By David Katz, M.D. HuffPost Healthy Living
If I were to say "_______ and eggs" to you (and in fact, I just did), what's the first thing that comes to your mind to fill in the blank?
I suppose you might come up with "heart disease," but I doubt it. I bet you will be in the overwhelming majority if you go with "bacon." If the eggs happened to be green, I suspect an even more overwhelming majority would settle on "ham." Please hold that thought, as I attempt to unscramble the latest research about eggs and health.
The study most recently in the headlines -- just published in the journal Atherosclerosis -- suggests not only that egg ingestion increases the risk of heart disease, but also that the association is as strong as that for cigarettes. I don't believe either is true.
The authors came up with the term "egg-yolk-years" as an analogue to "pack-years" of smoking, each representing the frequency of exposure multiplied by duration. They measured the volume of atherosclerosis in the arteries of patients attending the vascular clinics of University Hospital in Ontario, Canada, and asked them about lifestyle practices -- including such things as smoking and egg ingestion. Finding that the people with more plaque in their arteries reported eating more eggs, they reported that egg-yolk-years were a significant predictor of heart disease and that "regular consumption of egg yolk should be avoided by persons at risk of cardiovascular disease."
If you have been paying attention to research about eggs and dietary cholesterol over time, this report likely confused and frustrated you. You doubtless know that for years dietary cholesterol in general, and eggs in particular, were lumped in with sources of harmful fats as contributors to heart disease risk, to be avoided by those concerned for the health of their hearts. But subsequent research, much of it conducted and published over the past decade or so, focused on unbundling the effects of trans fat, saturated fat, and dietary cholesterol. And when this was done, adverse effects of dietary cholesterol all but disappeared.
But now, suddenly, eggs are as bad as cigarettes? How can that be?
The authors put what they presumably considered the sunny side of their data up to face the world, but frankly, it all turns over very easy. I have five reasons for thinking their conclusion is almost certainly wrong, and the implications rather badly scrambled. Those five reasons are: association, predisposition, intervention, aggregation, and adaptation.
This was a study of association, not cause-and-effect. This column began with a demonstration of what we "associate" with eggs -- other foods such as bacon. Imagine if the people who ate the most eggs also ate the most bacon, and sausage, and ham. Since the current study did not control for other aspects of diet, it could be that eating a lot of processed meats was contributing to arterial plaque, with eggs having nothing whatever to do with it. This is called confounding.
On the chance you think this is far-fetched, consider that in the 1970s, studies of "association" generated headlines that coffee consumption increased the risk of pancreatic cancer! When issues of confounding were resolved, however, there was no such effect. When Linus Pauling went looking for evidence that vitamin C could cure cancer, he found it. Those findings were refuted by the less biased studies that followed.
All research is biased, because all researchers are looking for something; all researchers have a predisposition. The only real defense against that is methods that eliminate bias -- such as randomization, double-blinding, and placebo control. None of this is possible in an observational study of associations -- which means there is a very high likelihood of finding whatever it is you are looking for.
I don't know the lead author of the current study, Dr. David Spence, personally. But his publication record indicates clear dedication to indicting dietary cholesterol for crimes against humanity's arteries.
I do, however, know one of the other authors, Dr. David Jenkins, quite well. He is both a friend and colleague. I consider Dr. Jenkins a pre-eminent nutrition researcher who has made seminal contributions to the field.
I also happen to know he is an ethical vegan -- whose devotion to this matter extends even to the avoidance of all leather. I consider this entirely admirable, although I must say it leaves Dr. Jenkins a bit fashion-challenged in the footwear department. He is an elegant gentleman, however, and puts a far better foot forward in this situation than most of us would manage.
But the point here is that Dr. Jenkins almost certainly has biases aligned with his own lifestyle choices. In research, unless we use robust methods to defend against it -- we are almost certain to find what we are looking for. The current study has no methods built in to defend against finding an association between eggs and heart disease.
Because Dr. Jenkins is a friend, I warned him in advance that I planned to write this column, and we exchanged opinions. We actually seem to agree that eggs may be less a direct cause for concern, than a marker of overall dietary pattern. In an email to me, Dr. Jenkins noted: "I believe the context in which eggs are eaten may be relevant. Eggs, bacon and rich pork sausages ... may be very different from scrambled eggs on whole wheat toast."
On this, Dr. Jenkins and I agree; we seem to disagree on the importance of distinguishing the nutritional baby and bathwater. If bleu cheese dressing contributes to heart disease risk, it doesn't mean we should advise people to avoid lettuce.
Intervention studies, which can determine cause-and-effect, tend to trump observational studies, which cannot. In the context of a large body of evidence challenging the association between eggs and cardiac risk, my own lab has conducted three intervention studies. We have already published the results of two of them, one showing no harmful effects of daily egg ingestion in healthy adults, and the other showing no harmful effects in adults with elevated cholesterol.
Results of the third study, just completed, are not yet published. I can't spill those beans here, but I can say something about it. It was a study of daily egg ingestion among adults with established coronary artery disease -- and the opinion I am expressing in this column is informed by my review of the findings. Enough said.
I hasten to add I don't really have a personal agenda about eggs, and certainly am not in favor of them. I banished them from my own diet for roughly 20 years when the research evidence seemed to incline that way. I don't like the way hens are treated on large "factory farms," and certainly don't want to take a position that fosters the ill treatment of any animal. But the hard-boiled results of research are what they are, and I don't think eggs contribute to cardiac risk. I have added eggs (organic, local, free-range) back into my own diet, although like Dr. Jenkins, my standard breakfast is, in fact, that combination of whole grain cereal, walnuts, and mixed berries -- or some variation on that theme.
My own lab's findings are, as noted, in the context of a clear aggregation of evidence over recent years suggesting that dietary cholesterol is mostly, if not entirely, innocuous when isolated from the company it often keeps in the diet. The weight of evidence about eggs has, in my opinion, tipped decisively toward a "not guilty" verdict.
And finally, there is the context that makes sense of this: adaptation. Paleoanthropologists tells us that eggs have been part of the human diet back to the Stone Age. And while saturated fat intake in the Stone Age was next to nil, cholesterol intake from both eggs and organ meats of hunted animals was substantial. It is, in a word, illogical that the human body would be harmed by something to which it is long adapted. It would be a bit like concluding that wildebeest is harmful for lions; bamboo is bad for pandas. Eggs have been part of the "native" human diet since long before the advent of the first domesticated chicken. Science is generally at its best when bounded by sense.
I am a strong proponent of a mostly plant-based diet. And it is the overall dietary pattern that matters most to health. But the truth about eggs and health is best revealed by research that subordinates such predilection and eggspectation to hard-boiled methodology. When that is done, I believe the egg is substantially... eggsonerated.
TIME magazine was right to show eggs on a plate with bacon; that's a robust association in our culture. When such associations are left unaccounted for, any apparent association between eggs and cardiovascular risk is certain to be deviled by them.