Headlines plastered across the web in the last couple days said that eating eggs is almost as bad for your heart as smoking:
- “No yolk: eating the whole egg as dangerous as smoking?” writes the LA Times
- “Eggs found to be as dangerous for the heart as smoking, new study suggests” says the Global Post
- “Eggs move back to the naughty list” notes The Globe and Mail
- Along with many more
These were prompted by a press release from The University of Western Ontario titled “Research finds egg yolks almost as bad as smoking.” The press release is based on a new study by Drs. David Spence, David Jenkins, and Jean Davignon. Maybe you recognize the university and authors from a similarly scary press release from 2010 titled “KFC’s Double Down or an egg – what’s worse?” In my opinion, both of these press releases are fear-mongering and don’t accurately portray the research. The new one, for example, implies causality in the first sentence, downplays the big limitations until the final sentence, and doesn’t put the study in context of existing research, as press releases often neglect to do. Those things aside, the study is worth looking at with existing research.
Two years ago I explored much of the egg literature and wrote a long post describing it. I concluded that epidemiological studies and short term interventional trials seem to show that eggs are safe for healthy populations, and given that they may be good for weight maintenance for breakfast (because of the protein- eggs have been studied specifically for this), they are inexpensive, and nutrient rich, it seems like a no-brainer to eat them if you enjoy them. Yet, some well designed observational studies showed an increased risk for cardiovascular disease and mortality in diabetics with higher egg intake, and I noted that caveat in the conclusion. I didn’t go into detail about these studies though.
About 6 months later, the review paper that inspired the KFC Double Down to egg comparison was published. The authors blame the egg industry for downplaying the risks of dietary cholesterol and emphasizing that postprandial effects of cholesterol and LDL oxidation rather than fasting cholesterol be considered, and explore dietary cholesterol and cardiovascular research. It is worth a read to gain their perspective. Incidentally, they hint at their new study:
“In our opinion, stopping egg consumption after a myocardial infarction or stroke would be like quitting smoking after lung cancer is diagnosed: a necessary act, but late.”
Onto their new paper, titled “Egg yolk consumption and carotid plaque.” To summarize, the study consisted of 1231 patients who had ischemic attacks or strokes who were attending vascular prevention clinics. At referral, the patients filled out a questionnaire that included smoking and egg yolk consumption (eggs per week and number of years consumed). Given that someone with vascular disease is probably aware of eggs and their cholesterol content and government recommendations, and our memories are bad as demonstrated on case-control studies, it seems like this retrospective recall is probably very biased. Unfortunately, data on alcohol intake, other dietary factors, and exercise were not accurately quantifiable so they weren’t included, another major limitation as I will discuss later. The patients had their carotid total plaque measured at referral as well. The authors split the data into quintiles of egg consumption. With each increasing quintile (and smoking group), plaque area increased after adjustment for age. Eggs remained associated with plaque area after adjusting for sex, total serum cholesterol, systolic blood pressure, diabetes, BMI, and smoking. They found no correlation between egg consumption and smoking. Shockingly, the effect from the upper quintile of egg consumers was equivalent to 2/3 of the upper quintile of smoking.
So does this provide evidence that eggs are causing plaque buildup and heart disease?
The authors casually mention that a weakness of their study is that they don’t have exercise or other dietary data. But they don’t comment on other research that has found tendencies of egg eaters to have different lifestyle habits. For example, Hu et al (1999) looked at eggs and cardiovascular disease in the Health Professionals Follow-up Study and Nurses’ Health Study cohorts. There is a consistent decrease in physical activity across quintiles in the HPFS as subjects consume more eggs (not in WHS though), and energy intake consistently increases across quintiles in both cohorts (the difference between highest and lowest is 500+ kcal). Red meat intake increases consistently in the HPFS, which has a small relationship with cardiovascular disease. Data from the Physicians’ Health Study I (Luc Djousse et al, 2008) shows a similar pattern for red meat, alcohol consumption, and (strikingly/oddly) whole milk. They were also less likely to exercise more than 1 time per week, albeit only slightly. I could continue, but the point is we know there are differences in the lifestyle habits of people who eat more eggs from other research. I understand that the data simply isn’t there, but I certainly wouldn’t make such strong conclusions knowing this as Dr. Spence did in the press release as:
“What we have shown is that with aging, plaque builds up gradually in the arteries of Canadians, and egg yolks make it build up faster — about two-thirds as much as smoking. In the long haul, egg yolks are not okay for most Canadians.”
