Here are two published statements about the additional burden of cancer which might be due to recent increases in the prevalence of obesity:
“Overall, a 1 kg m−2 increase in body mass index (BMI) is associated with an approximately 1% increase in incidence for all cancers combined (based on data from women) and about a 2% increase in total cancer mortality (based largely on data from men).”
“We estimated that a 1 kg/m2 population increase in mean BMI would lead to 3790 additional patients in the UK each year developing one of the ten cancers that were overall positively associated with BMI.”
These two statements are entirely consistent. The “3790 additional patients” is just over 1% of the roughly 330,000 people diagnosed with cancer each year in the UK.
The real difference between them is in how they are framed. The first focuses on a small, dimensionless, relative increase, and the second on a seemingly large absolute number of unspecified but real, live patients. Ultimately, the statements refer to the same finding, but they are supposed to mean different things to the reader.
The first statement was written by me and some of my colleagues, and published as an editorial in the British Journal of Cancer in 2011. We concluded by saying that the “obesity epidemic” is not a new epidemic of cancer:
“We might expect, other things being equal, that the 1 kg m−2 per decade increase in BMI should have resulted in 1–2% per decade increases in overall cancer incidence and mortality, although site-specific trends will have varied. This increase is small compared with the effects of changes in other risk factors, such as smoking, and improved cancer detection and treatment. The obesity epidemic is therefore unlikely to add greatly to the overall burden of cancer.”
The second statement was published yesterday in The Lancet by Krishnan Bhaskaran and colleagues, as the coda to a very large study of the association of BMI with cancer risk. They concluded that the increased burden of cancer due to rising obesity was important:
“To put this in context, based on data from 2003 to 2010, mean BMI in England has been increasing at a rate equivalent to a 1 kg/m2 increase taking around 12 years. Our data strengthen the rationale to assess and implement strategies aimed at stopping these trends and mitigating their public health effects.”
These are very different conclusions, but that is not so surprising, given that the basic facts were presented in such different ways.
My view is that the data haven’t changed (sure, this latest study wasn’t available in 2011, but the overall association with cancer was not a novel finding even then), so I stand by our editorial. We weren’t arguing that any person’s cancer should be ignored, let alone cancers in 3790 additional patients each year. Nor were we arguing that obesity is not an important cause of certain cancers. We did think, however, that to assess the public health importance of rising obesity, it would be a good idea to compare its effects with those of trends in other factors, like smoking and the detection and treatment of cancer. These have had a much greater influence on cancer incidence and mortality, which makes rising obesity (the “obesity epidemic”) seem much less important.
The broad press coverage for yesterday’s Lancet paper might suggest that not everyone agrees with my view. There is no doubting that the increased prevalence of overweight and obesity is an important public health problem, particularly for diabetes and vascular diseases, and a host of other disabling conditions. But how important is it for cancer? And is the answer to that question a matter of framing, or a matter of fact?