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November 26, 2007, 4:00 a.m.
Contestable Conclusions
The link between food, fat, and cancer does not follow from the available evidence.
The World Cancer Research Fund’s new report proclaims three truths about cancer, fat, and food. First, it asserts that being fat increases our risk for cancer; second, it claims that eating certain foods gives us cancer; and, third, it suggests that cancer is “mostly preventable.”
Before we condemn red meat, alcohol, sodas, shakes, chips, and other such “bad” foods to the realm of the inedible, it’s worth looking at the scientific evidence to see whether it really supports the claims.
The report’s authors tell us that they looked at over half a million studies, and then concentrated on the 7,000 that were most relevant. That is not quite true — actually they refer to 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 refer to 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. Indeed, the very nature of epidemiological studies means that the margin of error arising from the nature of the data exceeds the supposed relationships that the study has found.
What about the headline-grabbing claim that being fat gives one 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, 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, 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.
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 of breast cancer for those who are obese. 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 U.S. National Cancer Institute, such risks are so small that they may be due to “chance, statistical bias or the effects of confounding factors.”
The just-published Million Women Study from the U.K., which examined the evidence for a link between 17 of the most common cancers and Body Mass Index (the conventional yardstick for measuring overweight and obesity), found a similar pattern of results.
In this study, ten 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 nonsignificant 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 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.
It also concludes that eating certain foods increases our risk for cancer Of the 17 cancers discussed in the report, however, 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 the 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.” Given the limited nature of this evidence, it is difficult to see how the report justified its advice to avoid red 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, by contrast, was an increased risk of stomach cancer in women who consumed more vegetables!
Finally, the report claims that cancer is “mostly preventable.” This is perhaps the most curious claim of all — since there is massive evidence of the best kind that suggests precisely the opposite.
The Women’s Health Initiative Dietary Modification Trial is the most recent, and one of the largest, and most expensive, randomized, 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 in fat and high in fiber, with six servings of grains and five 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 even weigh 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, randomized, controlled intervention, found no evidence to support the claim that there is a connection between eating certain foods, being a certain weight, and preventing cancer.
This study is not unique. A new study published in the Journal of the National Cancer Institute analyzed 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, indeed.
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.”
— Patrick Basham is an adjunct scholar at the Cato Institute and John Luik is the Western Standard’s science columnist. They are co-authors of Diet Nation: Exposing the Obesity Crusade. |
