June 28th, 2012

What Reading That “Low-Carb Gives You Heart Disease” Paper Actually Told Me

The following guest post by Dr. Yoni Freedhoff is reprinted with permission from his blog, Weighty Matters. Dr. Freedhoff is an Assistant Professor at the University of Ottawa and the founder of the Bariatric Medical Institute in Ottawa, Canada.

The short version is it told me that — in this case — the British Medical Journal, and possibly the authors and this study’s peer reviewers, value the publication of unsubstantiated alarmist and media-friendly hype over true journalistic and scientific integrity.

Let me start the longer version of what it told me with a question: Is your style of eating — your dietary repertoire, if you will — the same today as it was 15 years ago? Mine’s certainly not. Fifteen years ago I was a bachelor doing my residency. My diet consisted of burgers, steaks, sausages, pizza, chicken wings, french fries, restaurants, take-out, pots and pots of coffee, and horrible, horrible hospital cafeterias. The fact is, whether it’s consequent to life changes, medical conditions, dietary fads, new relationships, or new jobs,  eating style for many varies wildly with time and circumstances.

Yet the researchers of this paper — which looked at the impact dietary style would have on cardiovascular risk among 43,396 Swedish women and concluded that women who followed low-carb diets had an increased risk of heart disease — used only one single baseline dietary data set upon which to base their 15-year-long study and its conclusions.

And about that single, solitary, data set: It consisted of a food-frequency questionnaire in which subjects were asked to identify how frequently they consumed 80 different food and beverage items over the course of the past 6 months.

Think that’d be accurate? Can you remember how many of anything, let alone 80 items, you’ve had over the past 6 months? And what if you ate and drank more than 80 items?

But let’s not guess about accuracy, let actually look at it.  Taking the study subjects’ reported average caloric intake, it’s immediately evident that their dietary recall data are inaccurate as the authors report an average daily calorie consumption of only 1,561 calories.  That’s an incredibly small number, and one which flies in the face of the Food and Agriculture Organization of the United Nations report of a Swedish per capita average calorie consumption of 2,990 calories during the same time period this study’s data were collected.

But let’s for a moment pretend that the data were in fact accurate and that you could fairly extrapolate that your style of eating today will remain the same for the next 15 years. My next questions would be: Do you think the quality of your diet’s various proteins, carbohydrates, and fats might have a bearing on your risk of developing cardiovascular disease over the next 15 years? Would eating a diet higher in trans-fats confer a different degree of risk than a diet higher in unsaturated fats? Would eating a diet rich in whole grains confer a different risk than a diet rich in ultra processed pulverized flours? How about if you ate out regularly versus cooked from scratch? Deep fried versus baked? Sausages versus salmon? Quinoa versus white rice? Kale versus potatoes?  Of course it would matter, and I’d bet even a straw poll of 10-year-olds would agree.

Not this paper’s researchers.  You see, they simply reported and analyzed the data from that single-time-point questionnaire by means of 10 different centiles of carbohydrate consumption. They didn’t pay any attention to the quality of the macronutrients being consumed, just their total quantities.  The only consideration they gave as to the quality of macronutrients was a very broad “animal protein” vs. plant sources. There was no consideration given to quality of fats (despite the well-known effect of trans-fats on increasing and unsaturated fats on decreasing cardiovascular risk), or carbohydrates (again, despite data that suggest cardio-protective benefits from whole grain and risks from refined grain).

But, okay, let’s even pretend for a moment that the data were well controlled for dietary factors known to affect cardiovascular disease. Is the risk reported in this paper worthy of concern and press release? The relative risk increase of 5% per increase in low-carb score means an extra 4 to 5 cases of cardiovascular disease per 10,000 women in the lowest carb group per year. Should we stop the presses?  Given how incredibly small that is absolutely, and given how incredibly complicated it is to control for confounding variables (and I’m not just talking about the unbelievably glaring confounders that were explicitly ignored by the researchers, but also the non-dietary and lifestyle variables of which this study looked at very few), can and should cautionary conclusions truly be drawn let alone publicized?

Last, let’s pretend for a moment that the data are conclusive and that the risk is real and scary enough to shout from the rooftops.  Do you think it would matter if the diets studied and being reported as risky weren’t in fact “low-carb” given that the caution being shouted is that low-carb dieting increases the risk of heart disease?   Looking at the data, the 1st quartile of lower carb consumption is in fact a diet where the 154.7g of carbohydrates account for 40% of calories. Simply put, that’s not a low-carb diet! Atkins, for instance, starts at just 20g and then over time people tend to work themselves up to between 50-100g. 154.7g?  Thats.  Not.  Low.  Carb.  Nor is their reported 10th percentile at 123.7g, or 32%, carbohydrate.

So, to review: The authors of this paper are  basing their 15-years-worth of conclusions off of a single, solitary — and clearly inaccurate — baseline food-frequency questionnaire; they didn’t control for clearly known smack-you-in-the-face dietary confounders; they found just a miniscule absolute increase in risk; and the diet they are reporting on can’t even be fairly referred to as a low-carbohydrate diet.

Useful?  Conclusive?  Press worthy?

It gets worse.

The BMJ didn’t just publish a completely useless paper, they gave this very clear, yet completely non-evidence-based, advice to clinicians in their accompanying editorial:

Despite the popularity of these diets, clinicians should probably advise against their use for long-term control of body weight.

Worse still, highly reputable, socially networked curators of medical information tweeted the resultant media stories as relevant, and even Physician’s First Watch — a news alert from Journal Watch and the publishers of the New England Journal of Medicine — reported it as valuable to scores of physician subscribers who trust JW to keep them abreast of the latest important journal studies.

As far as the true state of the evidence on the long-term risks or benefits of low-carbohydrate diets goes, this paper, and the BMJ‘s editorialized conclusions, add absolutely nothing.  Given the paper’s rather horrifying flaws, it is a useless contribution to the medical literature.  In fact, I’d describe it as worse than useless in that it unforgivably, irresponsibly, shamefully, and knowingly misinforms – something our incredibly, nutritionally confused world really doesn’t need.

[For another take by someone who also actually read the paper, have a peek at Larry Husten’s piece on Cardiobrief (I’m quoted there, too).]

2 Responses to “What Reading That “Low-Carb Gives You Heart Disease” Paper Actually Told Me”

  1. Uffe Ravnskov, MD, PhD says:

    From table 2 it is possible to calculate that if you are a woman below age 50 and eat little carb and much protein, your chance not to die from a CVD during the next fifteen years is 99.58%. But if you do the opposite, you can increase your chance to 99.87% !!!

  2. Uffe Ravnskov, MD, PhD says:

    I forgot to mention that the mortality figures are from their first report (J Intern Med 2007;261:366-374). In the recent BMJ-report the mortality figures aren´t given at all.
    Could it be that there were no differences? I mean, if a hazard ratio in such a study is significantly different between the groups, any responsible author should of course mention it in the paper.