I lost count of how many people sent me emails or posted comments about the latest Eggs Will Kill You! study. Here are a couple of sample headlines and lead paragraphs:
Just as you were ready to tuck into a nice three-egg omelet again, comforted by the reassuring news that eggs are not so bad for you, here comes a study warning that for those over 40, the number of egg yolks consumed per week accelerates the thickening of arteries almost as severely as does cigarette smoking. Server, can you make that an egg-white omelet instead, please?
Yolk or smoke — the first is almost as bad for you as the second, London researchers have found.
Egg yolks accelerate the thickening of arteries? As in cause and effect? Was this a carefully controlled clinical trial?
Of course not. It was yet another observational study based on a food questionnaire, as the LA Times article explains.
The study, published Tuesday in the journal Atherosclerosis, measured the carotid wall thickness — a key indicator of heart disease risk — of 1,231 patients referred to a vascular prevention clinic, and asked each to detail a wide range of their health habits, from smoking and exercise to their consumption of egg yolks. Just as smoking is often tallied as “pack-years” (the number of cigarette packs smoked per day for how many years), egg-yolk consumption was tallied as “egg yolk years” (the number of egg yolks consumed per week times the number of years they were eaten).
So what we’re looking at here is a group people who were referred to a heart-disease clinic – hardly a random sampling of the population – and a measure of their plaque levels compared to their answers on a questionnaire about their dietary habits. Here’s what you can reasonably conclude about cause and effect from a study like this:
But of course, that’s not how our intrepid media reporters interpreted it:
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.
Eating yolks triggered plaque build-up at two thirds the rate for people who are smokers.
It would be bad enough if we were just witnessing the usual media misinterpretation of an observational study. But in this case, the lead (ahem) researcher has been aiding and abetting that misinterpretation. Here are some quotes from his university’s own press release:
Newly published research led by Western’s Dr. David Spence shows that eating egg yolks accelerates atherosclerosis in a manner similar to smoking cigarettes.
No, you dimwits, Dr. Spence found a correlation. That’s all.
“The mantra ‘eggs can be part of a healthy diet for healthy people’ has confused the issue. It has been known for a long time that a high cholesterol intake increases the risk of cardiovascular events, and egg yolks have a very high cholesterol content. In diabetics, an egg a day increases coronary risk by two to five-fold,” said Spence, a professor of Neurology at Western’s Schulich School of Medicine & Dentistry and the director of its Stroke Prevention and Atherosclerosis Research Centre at the Robarts Research Institute.
Dr. Spence, even Ancel Keys admitted that the amount of cholesterol we consume in our diets has no effect on the cholesterol levels in our blood. How exactly does consuming cholesterol cause heart disease? What’s the biological mechanism?
As for his statement an egg per day increases coronary risk in diabetics, I dealt with that lousy study in a previous post.
“What we have shown is that with aging, plaque builds up gradually in the arteries of Canadians, and egg yolks make it build up faster – about two-thirds as much as smoking. In the long haul, egg yolks are not okay for most Canadians.”
No, Dr. Spence, you haven’t shown that eggs make it build up faster. You can’t possibly show any such cause and effect by conducting an observational study.
Spence added the effect of egg yolk consumption over time on increasing the amount of plaque in the arteries was independent of sex, cholesterol, blood pressure, smoking, body mass index and diabetes.
Excuse me, but did I just read that the artery-clogging effects of eggs were independent of cholesterol?!! The whole reason Dr. Spence has been warning us against consuming eggs is that they contain too much cholesterol. So is cholesterol the bad guy here or not?
Let me see if I can follow the logic so far: eating eggs doesn’t raise cholesterol levels in our bloodstreams, cholesterol was not a determining factor for plaque buildup in this study, but Dr. Spence doesn’t want us to eat eggs yolks because (as he’s been busy explaining to the media), eggs contain more than the recommended amounts of cholesterol.
Okay, got it.
Here’s another of my favorite bad-science interpretations of the study:
The cholesterol in delicious egg yolks accelerates atherosclerosis (the build-up of plaque in our arteries) almost as much as smoking.
Once again, that sure sounds like cause and effect to me.
