Flabby claims about food and cancer
A widely publicised report says that having a 'spare tyre' and consuming anything from bacon to milkshakes could increase your risk of cancer. Fat chance.
Professor Patrick Basham and Dr John Luik, authors of Diet Nation: Exposing the Obesity Crusade, pick apart a shocking report on food and cancer that hit the headlines around the world last week.
‘Scientists find link between body fat and cancer risk’, declared the UK Independent. ‘Cancer study sparks bacon sandwich backlash’, said the Melbourne Herald Sun. ‘To avoid the Big C, stay small’, warned The Economist. Publications around the world summarised the findings of the latest report on cancer to tell us that bad diets and expanding waistlines are a public health disaster.
But before committing ourselves to a dietary life of little red meat or alcohol, and few fizzy drinks, milk shakes, crisps or other such ‘bad’ foods, let’s look behind the scary headlines and ask whether the scientific evidence really supports these cancer truths.
The catalyst for these stories was the World Cancer Research Fund’s new report, Food, Nutrition, Physical Activity and the Prevention of Cancer: A Global Perspective (1). The report proclaims three truths about cancer, fat and food. First, being fat increases our risk for cancer; second, eating certain foods gives us cancer; and third, cancer is ‘mostly preventable’.
The report’s authors tell us they looked at over half a million studies, and then concentrated on the 7,000 that were most relevant. That is not quite true, for they actually reference slightly fewer than 2,500 studies on diet and disease. More importantly, they conveniently omit many major studies that don’t support their three truths theory.
Crucially, they almost exclusively reference epidemiological studies, which inherently cannot establish that being fat or that eating red meat gives you cancer, as that’s not what this type of study does. Instead, such studies look for associations between various factors and the risk of disease. For example, this report was interested in whether the variation in people’s weights or their diets were correlated with the development of cancer. But the very nature of epidemiological studies means that the margin of error arising from the nature of the data almost invariably exceeds the supposed relationships that the study has found. Only in a very few cases – like the link between active smoking and lung cancer – is the association between a lifestyle factor and disease strong enough for us to be reasonably sure that one causes the other.
Obesity and cancer
What about the headline-grabbing claim that being fat gives you cancer? The report actually claims that being overweight or obese increases your risk for six cancers – cancers of the oesophagus, pancreas, colon/rectum, breast, endometrium (the inner lining of the womb), and kidney. However, when you look at the report’s support for this conclusion, the evidence is extremely thin.
Take pancreatic cancer, for example. The report cites 20 case control studies. (These are studies where groups who already have a disease are compared to reasonably matched people who do not, in order to look for possibly significant differences. Case control studies have a greater risk of bias than cohort studies.) Only three of these studies show a statistically significant association between obesity and pancreatic cancer. Similarly, of 42 cohort studies (where you start with a group of healthy people and see who develops the disease) on colorectal cancer, only 13 show a link with obesity.
Of the 16 studies that the report documents on the relationship between breast cancer and obesity, only three are statistically significant, while eight actually show a decreased risk between breast cancer and obesity. Even for oesophageal cancer, the increased risk was largely confined to the morbidly, as opposed to the moderately, obese. With endometrial and kidney cancers, the relative risks were below two. According to the US National Cancer Institute, such risks are so small that they may be due to ‘chance, statistical bias or the effects of confounding factors’. Such results should be treated with extreme caution.
The just-published Million Women Study from the UK, which examined the evidence for a link between 17 of the most common cancers and Body Mass Index (BMI), the conventional yardstick for measuring overweight and obesity, found a similar pattern of results. In this study, 10 of the cancers do not show a statistically significant association with either higher levels of overweight or obesity. Of the remaining seven cancers, the association between overweight and the cancer is non-significant in four, and where the results are significant, the risks (except for endometrial and oesophagal cancer) are never stronger than two, except among the obese.
A new study from the National Cancer Institute and the US Centers for Disease Control also contradicts the obesity-cancer link. This study found that being overweight was not associated with those cancers previously considered obesity-related. The study found ‘little or no association of excess all-cancer mortality with any of the BMI categories’. Indeed, the study suggests that overweight might in fact be protective against cancer.
Diet and cancer
Is the second truth in the World Cancer Research Fund’s report – that eating certain foods increases our risk for cancer – really true? 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 which assessed the link between colon cancer and processed meat, 13 are not statistically significant. Despite those scary headlines about red meat, the report concludes that ‘there is limited evidence…suggesting that red meat is a cause of oesophageal cancer’. Again, ‘there is limited, inconsistent evidence…that grilled…or barbecued animal foods are causes of stomach cancer’. And ‘there is limited evidence suggesting that processed meat is a cause of stomach cancer’. Given the limited nature of this evidence, it is difficult to see how the report authors justified the advice to avoid red and processed meat.
Are these anomalous findings? On the contrary. Consider, for example, the American Cancer Society’s 2001 study of diet and stomach cancer, which looked at 436,000 men and women and found no increased risk of stomach cancer associated with eating processed meats. What that study did find, however, was an increased risk of stomach cancer with women who consumed more vegetables!
What of the report’s claim that cancer is ‘mostly preventable’? This is perhaps the most curious claim since there is massive evidence of the best kind that suggests that it is simply not true.
The Women’s Health Initiative Dietary Modification Trial is the most recent, and one of the largest and most expensive, randomised controlled studies of the effect of diet and weight on breast cancer, colon cancer, heart disease and stroke. It studied 49,000 American women over an eight-year period. The women in the intervention group ate diets that were low fat and high fibre with six servings of grains and five servings of vegetables and fruits per day.
There were no statistically significant differences between the intervention group and the control group in the incidence of breast cancer, colon cancer, strokes or heart attacks. Ironically, the women following the ‘healthy’ diet designed to reduce cancer and heart disease didn’t even weigh less than they did at the beginning of the study, or less than the women in the control group who continued to eat as they always had.
Unlike the epidemiological studies cited in the World Cancer Research Fund report, this gold standard, randomised, controlled intervention found no evidence to support the claim that there is a connection between eating certain foods and being a certain weight and preventing cancer.
This study is not unique. A newer one, published in the Journal of the National Cancer Institute analysed data from 14 studies involving 756,000 men and women who were followed from six to 20 years. The study found that fruit and vegetable intake was not associated with a reduced colon cancer risk. Some cancer prevention!
Contrary to recent media headlines, the World Cancer Research Fund report does not prove there is a causal connection between cancer and being fat, or cancer and eating certain foods, or diet and cancer prevention. Rather, the report merely demonstrates that, as epidemiologist Petr Shrabanek observed, ‘people who eat, die’.
Professor Patrick Basham and Dr John Luik are co-authors, with Dr Gio Gori, of Diet Nation: Exposing the Obesity Crusade, a Social Affairs Unit book. (Buy this book from Amazon(UK).)
Rob Lyons asked who’s afraid of bacon?, and interviewed John Luik on his book Diet Nation. Joe Kaplinsky warned of the dangers of lazy science reporting Brendan O’Neill broached the panic over the death of an obese toddler. Or read more at spiked-issue: Obesity.