The headlines are once again filled with advice to reduce egg and cholesterol consumption based on a study that found an association of egg and cholesterol consumption with increased risk for cardiovascular events. Sounds scary and persuasive, doesn’t it? After all, nearly 30,000 people were tracked over 17 years and the authors authoritatively declare that this proves that eggs and cholesterol are risk factors for heart disease.
There are several problems with this assessment. It is emblematic of the studies that confuse people, yield wildly conflicting conclusions, are used to craft absurd and ineffective dietary guidelines, and provide tantalizing headlines for media. And it is virtually all untrue.
1) This is an observational study, meaning people are asked a few questions, then followed for many years, i.e., they are observed. Over and over again, observational studies that do not involve any sort of intervention (i.e., people purposefully advised to eat, say, 3 eggs per day vs. people advised to eat no eggs, as would be done in a real clinical study—much more difficult and costly to conduct) cannot be used to generate cause-effect relationships. See this Undoctored Blog post about the fictions generated by such observational studies. 4 times out of 5, the conclusions drawn by observational studies are disproven by prospective randomized clinical trials, i.e., the real way to prove or disprove a cause-effect relationship. Or, stated differently, 80% of the conclusions drawn through observational analyses are false. In other words, the majority of conclusions drawn by observational studies are complete fictions.
If we were to believe observational studies, we would have to believe that Premarin is good for women’s health, since proven to increase endometrial cancer, breast cancer, and cardiovascular death despite the observational evidence suggesting the opposite; red meat causes colon cancer; coffee consumption leads to extended longevity, etc., all the absurd proclamations based on the fictions of observational studies and nutritional epidemiology.
2) Observational studies are unreliable because they are plagued by confounding factors—That is, people who eat eggs also do other things differently than people who don’t eat eggs. Perhaps they eat cured bacon, exposing them to nitrosamines like N-nitrosodimethylamine. Or perhaps they eat at restaurants that use soybean or corn oil to prepare the eggs. In other words, the nature of observational analyses is that you cannot untangle the oodles of other factors that can influence outcome and cannot say that one factor—egg consumption—is the cause. This is the reason why, by the way, the vegetarian community declares that being vegetarian is healthier: Being vegetarian is not healthier; being vegetarian is associated with a constellation of other behaviors such as less cigarette smoking, less alcohol, more exercise, more overall health consciousness, etc. that can improve health, but health has nothing to do with avoiding animal products.
3) That all said, observational studies can occasionally lead to firm conclusions if the association is overwhelming powerful. This happened with cigarette smoking: The increase in heart disease and lung cancer was so large that even the observational data proved conclusive. The very small increment in risk suggested by this study (17% increased cardiovascular disease, 18% increase all-cause mortality) is essentially no difference at all—much too small to be conclusive.
Observational studies have been the basis of all manner of nutritional misinformation. It is the basis for cutting total and saturated fat, increasing consumption of “healthy whole grains,” “move more, eat less,” all the fictions that have created the worst epidemics of obesity, heart disease, type 2 diabetes, gastrointestinal and autoimmune diseases ever in the history of humankind. Dietary guidelines have gotten it wrong for 50 years and I am not hopeful that they will ever get it right in our lifetimes.
What I find most shocking is that most of us in healthcare know that observational studies are virtually worthless and cannot be used to generate cause-effect conclusions, yet the authors of this study state authoritatively that their study should be used to resurrect the advice to limit dietary cholesterol. I cannot decipher their motivations beyond garnering attention and headlines. But don’t fall for it: This is yet another example of how far wrong dietary conclusions can be when drawn from the fictions of nutritional epidemiology.