What’s the deal with that egg study?

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One of the most common sources of nutrition-related frustration that patients express to me is the apparent fickleness of nutrition advice. It feels as though headlines and sound bites demonize a food that only yesterday was deemed the food of the Gods, or vice versa, leaving exasperated and confused eaters at a loss.

Eggs became the latest example when a recent Northwestern University study was picked up by mainstream media and turned into “clickbaity” headlines, such as “Bad news for egg lovers,” “Eating Eggs and Cholesterol Linked to Heart Disease and Death Risk,” “Are eggs good or bad for you? New research rekindles the debate,” and “Northwestern study cracks dietary guidelines for eggs.”

Unfortunately, disconnects often exist between headlines – which, remember, are sometimes sensationalized and designed to generate clicks, views, and shares – and the research behind them. For example, the Northwestern study in question is not actually bad news for egg lovers. Far from it. Let’s take a look at the study.

The study relied on self-reported dietary data, which are terribly flawed. Sometimes during the course of our work, I may ask a patient to keep a food journal and return it to me for analysis. Despite patients’ best efforts to keep accurate journals, their sources of error are ultimately numerous. People misremember what they consumed, forget to report some of what they ate, provide vague information that I can easily misinterpret, and purposely falsify data for fear of judgment.

Close to a decade ago, I was working on a research study that in part required that I interview people about what they ate the preceding day. As I sit here right now, I could not tell you what I ate for dinner last night, and the subjects were no different. One of the gentlemen I interviewed got frustrated because I had to drill down to such a specific level of detail that I was asking him for the measurements of the piece of lettuce he put in his previous day’s sandwich; meanwhile, he could not even be sure that he had eaten a sandwich at all. Eventually, my research team made the decision to drop the dietary recall portion from our study because the data were just so poor. Similarly, how confident can we really be that subjects included in the Northwestern study accurately reported their egg consumption?

Even if we take the data at face value and assume them to be completely accurate, we must remember that this study only found associations between egg consumption and disease, which is not the same as establishing a causal relationship. One of the most common mistakes that people make is to assume that correlation implies causation, but such an assumption is premature at best and can turn out to be just plain wrong.

Just because two events tend to occur together does not mean that one causes the other. Consider what our friend and colleague, Ragen Chastain, famously wrote in 2017. “Imagine if I got together everyone who had survived a skydiving accident when their parachute didn’t open and started looking for things they have in common. Even if every single one of them wore a green shirt and had oatmeal for breakfast, I cannot say that wearing a green shirt and eating oatmeal will allow you to survive a skydiving accident, nor can I ethically start Ragen’s School of No Parachute Skydiving ‘free green shirt and oatmeal with every jump!'”

In other words, even if it is true that people who consumed more eggs had a greater incidence of cardiovascular incidents and death, we cannot say for sure that the eggs were responsible, just as we cannot say that blueberries reduced heart attack risk, because it could be that another factor – or combination of factors – common to people who consumed more eggs is responsible for their disease and death as opposed to the eggs themselves.

Observational studies like these are great for developing hypotheses to be explored in subsequent research, but their design prevents them from establishing causal relationships. Unfortunately, this incredibly important point is often glossed over or ignored entirely when a study is distilled to pop culture news articles and then further condensed into headlines.

Consequently, the news that we see leaves us with the impression that nutrition information and guidance are always changing like early springtime New England weather. Don’t like seeing your favorite food being vilified? Just wait until tomorrow when a new headline will sing its virtues.

In reality, nutrition science moves at a more glacial pace. One study generates hypotheses that subsequent studies investigate, followed by yet more research that looks at the given questions from different angles in an attempt to confirm or refute the original findings and gain a deeper understanding that policymakers eventually take into account when issuing dietary guidelines.

If someone’s current egg consumption is working for them, I see no compelling reason – based on what we know at this point – for changing it.

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