Here we go again. The popular press gets ahold of a scientific study, misinterprets it, and runs a scary story. Of course, it’s much worse if the scientific study published in a respected journal seems to also misinterpret the study. As I mentioned before, a true skeptic needs to critically analyze whatever is written in the press by going to the original study whenever possible; but what happens if that study requires some critical analysis? Well, I never said it was easy. If you want easy, denialism is really easy!
So back to the eggs. All across the news during the past week or so, you probably saw a story that eating egg yolks cause arteriosclerosis, a chronic condition in which an artery wall thickens as a result of the accumulation of fatty materials such as cholesterol. Some people may have already believed that anecdotally, but a new article published in Atherosclerosis, Egg yolk consumption and carotid plaque, concluded that,
Our findings suggest that regular consumption of egg yolk should be avoided by persons at risk of cardiovascular disease. This hypothesis should be tested in a prospective study with more detailed information about diet, and other possible confounders such as exercise and waist circumference.
Well, newspapers and other news outlets jumped on this articles with full-throated arguments that we needed to avoid eggs. The Los Angeles Times published an article, No yolk: eating the whole egg as dangerous as smoking? As dangerous as smoking? Really? Nevertheless, the article reported that:
Smoking tobacco and eating egg yolks increased carotid wall thickness in similar fashion — which is to say, the rate of increase accelerated with each stair-step up in cigarette smoking or yolk consumption. By contrast, for those who did not smoke, or who rarely consumed egg yolks, carotid wall thickness increased after 40, but at a slow-steady rate.
Of course, smoking has other health effects like lung cancer, but sure, this might make sense. And of course, eating eggs may have benefits, according to the LA Times,
In recent years, nutritionists have begun to agree with egg purveyors that chicken eggs — cheap and packed with protein — have gotten a bad rap as a dangerous source of cholesterol. Some studies have suggested that eggs may increase HDL, or “good cholesterol” that protects against heart disease, even as it contributes to the artery-clogging LDL cholesterol, making egg consumption something of a wash. And regular egg-eaters may form larger lipoprotein particles that help clear the blood of fat particles and are not as likely to settle in artery walls.
So what are the facts? What’s really going on with eggs? Let’s go back to the Atherosclerosis article by J. David Spence, David J.A. Jenkins, and Jean Davignon. Unfortunately, it’s behind a pay wall (one day, these science articles need to be open-access, if we’re going to have open discussion of issues, but that’s a debate for another day), but the good people at Science Based Medicine just published a review of the Spence et al. article, Eggs and Atherosclerosis, and gave us an interesting graph with some data that seems to poke a few holes in the eggs are bad story.
To interpret this chart, “egg group years” is the number of egg yolks eaten per week times the number of years, so more eggs are eaten in each patient grouping across the chart. As you can see, the plaque area (in the last line) increases as more eggs are eaten which would seem to imply that as you eat more eggs, the more closed the carotid artery becomes in these groups of patients. That’s not good and is a very important indicator of cardiovascular disease. But there’s a really serious issue with this data, and that is that triglycerides, HDL cholesterol (what is considered the “good” cholesterol) and LDL (“bad” cholesterol) are unchanged across the groups. Though plaque area is significantly different, the assumed cause of the plaque change, bad cholesterol, isn’t changing as a result of the increase in eating eggs.
This goes to the age old problem in science of “correlation does not imply causation.” Maybe the authors missed some other factor that is causative, such as…nothing seems to be clear with this data. As Steven Novella says at Science Based Medicine, “This strongly suggests the association is not causal but is incidental or spurious – unless an alternate mechanism can be proposed and supported by evidence.” About the only thing I would consider is that eating more eggs could mean a higher calorie diet when combined with a more sedentary lifestyle might lead to higher plaque formation, but none of this data would be supportive of that conclusion.
It is so frustrating that these food myths are invented based on cursory reading of data. But it’s even more frustrating when the scientific study seems to make a conclusion that isn’t supported by the data that it presents.
But what do other studies say. Well, a British Medical Journal article, Dietary fat intake and prevention of cardiovascular disease: systematic review, in 2001 stated that:
There is a small but potentially important reduction in cardiovascular risk with reduction or modification of dietary fat intake, seen particularly in trials of longer duration.
And a Cochrane Review, Reduced or modified dietary fat for preventing cardiovascular disease, concludes that:
The findings are suggestive of a small but potentially important reduction in cardiovascular risk on modification of dietary fat, but not reduction of total fat, in longer trials. Lifestyle advice to all those at risk of cardiovascular disease and to lower risk population groups, should continue to include permanent reduction of dietary saturated fat and partial replacement by unsaturates. The ideal type of unsaturated fat is unclear.
The results seem a bit equivocal, and it’s suggestive of a general lifestyle change that include dietary changes that are not of the level of what was suggested in the “eggs are as dangerous as smoking” article in the Los Angeles Times.
Published articles that directly studied effects of eggs on overall cardiovascular health are all over the place:
- The Effect of Egg Consumption in Hyperlipidemic Subjects during Treatment with Lipid-Lowering Drugs. “…the consumption of additional 3 eggs per day to their regular diet will increase the level of HDL-cholesterol and decrease the ratio of LDL-cholesterol to HDL-cholesterol.” In other words, eating eggs while on lipid lowering drugs might be beneficial.
- Effects of eggs on plasma lipoproteins in healthy populations. ”Egg intake has been shown to promote the formation of large LDL and HDL subclasses in addition to shifting individuals from the LDL pattern B to pattern A, which is less atherogenic. For these reasons, dietary recommendations aimed at restricting egg consumption should be taken with caution and not include all individuals.” So, egg intake should be analyzed from individual to individual.
- Egg consumption, serum cholesterol, and cause-specific and all-cause mortality: the National Integrated Project for Prospective Observation of Non-communicable Disease and Its Trends in the Aged, 1980 (NIPPON DATA80). “In men, egg consumption was not related to age-adjusted total cholesterol. Cox analysis found that, in women, all-cause mortality in the 1-2-eggs/wk group was significantly lower than that in the 1-egg/d group, whereas no such relations were noted in men.”
- Dietary fat and cholesterol and risk of cardiovascular disease in older adults: the Health ABC Study. ”Dietary cholesterol and egg consumption were associated with increased CVD risk among older, community-dwelling adults with type 2 diabetes. Further research on the biological mechanism(s) for the increased CVD risk with higher dietary cholesterol and frequent egg consumption among older adults with diabetes is warranted.” Of course, these results are in a more at risk group. And even then, those who have their diabetes under control and may keep their LDL under control may have different outcomes.
So what’s the overall conclusion? I don’t see any definitive data (I don’t even see equivocal data) that there is anything wrong with eating eggs in healthy adults. I don’t even see definitive data that eating eggs in those individuals with chronic diseases are increasing their risks, although the extra calories might not be necessarily a good thing. Once again, dietary claims seem to be more shrouded in myth and magic than it is in science. Once you look for evidence in dietary claims, you either find contrary evidence or varying evidence (which, unless it is conclusive, does not mean you can pick out the evidence that supports your on beliefs).
Eat eggs if you want, but eat them in moderate quantities. Get exercise. Burn more calories than you consume. It’s not that hard.
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