Wednesday, August 15, 2012

Will eggs kill you? The truest answer is "It depends"

Dueling conclusions
Headline in The Atlantic:
Study: Eggs Are Nearly as Bad for Your Arteries as Cigarettes
Conclusion from the actual paper in Atherosclerosis:
Our findings suggest that regular consumption of egg yolk should be avoided by persons at risk of cardiovascular disease.  
What's the story?  Other than that journalist gets it wrong again.  Or actually, some right some wrong.  The headline is literally correct, but the story avoids the nuances.  Not surprisingly.  So we'll try to fill in.

Knowing that the effect of dietary cholesterol, particularly eggs, is increasingly considered insignificant -- because the relationship between dietary cholesterol and serum cholesterol levels is not at all a linear one -- a group of Canadian researchers undertook to determine once and for all whether in fact eating eggs does increase serum cholesterol levels.

That was then
Eggs were first shown to increase serum cholesterol way back when we first were being told to watch our cholesterol intake, back when the Framingham Heart Study showed us this something like 40 years ago.  (Indeed, here's your handy heart attack risk calculator, based on data from that study.)  So eggs became one of those guilty pleasures we consumed knowing we were knocking hours off our lives with every bite of that runny cholesterol-laden yolk.  Same with that hunk of marbled steak we couldn't resist.

But then we were told six years ago or so to forget all that -- at least the eggs bit -- when a study by Christine Greene et al. of the effects of eggs on cholesterol showed that "most people's bodies handle the cholesterol from eggs in a way that is least likely to harm the heart" as described in a 2012 piece in ScienceNews. And in fact the more eggs you eat, the bigger the HDL and LDL lipoproteins you make, which is good because large LDLs (the 'bad' cholesterol) are less likely to enter artery walls and contribute to plaque, and large HDLs (the 'good' stuff) are better at transporting plaque-producing cholesterol out of the body. Eat more eggs! That is, if you aren't already at high risk of heart disease, which you've determined with the heart attack risk calculator in the paragraph above. 

This is now
But now we're being told we should go back to an egg free existence -- or the guilty egg indulgence of earlier years.  At least according to The Atlantic.  The Canadian study of 1200 people, based on lifestyle and dietary questionnaires and assessment of arterial plaque build-up, and including recall data on egg consumption and smoking, found that arterial plaque increased linearly after age 40, most in smokers and then second in people who consumed more than 3 eggs per week.  But, they, naturally, recommend further research, including more detailed dietary information (i.e., more reliable dietary information?), but in particular they want to account for the "possible confounders," waist circumference and exercise.  

Possible confounders they call these?  These are all factors that have been shown over and over again to be associated with heart disease risk, and they didn't include them?  Not even exercise?!  "Possible confounders" is the scientific way to say "Even we don't believe this study!"?  Of course, it also diverts attention from what they really mean, which is that their study is of the additional risk, after all the known major causes are accounted for.  Anyway, it's all based on dietary recall, which as we've said numerous times before, is not a greatly accurate way to collect reliable data.  So, really, should we believe any of the conclusions of this study?    

It's complicated
But let's go back to that "most people" part of the sentence about bodies and how they handle cholesterol from eggs. That's the crucial bit here, as Greene's work and others have pointed out -- but The Atlantic did not.  It seems that most bodies can handle the cholesterol from eggs just fine (unless you eat 42 a week, as reported (paywall) in Atherosclerosis this month -- cutting down to 6 a week brought the patient's cholesterol levels down to levels that no longer worried her doctors). People with risk factors like diabetes or existing heart disease tend to have smaller lipoproteins than most other people, which may indicate that they process dietary cholesterol in a way that can lead to arterial plaque, the risky consequence of excess cholesterol levels. Or, it may indicate that these diseases lead the body to process cholesterol differently. And Greene has found that some people are "hyper responders," which means that the pool of study subjects is heterogeneous and should be stratified by how they process cholesterol in any study of the effects of dietary cholesterol.  But then, at least some of Greene's work has been funded by the American Egg Board, and it's not uncommon for industry-supported work to come out in favor of the industry.  So yet more caveats (not proof, but issues to be aware of).
As the Canadian study itself points out, many studies have shown that dietary cholesterol, including eggs, had no effect on blood levels, some that it raised some lipoproteins and not others, others that the effect depended on genetic background.  In short, it wasn't possible to issue blanket dietary advice that was true for everyone.  Do we just ignore all of those results now? And indeed, the Canadian researchers themselves acknowledge that the effects of dietary cholesterol are different for different people because they fall short of recommending that we all limit our egg consumption, only suggesting this for people who are already at high risk of heart disease.  

So, we venture to say as with all complex diseases, there's no one-size-fits-all answer here. It wouldn't be at all surprising if the cholesterol in eggs were actually protective against heart disease for some, but risky for others.  Population-level statistics which are the basis of all recommendations about diet are hard to interpret clinically. 

By the way, 50 years on the Framingham study is still going at a nicely funded level....

Should you eat eggs?  The answer is "It depends."


Jennifer said...

yup, gonna go fry up my homeraised, pastured bacon, throw in a couple free range eggs from my yard and toast a slice of homemade whole grain bread with local flour to go with it. Yum.... Proper breakfast!

Anne Buchanan said...

We should all be so lucky!

Louisa said...

I believe that if you have gallbladder problems, eggs should be avoided.