Last week, my husband sent me a link to a press release. (He loves sending me press releases that he thinks will incense me.) This one was a doozy: “One Egg Yolk Worse than a KFC Double Down When it Comes to Cholesterol,” the headline read. Um. Wow.
Intrigued (and, I’ll admit, a little incensed), I looked up the study—which actually turned out to be a review article—published in the Canadian Journal of Cardiology. I also looked up what I could find in the scientific literature about the risks associated with eating too many eggs. The general idea is that egg yolks contain upwards of 200mg of cholesterol, and dietary cholesterol is thought to increase blood cholesterol levels and therefore heart disease risk. But dietary cholesterol only contributes a tiny amount to your blood cholesterol—about 70 percent of your cholesterol is actually made by your liver. And I won’t go into this right now, but earlier this year in Slate, I discussed some of the reasons we should question the link between blood cholesterol levels and heart disease risk.
Here’s what I found: according to a handful of epidemiological studies, eggs aren’t so bad (and may even be good) for the heart. A study based on the famous Framingham Heart Study, which investigated the effects of host and environmental factors on the development of coronary heart disease, concluded that there is “no relationship between egg intake and coronary heart disease incidence.” A 1999 study published in the Journal of the American Medical Association drew on data from the Nurses’ Health Study and the Health Professionals Follow-Up Study and reported “no evidence of an overall significant association between egg consumption and risk of CHD or stroke in either men or women,” though the study did find that diabetic subjects had an increased risk of developing heart disease if they ate more than one egg per day. A 2008 study published in the American Journal of Clinical Nutrition analyzed data from 21,327 subjects who participated in the Physicians’ Health Study and concluded that egg consumption was not associated with heart attack or stroke. And finally, an analysis from the National Health and Nutritional Examination Survey (NHANES) III found that people who reported eating more than 4 eggs per week had significantly lower mean serum cholesterol levels that than those who reported eating less than 1 egg per week.
The epidemiological data seem pretty convincing to me, so I wondered how the authors of the review paper were going to address the findings. Interestingly, I found, they chose not to delve immediately into the science—instead, they began by attacking two studies published earlier this year that reported that eating eggs had health benefits, pointing out that the studies were funded by egg marketing agencies. There’s nothing wrong with questioning conflicts of interest in research, of course, but something about the vigor with which they made the point felt funny to me. I glanced to the end of the paper to see whether the authors had any conflicts of interests themselves. Here’s what I found:
None of the authors receives funding from purveyors of margarine or eggs. Dr Spence and Dr Davignon have received honoraria and speaker’s fees from several pharmaceutical companies manufacturing lipid-lowering drugs, and Dr. Davignon has received support from Pfizer Canada for an annual atherosclerosis symposium; his research has been funded in part by Pfizer Canada, AstraZeneca Canada and Merck Frosst Canada Ltd.
Okay, so it seems that Dr. Spence and Dr. Davignon have their own potential biases to contend with. Maybe they’re motivated by more than just concern for the health of their fellow Canadians? But back to the science: what do the authors say? “Concern about dietary cholesterol has been developing over the past 40 years,” they write. “This concern is based on the careful and independent conclusions of Ancel Keys and Mike Hegsted, who formulated our two most commonly used equations relating dietary saturated and polyunsaturated fat and cholesterol to serum cholesterol.” Now, I can’t say that I myself have closely looked at Keys’ and Hegsted’s work, but I know that science writer Gary Taubes has. His famous 2001 piece in Science, for instance, suggests that these “careful conclusions” aren’t actually based on sound data. I’m not going to go into all that here, but suffice it to say, I don’t think an equation that two scientists came up with 40 years ago trumps recent epidemiological data.
Thankfully, towards the end of the article, the authors do address the epidemiological findings. They mention the findings from the Physician’s Health Study suggesting that diabetics may for some reason be adversely affected by high egg consumption. Fine. But what about the risks posed to people without diabetes? “Failure to show harm from eggs in healthy people is likely an issue of statistical power,” they write. “In healthy people, a larger study with longer follow-up would be required.”
In other words, maybe all those studies just aren’t big enough to find evidence that eggs are harmful. Really, guys? The Health Professionals Follow-up Study included 37,851 people. The Nurses’ Health Study had 80,082. NHANES had 27,378. Surely if eggs were deadly, these studies would have found a signal amongst the noise. And as for your implication that people might be better off starting the morning with a KFC Double Down than a single egg yolk, well, that I can’t even dignify with a response.
JD Spence, DJ Jenkins, J Davignon (2010). Dietary cholesterol and egg yolks: Not for patients at risk of vascular disease The Canadian Journal of Cardiology, 26 (9)
Dawber TR, Nickerson RJ, Brand FN, & Pool J (1982). Eggs, serum cholesterol, and coronary heart disease. The American journal of clinical nutrition, 36 (4), 617-25 PMID: 7124663
Hu, F. (1999). A Prospective Study of Egg Consumption and Risk of Cardiovascular Disease in Men and Women JAMA: The Journal of the American Medical Association, 281 (15), 1387-1394 DOI: 10.1001/jama.281.15.1387
Djoussé L, & Gaziano JM (2008). Egg consumption in relation to cardiovascular disease and mortality: the Physicians’ Health Study. The American journal of clinical nutrition, 87 (4), 964-9 PMID: 18400720
Song WO, & Kerver JM (2000). Nutritional contribution of eggs to American diets. Journal of the American College of Nutrition, 19 (5 Suppl) PMID: 11023007
Taubes, G. (2001). NUTRITION: The Soft Science of Dietary Fat Science, 291 (5513), 2536-2545 DOI: 10.1126/science.291.5513.2536