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The Canadian Yolk Study's Scrambled Science

Egg Yolks, Smoking Clog Arteries Similarly, Says Study - The Huffington Post, August 16 “Who did that study?” Paula Poundstone, the comedian, was asking on the radio a few days later. She’s a panelist on NPR’s satiric news-quiz show “Wait, Wait. . . Don’t Tell Me!” and she was not amused by the “shocking new study,” as the cheeky host Peter Sagal put it, that suggested egg yolks are almost as bad for you as smoking. “This concerns me,” Poundstone said. Like millions of people, she likes her eggs. “This was an actual, legitimate medical study,” Sagal assured her. “Who says?” she demanded. “The science people!” another panelist, Faith Salie, chimed in. “They studied thousands of people who have been eating a certain amount of eggs,” Sagal explained, “and they discovered that the plaque in their arteries was two-thirds as bad as people who had been smoking.” For Poundstone, it was a case of Wait, wait. . . don’t tell me what’s bad for me now. She’d already given up red meat, she said. “Now to hear this yolk thing.” Let me congratulate Poundstone on her skepticism and be perhaps the first medical professional to tell her it’s okay for her to keep eating eggs. The “yolk thing” is the latest hysterical medical warning based on simplistic and absurdly broad interpretations of data.  That makes it not just the latest Don't-eat-(your favorite food) story but an object lesson for consumers of health information. The report in question was published in the August 14 issue of the journal Atherosclerosis.  To answer Paula Poundstone’s question—Who did that study?—it was led by a Canadian neurologist, Dr. J. David Spence, who conducts clinical research of hypertension and stroke at Western University in Ontario. His and his colleagues measured the thickness of the carotid artery walls of 1,231 middle-aged people and asked them about various aspects of their diets and lifestyles, including how many eggs they eat and whether they smoke. Spence et al reported that subjects whose diets were in the top 20 percent in the consumption of whole eggs had two-thirds the narrowing of the carotid artery as those who smoked. They concluded that egg yolks, like smoking, can lead to atherosclerosis, an accumulation of plaque that thickens the walls of arteries and increases the risk of heart attack and stroke. There are so many problems with these conclusions that it’s disconcerting that the study was accepted by a peer-reviewed journal. But to me, topping the list is the basic fact of  where the researchers found their 1,231 study subjects: All were patients of vascular clinics—to which they had been referred after suffering a stroke or a mini-stroke. Obviously, this means that, at best, the conclusions about egg yolks, smoking and arterial damage can only be legitimately applied to people with established cerebrovascular disease.  What the researchers wrote, the journal published, the media reported and the public consumed is a classic example of using data from a select group of subjects to support broad conclusions and apply them to everyone. The reason this story leaped out to me is that this is what happened in 2002, when the Women’s Health Initiative began the 21st Century by promulgating one of the great medical myths of modern times. The WHI drew a connection between hormone replacement and breast cancer by conducting a large-scale study with distinct—and distinctly important—subgroups. And then ignoring the crucial distinctions that resulted.  Fully half the subjects were older women who were more than 13 years past the onset of menopause when they first began taking hormones, and this was the group that showed a slightly higher incidence of breast cancer. No such association was found in the other half—women who began hormone replacement therapy at or soon after menopause. Yet the association with breast cancer was applied to every woman in the study—and by implication every woman in the world. A decade of misguided medical opinions and decisions was the result.

Sunny side up

Back to the egg yolk. The Canadian researchers said they drew their subjects from “vascular prevention clinics.” Syntax aside—we can assume these are not clinics that prevent blood vessels—it seems an odd way to set up an objective study. Indeed, it suffers from the classic research flaw known as selection bias. These individuals had had a significant vascular event and were referred to the clinic to prevent another.  So they were different from the average person because they had a demonstrated tendency to accumulate atherosclerosis. The researchers asked their subjects what their egg yolk consumption patterns were prior to their stroke or mini-stroke. But that’s a fundamentally flawed way to go about the question, and it’s a mistake to think the results imply cause and effect. It’s true that yolks may cause an increase in carotid plaque, but this type of study cannot prove it. The subjects’ arterial damage might have been caused by any number of factors other than (or in combination with) their consumption of egg yolks. What they should have done was compare the egg consumption and carotid plaque of the clinic subjects to that of a group of that hadn’t had any clinical evidence of atherosclerosis.  Even if it is true that egg yolks cause an increase in plaque area in people who are proven to be plaque formers, that doesn’t necessarily mean that it will cause it in the general population. The good news is that the Canadian study has rightfully come in for a lot of criticism for its unsophisticated comparison between egg yolks and cigarettes. Cardiologists have pointed out the danger of likening smoking—which has long been directly linked to atherosclerosis—to eating one  food that is part of an entire array of factors.  “Eggs are part of the diet and the diet has an effect on overall blood cholesterol,” Dr. David J. Frid, an expert in preventive cardiology at the Cleveland Clinic, told the Huffington Post. “A high level of blood cholesterol can lead to arterial plaque, but there are so many factors that can affect your cholesterol above eating eggs. There's the rest of your diet, whether you're overweight, whether you exercise, genetics." It is probably true that many people can eat egg yolks to their heart’s (and arteries) content because they don’t share some factor with people such as those in the Canadian study who have had vascular events. And, of course, there are many people in between. Whatever group Paula Poundstone falls in, the one thing I can say with confidence is that she can relax about this study. It makes a poor case against egg yolks.

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