That sentence came from The Atlantic … which is a bit ironic, since the same magazine had the good sense last year to publish an excellent article titled Lies, Damned Lies and Medical Science. Perhaps whoever wrote the sentence above should read that article, which describes how Dr. John Ionnidis — an M.D. and mathematical genius who has spent years studying studies – has been exposing bad science in the health and medical fields. Here are some quotes:
He’s what’s known as a meta-researcher, and he’s become one of the world’s foremost experts on the credibility of medical research. He and his team have shown, again and again, and in many different ways, that much of what biomedical researchers conclude in published studies—conclusions that doctors keep in mind when they prescribe blood-pressure medication, or when they advise us to consume more fiber or less meat — is misleading, exaggerated, and often flat-out wrong. He charges that as much as 90 percent of the published medical information that doctors rely on is flawed.
When it came to cancer, heart disease, and other common ailments, there was plenty of published research, but much of it was remarkably unscientific, based largely on observations.
Good scientists don’t jump to conclusions based on observational studies, and with good reason: as I mentioned in my Science For Smart People speech, Dr. Ionnidis determined that 80 percent of the conclusions drawn from observational studies have turned out to be wrong. Yup, 80 percent. Here’s more from the article:
Consider, he says, the endless stream of results from nutritional studies in which researchers follow thousands of people for some number of years, tracking what they eat and what supplements they take, and how their health changes over the course of the study …
For starters, he explains, the odds are that in any large database of many nutritional and health factors, there will be a few apparent connections that are in fact merely flukes, not real health effects—it’s a bit like combing through long, random strings of letters and claiming there’s an important message in any words that happen to turn up.
Apparently in combing through his long, random strings of letters, Dr. Spence found the message Eggs Yolks Will Clog Your Arteries! – followed by another one that read: Forget What Real Scientists Believe – Correlation Does Too Prove Causation!
I don’t have a copy of the full study (and I’m not paying $32 to buy one), but Zoe Harcombe has one and wrote a spot-on analysis. One interesting bit of data she pulled from the study is that the people who ate the most eggs (or had the most “yolk years” under their belts) also had the lowest total cholesterol levels. Hmmm … once again I have to ask myself how these killer eggs yolks are clogging Canadian arteries if not through cholesterol.
She also noticed that even in the highest quintile of “egg yolks years,” the people surveyed were consuming an average of 4.68 eggs per week. According to a USDA table I downloaded, Americans in the 1920s consumed between 6 and 7 eggs per week on average. I guess that explains the sky-high rate of heart disease in the 1920s.
So what’s going on with this study? Why did Dr. Spence find a correlation? As Dr. Ionnidis points out in the Atlantic article, researchers have a way of finding the results they want to find. But let’s suppose this was a totally unbiased analysis and the correlation between “egg yolk years” and plaque buildup is really and truly right there in the data. Does that prove egg yolks cause plaque?
As I’ve said before, if I could get the media to go along and convince everyone that celery will clog your arteries, in a decade or two we could conduct an observational study and find that – lo and behold – people who ate more celery had more heart disease. The reason we’d find that correlation is that health-conscious people would be avoiding celery, while the “I don’t give a @#$%” people wouldn’t.
We saw that effect (in reverse, anyway) with the estrogen pill fiasco. A large observational study showed that women who took estrogen pills had a 40% lower rate of heart disease. But in two large clinical trials (the kind that matter), women who took estrogen pills ended up with higher rates of both heart disease and strokes. The estrogen pills weren’t protecting women’s hearts, but health-conscious women were more likely to take estrogen pills. Health-conscious people routinely gravitate towards what they’re told is good for them and avoid what they’re told is bad for them. In doing so, they can create all kind of correlations that have nothing to do with cause and effect.
I know that, even Dr. Spence doesn’t.
We’ve been told for 35 years now that eggs yolks are bad for us. So who is going to eat fewer eggs? Health-conscious people. The “I don’t give a @#$%” types will eat more – probably with a couple of pieces of white-bread toast.
My advice to the Canadians (and anyone else who wants to avoid heart disease) is to enjoy your eggs but dump all the sugar, wheat and other refined carbohydrates from your diet.
I had four egg yolks today, courtesy of our chickens. I had three yesterday and four on Tuesday. I’ll eat more eggs tomorrow. In “egg yolk years,” I’m probably coming up on my 237th birthday. And I’m not the least bit worried about it.