A study links egg consumption with a reduced risk of heart attack. Don't believe it.
All those egg-bashing scientists are going to have egg on their face when this study, which is no yolk, cracks the case on the dietary benefits of those calcium-carbonate covered comestibles.
This ends the egg humor part of the presentation.
I need to joke a bit though, because honestly, it sort of pains me to talk about studies like this. But I need to talk about them, because other people talk about them, and I feel like as a physician part of my job is to say no, don’t take these studies seriously.
But before I rant, here’s what you need to know: a huge study of 500,000 Chinese people found that those who ate more eggs had less cardiovascular death.
So, are eggs back on the menu? I don’t know – these studies really don’t tell us whether adding eggs or removing eggs from your diet will make any difference.
I’ll put it really plainly: Studies that use responses to a food-frequency questionnaire to link to some health outcome are not worth the paper they are printed on. And so let me use this recent egg study as an object lesson in the problems with dietary epidemiology research.
First – no one eats randomly. Except my two year old who asked for pasta this morning.
The rest of the world chooses what they eat based on a variety of social, economic, practical, and gustatory factors. These confounders can not be controlled for with simple statistical adjustment. If I told you that American eaters of foie gras live longer than those who don’t, would you attribute that to the foie gras, or to the fact that these 1 percenters have access to quality healthcare and other good things?
And, as I’ve discussed before, adjustment for things like “income range” does not fully account for the complex socioeconomic web we weave.
Second – Eggs, like coffee, marijuana, wine, chocolate, and many other exposures are not really one thing. While the macronutrient composition of eggs is somewhat stable, the micronutrient composition is all over the map. When researchers say coffee protects against colon cancer, are they referring to black coffee or a double-tall mocha Frappuccino? When the exposure is muddy in this way, inferences about effects become much less reliable.
Third – multiple comparisons. There are over 130 food items on the dietary health questionnaire.
The chance that one of those 130 items will appear to be statistically linked to any health outcome is near 100% - I just have to try them all. In fact, given no true relationship, there will be, on average, 6 or 7 items on the questionnaire that nevertheless fall below our conventional statistical significance threshold of 0.05.
Fourth – I told you there are 130 items on the food frequency questionnaire. But I can make even more. I can combine items to calculate your total calorie intake, or fat intake, or magnesium intake, or even your pesticide intake. More potential exposures!
Fifth – many of these dietary studies use huge datasets, like the China Kadoorie biobank with its 500,000 participants in the egg study. This means it is trivially easy to find statistically significant effects that are not remotely clinically interesting. Even if you believe the primary findings of this study – that consuming an egg per day reduces your risk of cardiovascular death by 18% compared to rarely eating eggs, the absolute effect is tiny. You’d need to treat nearly 800 people with daily eggs to prevent one cardiovascular death per year. That’s a lot of eggs.
The bottom line: what you put into your body matters, but you put a lot of stuff into your body. No one thing is going to keep you alive, and conversely, no one thing is going to kill you. So when your friend tells you that you should eat more eggs based on this study, remember how Homer Simpson handled it:
So the next time you see a study that uses a food frequency questionnaire to make some inference about something you eat, remember: it’s all a shell game.