Here in the UK this week the media has been feasting on a recent pronouncement from the World Cancer Research Fund (WCRF) which urges us to strictly limit our consumption of red meat to protect ourselves from bowel cancer. The WCRF’s statement is based on research conducted recently at the Imperial College London, in the UK. It updated with more studies a similar review undertaken in 2007.
Here’s the opening salvo from the report linked to above: “The most authoritative ever report on bowel cancer risk has confirmed that red and processed meat increase [emphasis mine] risk of the disease…” Those are strong words, and the media has been only too happy to parrot them. Let’s have a look, though, and see if it’s desperately important for us to cut back on red meat for the sake of our health.
The first thing to note, I think, is that the evidence used as the basis for this recent review is epidemiological in nature. Such evidence looks for associations between things. However, just because two things are associated, does not mean one is causing the other. Owning a television is associated with an increased risk of heart disease. Does owning a television cause heart disease? If you bought a TV and left it in the boot of your car or buried it in a field, would you be at increased risk of heart disease? Probably not. The issue is not owning the television, it’s likely something associated with owning it (e.g. watching it and therefore being more sedentary). Related factors that can incriminate an innocent factor in this way are referred to as ‘confounding factors’.
We could have hundreds of studies linking TV ownership with increased risk of heart disease, but they still prove nothing. Likewise evidence regarding red meat and colon cancer.
The fact is, even when properly conducted, epidemiological evidence just simply does not tell us a great deal. And it can’t, honestly, be used to conclude that “red and processed meat increase risk of [colon cancer]”. Such conclusions are scientifically untenable, and are generally made by people who:
1. don’t understand the limitations of epidemiological evidence.
2. do understand the limitations but draw inappropriate conclusions anyway.
That latter category tends to be well populated by epidemiologists, by the way…
However, not all epidemiologists appear not to know better. Earlier this month saw the publication of a review of the evidence regarding red meat consumption and colon cancer in the European Journal of Cancer Prevention [1]. The review points out a range of inconsistencies and relevant issues in the evidence including:
- significant variation in results from study to study
- links with red meat and colon cancer but not rectal cancer (the rectum is the lowest part of the colon)
- links in men but not in women
- considerable potential for confounding, through lifestyle and other dietary factors
The authors conclusion? “The available epidemiologic data are not sufficient to support an independent and unequivocal positive association between red meat intake and CRC [colorectal cancer].”
The issue of confounding is particularly relevant here. We’ve heard about the perils of red meat for a long time now. Many health conscious people have acted on the warnings by eating less (or no) red meat. Others, perhaps less health conscious people, have ignored the advice. So is it possible that any increased risk of colon cancer in red meat eaters has nothing to do with the red meat, but the fact that they are more likely to engage in other ‘risky’ behaviours like smoking and being sedentary? You can read more about this concept here.
Also, it’s perhaps worth bearing in mind that not all epidemiological evidence supports a link between red meat consumption and colon cancer. In one study, for instance, vegetarians and vegans turned out to have an increased risk of colorectal (cancer in the colon or rectum) compared to those eating [2].
Another issue I have with the WCRF report is that it focuses on one disease (cancer) in one site (the colon). Does making recommendations on the basis of its relationship with a relatively rare disease make sense? Wouldn’t it make better sense to make recommendations based on a wider view, such as overall risk of death? Much that politically correct epidemiologists and scientists hate to admit this, but the fact remains those who eat meat are at no increased risk of disease or death, overall, compared to vegetarians. See here for more about this.
Put all of this together and what do we have? In my view – much ado about nothing.
References:
1. Alexander DD, et al. Meta-analysis of prospective studies of red meat consumption and colorectal cancer. Eur J Cancer Prev. 2011 May 2 [Epub ahead of print]
2. Key TJ, et al. Cancer incidence in vegetarians: results from the European Prospective Investigation into Cancer and Nutrition (EPIC-Oxford). 2009;89(suppl):1S-7S