By the way, there are two nice tables in this paper on page 405 & 406 with relative risks of various lifestyle habits that influence coronary heart disease risk. Obviously, diet quality (which can be estimated with the Healthy Eating Index or other tools), exercise, alcohol have strong influences. The new study measures none of these or the numerous other differences that could be apparent.
Also not evident in the new paper is a discussion on mechanisms in which eggs are doing harm. Between quintiles of egg consumption, HDL, LDL, triglycerides, and total cholesterol are not different. This isn’t surprising based on other research I’ve read. As I noted before in their previous review paper the authors lay out mechanisms independent of these measures that could influence risk; it is too bad none of these were quantified for this study.
Even though I don’t think this paper is strong enough to demonize eggs, there are still other well designed prospective studies that suggest risk for diabetics, though there are limitations here as well. I will quote from the 5 papers that Spence, Jenkins, and Davignon cite as evidence for harm:
Hu et al (1999): “In subgroup analyses, egg consumption appeared to be associated with increased risk of CHD among individuals with diabetes. This result should be interpreted cautiously due to numerous subgroup analyses, but the consistency of the association in the 2 cohorts argues against a chance finding. The increased risk may be related to abnormal cholesterol transport due to decreased levels of apolipoprotein E37 and increased levels of apolipoprotein C-III38 among patients with diabetes.”
That study found no increased risk for healthy people at 1 egg per day.
Djousse and Gaziano (2008):“At present, we can only speculate that perhaps among diabetic subjects, dietary cholesterol might lead to a less favorable lipoprotein profile in terms of serum concentration and particle size, with a shift to smaller and dense LDL particle size, leading to accelerated atherosclerosis and its complications. Testing of such hypothesis in an experimental design among diabetics is warranted.
Our study has additional limitations. We cannot exclude unmeasured confounding or residual confounding as possible explanation of the observed positive association among diabetic subjects. In particular, we were not able to examine the effects of saturated fat, markers of insulin resistance, lipids, and other nutrients or relevant biomarkers on the observed association.”
This was not as strong as a design as other studies and the authors acknowledge it. They have a good discussion on the diabetic issue that I recommend reading. Also, in healthy male physicians, up to 6 eggs per week was not related to cardiovascular disease and mortality but 7+ was associated with a small increased risk of total mortality.
Eckel (2008): I discussed this editorial in my previous egg blog post- it points out the dramatic differences in consumption patterns in these physicians compared to the general population. Also of note:
“It is of interest that the trend to higher risk in subjects with diabetes begins at lower egg (yolk cholesterol) intakes. The expected effect on plasma LDL cholesterol would be expected to be small. It is noteworthy that this relation between egg consumption and CVD risk in patients with diabetes is not a novel finding (7, 8). Cholesterol absorption has been shown to be higher in patients with type 1 diabetes (9) but not in patients with type 2 diabetes (10). It is not stated—but may be presumed—that most of the subjects with diabetes in the study by Djoussé and Gaziano had type 2, not type 1, diabetes.”
Trichopoulou et al (2006): This seems like a well designed study that showed exercise reduces mortality in diabetics and saturated fat and eggs increase it- but they only had 1013 subjects, 4579 person-years, and only 80 total deaths!
Djousse et al (2009): This one suggests eggs increase the risk for diabetes, using large cohorts and lots of data. But it is a novel finding and there is a lack of good evidence for biological mechanisms.
On the other hand, Spence, Jenkins, and Davignon don’t cite the following prospective study that found no problem in diabetics:
Scrafford et al (2011): Using NHANES data, this study included a subgroup analysis on diabetics which found no association with eggs and coronary heart disease or stroke mortality, though there were very few deaths in the diabetic group so statistical power is limited.
I’d love to completely absolve eggs from any wrong-doing, but the data in diabetic populations can’t be ignored, and there are some plausible mechanisms. I certainly do not think it is strong enough right now to recommend that these populations stay away from eggs, but it deserves further study, and randomized trials aren’t practical in this case so we need more well designed prospective studies. But I do still think in healthy people the evidence that they are safe greatly outweighs that suggesting they are of risk.