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.”