Absolute risk, relative risk – and red meat

“Red Meat Consumption Increases Risk of Early Death”  Scientific American

The Deadly Dangers of Eating Red Meat” TIME magazine

“All Red Meat is Risky, a Study Finds”  Los Angeles Times

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I preface what follows next by this full disclosure: I’m not a scientist, although, because I spent 20 years of my life with one, I’ve endured countless scintillating conversations about things like zinc and copper sediment in the Fraser River estuary. Does that count at all?

As a heart attack survivor, I rarely if ever eat red meat anymore – with the very occasional exception of bacon, of course (referred to by some as the “gateway drug” that lures vegetarians like me back to the dark meat-eating side).  

You may have recently caught the sensational media headlines listed above after researchers at Harvard School of Public Health announced these findings in The Archives of Internal Medicine:*

“Red meat consumption is associated with an increased risk of total, cardiovascular disease, and cancer mortality.”

Not much ambiguity about that scary claim. Yet amid the media explosion  surrounding the study’s publication has arisen a number of small but important voices asking questions and pointing out potential flaws in the study’s findings.

These voices, however, were almost too late because, as described by physician, author, researcher and UMass professor of epidemiology Dr. Marya Zilberberg:  

“These headlines have been a runaway train. And now it is near-impossible to get ahead of that load of misinterpretation with the real story.”

Mark Bittman, writing in his New York Times column, was one of the very few journalists I could find who hadn’t jumped on the red-meat-equals-death bandwagon immediately following this study’s publication. He wrote:

“When you parse the results, the actual increase in the subjects’ risk of death from red meat-eating (at least for white health professionals, the overwhelming majority of the subjects) was maybe 1 percent a year. Other studies that have been shown to potentially shorten your life at a similar rate are divorce, working at a computer and watching television a few hours a day.

“The red meat study, like these, is ‘statistically significant’, but perhaps not enough for you to cancel your grilling for the weekend.”

In her blog article called Unpacking The Meat Data, Dr. Zilberberg – who describes herself as “sailing the seas of evidence-based medicine for nearly two decades” – takes aim at the way the results of this red meat study have been reported, starting with basics like the actual quality of the meat being consumed:

“All meat is not created equal: the cows that graze on the farm around the corner from where I live make meat that is quite different from that reconstituted slime used by fast-“food” restaurants.

“Cows that are raised on Concentrated Animal Feeding Operations and fed corn-based diets are practically different species from those guernseys down the street.

But the microscopic effect in the study had already been inflated to gargantuan proportions in the press echo chamber. Essentially just about everyone in the media copied from the press release, it seems.”

But even aside from these issues, there are other compelling concerns identified with the way the Harvard red meat study was done, including these:

  • study participants reported their food histories by completing personal  dietary habit surveys – a less-than-ideal way to ensure accuracy due to issues with data collection and subjects’ reporting bias
  • this was an observational study that cannot prove cause and effect;  it  can show only a statistical association, but can’t prove benefit or risk reduction
  • there are many variables not taken into account, including the quality of the meat and lifestyle factors associated with the red meat eaters including smoking, obesity, lack of physical activity

Dr. Jose A. Hernandez is a physician and Director of Medical Education at Mercy Hospital in Miami, Florida. He shares this basic concern about how research statistics can be interpreted and shared, or (as in the case of those journalists who merely repost the original news release) ignored completely.

Dr. Hernandez told me recently:

“We are confused when confronting statistical data. Our confusion in interpreting statistics is not because we lack a ‘math gene’ – a popular alibi. Instead, the root of the problem is how the information is presented to us.

A prime example is the frequent publication of relative risk, which tends to highlight differences – making it quite seductive and popular. In contrast, absolute risk is dull and often missing from reports even though it gives a more measured perspective of the differences.”

Dr. Hernandez adds that in much of the media coverage of Harvard’s red meat study, not once was an absolute number provided in interpreting the study’s findings. And, even more perplexing, he says, even the medical journal article abstract failed to include absolute risk.

Here’s a quick course in absolute vs. relative risk:

◊   Absolute risk is risk stated without any context. For example, you have a 50 percent chance of flipping a coin and getting heads.  This risk is not compared to any other risk; it’s just the actual probability of something occurring.

◊   Relative risk is a comparison between different risk levels. For example, your relative risk for lung cancer is (approximately) 10 if you have ever smoked, compared to a non-smoker. This means you are 10 times more likely to get lung cancer. But the relative risk (or risk ratio) is not the same as an increase in risk. In fact, it tells us nothing about the actual risk.

