Eggs are one food that have had wildly different press from the nutritional community over the years. Back in the 1950s, here in the UK, we were encouraged to ‘go to work on an egg’
All that changed, though, when we got all cholesterol-conscious and fat-phobic back in the 70s and 80s. Recently, there was a chance of eggs being rehabilitated, mainly on the basis that egg eating did little to raise cholesterol levels, so was unlikely to raise heart disease risk. But this week saw the publication of a study which give a damning verdict on eggs [1]. Three doctors based in Canada warn, in this review, that “patients at risk of cardiovascular disease should limit their intake of cholesterol,” and that “Stopping the consumption of eggs yolks after a stroke or myocardial infarction [heart attack] would be like quitting smoking after a diagnosis of lung cancer: a necessary action, but late.”
The review focuses on so of the biochemical changes induced by egg eating that might, in theory at least, cause cardiovascular disease. Cholesterol, inevitably, features heavily. The authors can’t quite bring themselves to be up front about the fact that egg eating does not raise cholesterol levels. The focus is shifted to things like cholesterol oxidation, raised fat levels in the blood after meals, and the idea that cholesterol “potentiates the adverse effects of dietary fat.”
Well, saturated fat doesn’t appear to have any adverse effects regarding cardiovascular disease risk, so that idea is essentially dead in the water. And so what if eating food with fat in it causes blood fat levels to go up? It’s what you’d expect. What is really important is the impact a food has on health and disease risk. ‘Surrogate markers’ such as blood fat levels are not to be relied upon to judge the impact of any factor on health. If arsenic or cyanide reduced cholesterol levels, would we recommend that everyone swigs back these poisons everyday?
To really judge the impact of eggs on health we require intervention studies. For example, we could take a group of individuals and instruct them to eat lots of eggs over a long period of time and then assess their health compared to a group eating fewer eggs. We don’t have such studies.
So, inevitably, scientists have turned to ‘epidemiological evidence’. This sort of evidence at best can tell us about relationships between things. It cannot prove that one thing causes another (or that one thing does not cause another, for that matter).
Anyway, the point is there have been some epidemiological studies that have found in healthy individuals, higher egg consumption is not associated with an enhanced risk of cardiovascular disease [2,3]. The authors are dismissive of this finding though, and suggest it’s most likely caused by the studies lacking the ‘statistical power’ to detect a link.
Interpretations of this type, in my mind anyway, smack of ‘bias’. It’s just the sort of comment authors who have made up their mind about what they want to find make. Is there any other evidence of bias in this review? You bet.
For a start, the authors are dismissive of these epidemiological studies ‘lacking statistical power’, but quote one of these studies as evidence of a link between egg eating and cardiovascular risk in diabetics (suggesting that the power here was sufficient).
They also do not pause to think about what might have led up to such a finding. It could be that individuals who are not very health conscious continue to eat relatively large amounts of eggs, and it’s not the eggs, but the laissez faire attitude about their health that explains this association. I wrote about this recently here.
The authors also quote a study which found that eggs eating was not linked with an increased risk of cardiovascular disease [4], but was associated with an increased risk of death. Now the authors spend a lot of time highlighting the supposed perils of eggs with regard to cardiovascular health. And then quote a study linking eggs are linked with an increased risk of death but not cardiovascular disease. So if it’s not cardiovascular disease that’s killing people, what is it? The authors don’t even suggest an explanation here, so let me do their job for them: the likely explanation is that egg eaters are, generally speaking, less health conscious (see above), and are at increased risk of dying as a result of other lifestyle factors associated with not being very health-conscious (such as being sedentary and smoking).
More evidence of bias in this review comes in the form of the language the authors sometimes use. Look at these two sentences:
“A recent re-analysis of the smaller Physician’s Helath Study…[showed] that regular egg consumption doubled all-cause mortality.”
“Two recent studies also showed that consumption of eggs increased new-onset diabetes, independent of other dietary factors.”
To read these sentences you’d think that the observations they refer to had been made via intervention studies that can discern cause and effect. But, in reality, they refer to epidemiological studies, the results of which we can be circumspect about at best. Essentially the authors of review have taken some quite weak evidence, and expressed in a way that suggests eggs cause diabetes and can kill.
Bias creeping into science is not a good thing, and it’s sometimes useful to ask where such bias may come from. Well, here’s the conflict of interest statement from this review:
“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 Inc and Merck Frosst Canada Ltd.”
Seems like two of the authors of this review have some potential interest in keeping the cholesterol theory alive and well.
Look, we simply don’t have definitive evidence which tells us whether eggs are healthy or not. But in the presence of evidence which does not incriminate them, as well the weak and inconsistent nature of evidence used to damn them, I’ll continue to ‘take my chances’ with this natural and nutritious food.
References:
1. JD Spence, DJ Jenkins, J Davignon. Dietary cholesterol and egg yolks: Not for patients at risk of vascular disease. Canadian Journal of Cardiology, 2010; 26 (9): e336-e339
2. Hu FB, et al. A prospective study of egg consumption and risk of cardiovascular disease in men and women. JAMA. 1999;281(15):1387-94.
3. Quereshi AI, et al. Regular egg consumption does not increase the risk of stroke and cardiovascular diseases. Med Sci Monit. 2007;13(1):CR1-8.
4. Djousse L, et al. Egg consumption in relation to cardiovascular disease and mortality: the Physicians’ Health Study. Am J Clin Nutr 2008;87(4):964-9.
