Four Fat Myths about Obesity and Cancer

The fat police have tried to frighten us for so long they've used up most of their stock of scary images. Yet the media still run with every 'The Fat End is Nigh' story, no matter how absurd.

Exhibit A is today's World Cancer Research Fund (WCRF) report, Policy and Action for Cancer Prevention - Food, Nutrition, Physical Activity, and the Prevention of Cancer: A Global Perspective, which warns of a global catastrophe from obesity-induced cancer.

As the Observer's David Smith breathlessly previewed the WCRF report last weekend, 'Cancer cases are now rising at such a rate in Britain and the rest of the world that the disease poses a threat to humanity comparable to climate change'. Not to be outdone, the Mail on Sunday's David Derbyshire wrote that the 'obesity epidemic will double the number of cancer deaths within 40 years... At least 13,000 cases of cancer are caused by obesity in Britain each year'.

The new WCRF report is largely based on a report published two years ago by the same group that claimed that a third of cancers were caused by diet and lack of exercise. That controversial report advised people to be as thin as possible, and to avoid red and preserved meat and alcohol. The problem with this latest effort from the WCRF is that it is as blatantly and foolishly wrong as its 2007 version. This is especially evident in four areas.

Are cancer and obesity linked?
The report's headline-grabbing claim about the link between obesity and cancer is not supported by the majority of the WCRF's own data from the 2007 study, or by other, more recent British and American studies. Once you dig beyond the bold claim, you find that what is being claimed is that overweight and obesity can increase your risk for six cancers (cancers of the oesophagus, pancreas, colon-rectum, breast, endometrium and kidney). But even this more limited claim has little scientific support.

Take pancreatic cancer, for example. The 2007 report cites 20 case control studies, but only three show a statistically significant association between obesity and pancreatic cancer. Similarly, of 42 cohort studies on colorectal cancer, only 13 show a link with obesity. Again, with breast cancer and obesity, of 16 studies only three are statistically significant, while eight show a decreased risk between breast cancer and obesity.

Even for oesophageal cancer, the increased risk was generally found in the morbidly obese (ie, those with a body mass index, or BMI, of at least 40 - a very small percentage of the population). And with endometrial and kidney cancers the relative risks were below two. According to the National Cancer Institute, relative risks below two (that is, two times the risk compared to a control group) are so small that they may be due to 'chance, statistical bias or the effects of confounding factors'.

These claims about a link between obesity and cancer are further called into question by the UK's recent Million Women Study, which examined the link between 17 of the most common cancers and BMI. In that study, the incidence of 10 of the cancers does not show a statistically significant association with either higher levels of overweight or obesity. Of the remaining seven cancers, the association between overweight and cancer is non-significant in four, and where the results are significant the relative risks (except for endometrial and oesophageal cancer) are never stronger than two, except in the obese.

The supposed link between cancer and overweight and obesity is again called into question by a study from the US National Cancer Institute and the Centers for Disease Control published in 2007 in the Journal of the American Medical Association. This study found that being overweight was not associated with increase mortality from cancers considered obesity-related, and further noted that 'little or no association with excess all-cancer mortality with any of the BMI categories'. In other words, the overall risk of dying from cancer was not related to body weight.

Indeed, the study suggested that being overweight might be protective against cancer. For example, in individuals aged 25-59, obesity appeared to be protective against death from cancer. Even for those individuals aged 70 and over, BMIs in excess of 35 were not significantly linked with a higher risk of dying from cancer.

Will 'five a day' keep cancer away?
The WCRF report claims that there is a link between a certain kind of diet, obesity and cancer, and that fruits and vegetables, which fat people tend to eat less of, can protect against cancer. Once again, the scientific evidence contradicts such claims.

For example, of the 17 cancers discussed in the report, virtually all have statistically non-significant associations with every type of food, which means that they provide no evidence of a link between a particular food and a particular cancer. For example, of the 17 studies cited that looked at a link between colon cancer and processed meat, 13 are not statistically significant.

Despite the advice to avoid red meat, the report itself concludes that 'there is limited evidence...suggesting that red meat is a cause of oesophageal cancer'. Or, again, 'there is limited, inconsistent evidence...that grilled or barbecued animal foods are causes of stomach cancer'.

If the evidence is so limited and inconsistent, how can the advice to entire populations to reduce red meat consumption or avoid it entirely be so dogmatic?

What of the extraordinary claim that, since fat people tend to eat less fruit and vegetables, they are more likely to get certain cancers? This claim is contradicted by the largest and most expensive randomised controlled studies of the effect of eating certain foods and weight on the risk of getting breast cancer, colon cancer, heart disease, and stroke, that is, the Women's Health Initiative Dietary Modification Trial.

Almost 49,000 American women were followed over an eight-year period in terms of eating, weight, and disease. The women in the intervention group ate 'healthy' diets that were low fat and high fibre. The results? There were no statistically significant differences between the intervention and the control group in the incidence of breast cancer, colon cancer, strokes, or heart attacks.

In fact, the women following the healthy diet didn't even weigh less than they did at the beginning of the study, or less than the group that continued to eat as they always had. So much for the claim that there is a link between eating certain foods and avoiding cancer.

Are fat children more likely to get cancer in later life?
The WCRF's Martin Wiseman told the Mail on Sunday that 'the increase in the number of overweight children is deeply troubling because the more overweight a child is, the more likely they are to be overweight as an adult. And the more overweight the population becomes, the more cases of cancer we are storing up for the future.'

But if Wiseman had simply looked at the official figures on childhood overweight and obesity in the most recent Health Survey for England, he would have found that since the previous survey in 2006, there was a decrease in obese girls aged 2-15, from 18 per cent to 15 per cent. Among boys aged 2-10, the prevalence of overweight declined from 16 per cent to 12 per cent. Indeed, according to the Health Survey, amongst boys and girls aged between 2 and 15, overweight and obesity has been declining since 2004.

Nor is Wiseman's claim about fat kids becoming overweight adults true. Only morbidly obese children - that is, a very small minority of all children - are at risk for adult obesity. As Charlotte Wright in her Thousand Families Study in Newcastle found, there is 'little tracking from childhood overweight to adulthood obesity'.

Does exercise protect against cancer?
The final problem with the WCRF report is its assumption that there is a scientifically established link between physical activity, obesity, and cancer prevention. To put it charitably, this assumption is open to significant question.

Most of the summaries of the relevant scientific literature on this supposed connection are either non-committal or highly skeptical, despite the common sense claim that exercise is good for one. As one reviewer wrote, 'It is important to emphasis at the outset that most of what can be written on this topic remains speculative. No study exists which has recorded adequate birth-to-death information relating physical activity to health'.

Moreover, the evidence of a specific cancer-physical activity link is difficult to establish. Commenting on the supposed association between breast cancer and physical activity, Rissanen and Fogelholm wrote that due to the lack of evidence one could not make a public health recommendation for women to exercise to reduce their risk of breast cancer.

A recently published meta-analysis on 52 studies that looked at the association between colon cancer and physical activity reported that 44 were not statistically significant. So the supposed, obvious link between physical activity and preventing cancer dissolves upon closer examination.

Once you get beyond the scary headlines, the WCRF's claims about diet, weight, and cancer turn out to be, at best, dubious and, at worst, simply untrue.
-- Patrick Basham and John Luik

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