That’s what’s disturbing about the sensational attention around the Harvard red meat study’s report based on relative risk:  it tells us nothing about the actual risk of eating red meat.

Here’s another example from Dr. Hernandez on the power of absolute numbers in some types of health care statistics:

“The relative risk of prostate cancer, given the new testing methods, may have increased, but the absolute numbers of death per year related to prostate cancer have not changed.”

To help us better understand how important this difference between relative vs absolute risk is, Dr. Hernandez recommends the work of Dr. Gerd Gigerenzer, author of the book, Calculated Risks: How To Know When Numbers Deceive You.

For example, Dr. Gigerenzer explains:**

“A review of experimental studies showed that many patients do not understand the difference between relative and absolute risk reduction, and that they evaluate a treatment alternative more favorably if benefits are expressed in terms of relative risk reduction.”

No wonder many researchers express their study results in terms of this  “seductive and popular” relative risk!

So what’s a dull-witted and (occasional) bacon-loving vegetarian supposed to make of this Harvard warning that all red meat is deadly?

The final answer might come in Dr. Marya Zilberberg’s wise observations:

“Moderation in everything, including meat consumption, is probably still the best course of action. I am far more likely to die next time I get into my car than from eating burgers.”

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* An Pan et al. “Red Meat Consumption and Mortality: Results From 2 Prospective Cohort Studies”.  Arch Intern Med. 2012;172(7):555-563. doi:10.1001/archinternmed. 2011.2287

** Gigerenzer, G et al. “Helping Doctors and Patients Make Sense of Health Statistics”. Psychological Science. 2008;8:53-96

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12 thoughts on “Absolute risk, relative risk – and red meat

  1. Very well done piece on an important and timely subject – given ASCO meetings are happening even as we speak – and the reporting of oncology trial results is normally expressed in relative terms. This is a practice that confuses some and annoys others.

    I myself wrote about this very subject back in Feb on my own blog.

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  2. I remember in the 70’s there were many headlines about “fats’ – first cholesterol was a bad thing; then HDLs, then LDLs, were all bad for you; then it all changed again to something else.

    I just told my patients to ignore the headlines because they would only change again the following year. I prefer to live according to the words carved into the walls at the Temple of Delphi in Greece: “Nothing in excess”. (At least, I try to).

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    • I agree, Dr. Ruth. Just like eggs: in the 70s, the advice from doctors was apparently NO EGGS. Then it became okay to eat three eggs per week max. Now my cardiologist tells me: “One egg a day is fine!”

      With bacon, even better . . . 😉

      cheers,
      C.

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    • That’s true. It wouldn’t hurt to enjoy a hamburger once in a while. But if you were to eat red meat for every meal and dessert, all I can say is it was nice knowing you.

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  3. I am confused how this reflects on the work of Dean Ornish and others who have some impressive research as to the benefits of cutting out animal fats and the improvements in clogged arteries and diabetes…. ??

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    • I’m easily confused, too, Suzie. But here’s my understanding of this very widely-publicized red meat study that Dr. Ornish and vegans often cite, using data from the Harvard Health Professionals Follow-Up Study and the Harvard Nurses Health Study.

      In this study, men and women with higher self-reported intakes of red meat did show higher all-cause mortality rates, but they were also LESS likely to be physically active and were MORE likely to smoke, drink alcohol and have a higher body mass index – all considered risk factors for cardiovascular disease and diabetes. But those factors somehow didn’t make it into the media headlines although they are clearly stated by study authors A.Pan et al.

      It’s also important to separate out less-reliable mice studies (Dr. Ornish likes these!) from human studies, which are notoriously challenging for researchers due to their self-reporting nature usually over an extended period of time.

      Perhaps a real live scientist can help us by wading into this question for Suzie?

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      • I am familiar with those studies…of mice and men! But I am more aware of Dr Ornish’s local work here in the Bay Area and his diet, exercise, support group, meditation programs with people who show marked improvement in their arteries. I read that after a gruelling review, Medicare is going to start covering Ornish’s program, which i was amazed!

        I am sure you are familiar with The China Study also. This is not to say that the red meat studies were not flawed, but I have the strong impression that animal fats should not be first choice for certain advanced heart disease patients in particular. And that the elimination of animal fats can render some dramatic improvements.

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