attributable to overweight and obesity will exceed those attributable to smoking. Cancer due to obesity is portrayed as a rising public health threat, despite the fact that it is projected to change hardly at all and despite the fact that the great success in combating tobacco consumption still leaves tobacco as the leading cause of preventable cancer now and for the next 25 years, even if we accept the study’s methodology.

As for the methodology: the largest number of female cancers attributed to overweight in the main source cited in the report are breast cancers. The source assumes without comment that smoking has no effect on breast cancer. Multiple studies from other countries have found, by contrast, a substantial effect, which would raise the cancer burden of tobacco, and so push this crossover point into the distant future.

Furthermore, lumping all of these very different malignancies together to compute a trend for cancer is misleading. The two most common cancers attributable to obesity in women – cancer of the breast and uterus – are among the most survivable, with ten-year survival above 75%. The next two most common would be bowel and bladder cancer, with ten-year survival above 50%. The cancer caused by smoking, on the other hand, is primarily lung cancer, with ten-year survival around 7%, followed by oesophageal (13%), pancreatic (1%), bowel and bladder.

While increasing corpulence is doubtless detrimental to public health, all else being equal, it is probably better to stay off the obesity-apocalypse bandwagon until the data objectively carry us aboard.
David Steinsaltz
Associate professor of statistics, University of Oxford

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