Thursday, November 5, 2015

Red meat makes a good, scary cancer story....but is it?

It's off again, on again:  don't eat processed meat, don't eat red meat, or you'll get colon cancer!! Eat fish (well, unless it has mercury) or chicken (unless it has salmonella), or 'the other white meat': pork (remember the billboards?). They're safe!

A few years ago we seemed to have been given some relief when stories suggested that red meat (beef) was OK after all (of course, the lives of the cows were awful, and eating beef meant you doped up on antibiotics, but at least it didn't give you colon cancer).

Recently, a statement (now apparently offline) released by the International Agency for Cancer Research, a part of the World Health Organization, asserts that eating processed meat and red meat, 'causes' cancer.  Actually, the report was a bit more nuanced than the headlines, but journalists have to make a living, no?

Bacon, Stock photo

In response to strong backlash, the WHO quickly was forced to 'clarify' their clarion call to vegetarianism -- here's a link to their Q&A on the subject.  They now acknowledge, or 'clarify' that what they had done was simply add the meats to a list of known nasties, that cause cancer.  Putting meat on a causal list is one thing, but dishing it out to the media is another, and a rather irresponsible way to play for publicity (of course, if the news media made an exception and actually did their job of being skeptics, this wouldn't have unwound as it did).

In any case, the bottom line was basically that even two strips of bacon a day increases your colon cancer rate by 18%.  That sounds like a whopping and terrifying difference!  The WHO put this in the same carcinogenic-substance category as asbestos and tobacco. As they quickly clarified, that is in a sense a warning list, but the 18% figure is what got in the news and may have, at least temporarily, slammed the bacon and hamburger industry, if anybody still listens to the daily Big Warnings. However, let's hold all cynicism for the moment, hard as that is to do, and look a bit more closely at was said.

First, there seems little doubt that processed meats 'cause' cancer.  That doesn't mean an innocent-looking strip of bacon will give you cancer.  Instead, what it means is that various high quality studies have found a dose-response pattern in which higher or longer exposure levels earlier in life are associated with higher cancer incidence later on.  We know that correlation is not the same as causation, and that lifestyle factors are highly correlated.  Thus, for example, those in dire poverty don't eat tons of processed meat, and those who eat less salami also eat more brussels sprouts, take vitamins, don't smoke, lay off the double gin tonics.....and of course, go to the Ashram regularly to get your mind off the bacon you didn't eat at breakfast and the aftertaste of your dinner's brussels sprouts, and say a mantra to stay calm after you've given up everything that's fun.

Now, in the west, the lifetime risk of colon cancer is about 5%.  That means that if you tote up the probability of having cancer at age 40, 45, 50, .... 100, if the 18% figure is credible, it means that risk is about that much higher in those who dose up on pastrami and burgers.  Actually, this was the estimate based on eating 2-strips of bacon or the equivalent every day.  Of course, by far most of these cancers occur in older people (over the age of 60, say).  That means that the risk figures mainly apply to you if you live to old age, and of those who die earlier of other things their actual risk turned out to be zero--they enjoyed their visits to McD's and the deli!  That's why smoking is, in a literal epidemiological sense, a preventive relative to colon cancer (smoking will kill you of something else first).  There's no joking about cancer, but the basic idea is that for those who lived long enough, about 5% get colon cancer at some age.  Actually, while we don't know about meat-eating habits, but risks have been declining in recent years in developing countries (and, I think, increasing in other countries as they westernize).

Eat meat and lower your risk!
At a baseline of 5%, an 18% increase means a lifetime risk of about 6%.  Now if you hog up even more, your risk will go higher, perhaps much higher.  But wait a minute.  How many people actually dish up so heavily on processed meat (including steak and burger)?  Surely some do.  In fact, we don't know exactly where the lifetime risk estimate of 5% comes from; if from a population sample, then it wouldn't have regressed out meat-eating, and the figure would already include meat-eaters. However, let's ignore all these potential confounding or confusing issues and just consider the 18% figure on its own, as a given, as risk differences between abstainers and sausage gluttons.

Now in modern countries with health care systems, one routine health-care procedure is regular colonoscopy in older adults.  There was a recent estimate that regular colonoscopy can prevent about 53% of colon cancers; the reason is that precancerous polyps are found and excised so they can't transform into cancer.  Actually, you can find even more dramatic estimates of the preventive effectiveness if you scan at the web.  Likewise, you'll find many other lifestyle factors widely cited as having protective effects, including exercise, vitamins, eating vegetables, and the like.

Let's just do a bit of back-of-the-envelope numerology to make the point that if you're a bacon hog but have regular screening, get your exercise and all that, and you reduce your meat-elevated risk by 50%, then your net risk is around 3%, about half the 'average' of 5%.  One can surmise that if you stop your bacon fix, but then figure you're fine and don't do the other preventives, many of which are likely to be wanting in the meat-hog's normal lifestyle, then the actual effect of your 'healthy' baconless diet change will be to increase your cancer risk!

This is a lesson in complex causation and oversimplified news stories.  Processed meat may be a risk factor for colon cancer, but throwing irresponsibly simplified figures like raw meat to the news media leads to worse, rather than better information for the public.

So, as Hippocrates said, moderation in all things.  Eat your reuben (OK, yes, along with some broccoli).  But go one better than Hippocrates:  get scoped!

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