Blog | Monday, April 23, 2012

Epidemiology is much worse for you than red meat

"Red meat is not bad for you. Now blue-green meat, that's bad for you!"
--Tommy Smothers

I generally try to avoid writing about meaningless studies that should be ignored. First, there are a lot of them. Second, I don't want to attract more attention to them than they already get in the media. But sometimes a meaningless study seems to perfectly confirm what we already wanted to believe. Then a feedback loop of reader gullibility and media misunderstanding leads inevitably to reaching a conclusion entirely unsupported by the science. Then I feel obligated to shine some light on the confusion.

Red Meat by Gonmi via Flickr and a Creative Commons licenseThis week's expedition into folly was occasioned by a study published in Archives of Internal Medicine that attempted to find a link between eating red meat and mortality.

My regular readers know that the only way to test whether some substance has some effect is to do a randomized study. That means if we wanted to know whether eating more red meat caused people to die sooner than eating less red meat we would need to do the following: Recruit a few thousand people with moderate meat intake and get their permission to control their diets. Then randomize them into two groups. One group eats a vegetarian diet, and the second group eats a whole lot of red meat. Follow them all and count deaths.

Voila! This would be good science and would teach us a lot about any link between eating red meat and longevity. It would be expensive and logistically difficult, but nature does not yield her secrets easily.

Is this what was done in the study published this week? Not even close. The study looked at data collected in two previous large epidemiologic studies, the Health Professionals Follow-up Study, which started in 1986, and the Nurses' Health Study, which started in 1980. Neither of these studies was randomized. They simply followed large groups of people and assessed their health periodically. There was absolutely no intervention done, just observation. They were given questionnaires every few years about their diet, from which their meat consumption was estimated. Then the deaths among the participants were recorded, and calculations were done to see if there is a correlation between meat ingestion and mortality.

And guess what? There is. The people who ate more red meat had a slightly higher mortality than people who ate less red meat. That means that eating red meat is correlated with increased mortality. It does not mean that eating red meat is what kills people. Meaning, it doesn't mean that changing your diet changes your risk. The authors of the study, of course, know this and never use words like "cause," but media coverage that followed completely missed this distinction and waxed hysterically that "all red meat is bad for you."

Observational studies have almost never steered us towards the truth. Remember that observational studies suggested that estrogen prevents strokes and heart attacks. It took a randomized study to show that it doesn't. That's because without randomization you never know if the people that are choosing to eat red meat are different from the people who don't in some important way that increases their mortality but has nothing to do with the meat.

For example, in this study the people who ate more meat were less likely to be physically active, more likely to be current smokers, to drink alcohol, and to be overweight than those who ate less meat. The authors of the study used statistical methods to account for these differences, but there were almost certainly other differences that could not be guessed or accounted for.

Also, an observational study can't tell us in which direction the causal arrow points. Meaning, if sick people craved more meat, then the link between the two would be due to high mortality causing more meat eating, not the other way around.

So this study teaches us absolutely nothing about a putative link between eating meat and death. It should have been completely ignored by the media, and it doesn't deserve a moment of your attention.

But let's take the study's data at face value and see what all the media hullabaloo is about. The study found that an increase of one serving of unprocessed red meat per day was associated with a 13% increase in mortality, and a 20% increase for processed meat. Let's take the higher number, 20%, that's terrible right? That must amount to people dropping dead in droves soon after biting into their hot dogs.

The study followed people for a total of 2,960,00 person-years, during which almost 24,000 deaths were counted, for an average of 0.0080 deaths per person-year. 20% of that is 0.0016 deaths per person year, which is one additional death for 619 person-years.

So let's pretend that the link between red meat and death is real (which is completely unsupported by this study) and let's imagine two groups of people. The first group is composed of 100 vegetarians. The second group is 100 people who eat one serving of red meat daily, perhaps a delicious hamburger. The group of meat eaters would have one additional death after six years and two months. In that time they would have consumed 225,935 burgers. So 225,935 servings of meat correlate to one additional death.

That makes a burger a lot less dangerous than, say, having general anesthesia, and about as dangerous as driving 300 miles, but much yummier.

So we've learned nothing about whether eating more red meat affects longevity, but we've learned a lot about what happens when preconceived opinions seem to be confirmed. People attach a lot of weight to arguments that purport to demonstrate what they already think should be true. We feel that red meat should be bad for us. We feel guilty because cows are so cute and meat is so tasty. There must be a health risk to balance the scales and atone for our guilt.

If there is, it will take a well-designed randomized study to prove it. Until then, skepticism, and a slice of brisket, is in order.

Learn more:
All red meat is bad for you, new study says (Los Angeles Times)
Risks: More Red Meat, More Mortality (New York Times Vital Signs)
Red Meat Consumption and Mortality (Archives of Internal Medicine)
I calculated the risk of driving from the WolframAlpha calculations for "U.S. auto fatalities per year" and "U.S. auto miles driven per year"

Albert Fuchs, MD, FACP, graduated from the University of California, Los Angeles School of Medicine, where he also did his internal medicine training. Certified by the American Board of Internal Medicine, Dr. Fuchs spent three years as a full-time faculty member at UCLA School of Medicine before opening his private practice in Beverly Hills in 2000. Holding privileges at Cedars-Sinai Medical Center, he is also an assistant clinical professor at UCLA's Department of Medicine. This post originally appeared at his blog.