First eggs were bad, then they became good, then just a couple of weeks ago, they became bad again. If you're scratching your head because you can't keep up with the conflicting information concerning egg consumption and the risk of cardiovascular disease (CVDs) or heart disease, you're not alone. Many consumers around the world are asking the question “should I continue to eat eggs?”. Part of this confusing stems from the media's sensationalization of research findings. Another part of it is that the science also actually does change based on how much evidence is available at any given point in time.
In this article, I will attempt to summarize the historical context of egg consumption or dietary cholesterol and its link to heart disease, as well as provide a critique of the latest article published in the Journal of American Medical Association (JAMA) by Zhong W. Victor et al.
What is Cholesterol? And what is the deal with eggs?
Cholesterol is a type of fat that is produced in the human body. It is important because the body uses it for many functions including making cell membranes,production of hormones, and some vitamins like Vitamin D which is important for bone health. A deficiency of cholesterol could lead to fertility problems, poor skin, and deficiency of fat soluble vitamins. Only animals produce cholesterol, so the sources of cholesterol in meals are from animal and animal products like meat, butter, eggs and milk or milk products. However, what is good for you could also be bad for you. When there is too much cholesterol in the body, it begins to get deposited in your blood vessels and could subsequently block them through a series of inflammatory reactions, leading to high blood pressure, heart attacks and stroke. These are collectively referred to as cardiovascular diseases (CVDs).
The recommended amount of cholesterol intake for one day, based on what had been associated with a lower risk of CVDs is 300mg. In terms of food, this translates to about 10 tablespoons of butter, 12 matchboxes (4 servings) of beef , 28 slices of bacon and, about 2 cups of soft serve ice cream (all values computed with MyDietAnalysis from Pearson). Eggs got a bad rep because one single hard-boiled egg contains 186 mg of cholesterol. That's about 60% of 300mg, which means that just 1½ eggs met the recommended value. For many years, scientists believed that the amount of cholesterol in your food could affect the amount of cholesterol in your blood, leading to an earlier recommendation of no more than 3 eggs a week.
However, with growing evidence, it was observed that the body produces its own cholesterol which accounts for almost 80% of the total amount of cholesterol in the blood, and that dietary cholesterol didn't increase blood cholesterol significantly. It was later also observed that other fats, and not necessarily dietary cholesterol, could lead to high cholesterol levels in the body. These are saturated fats and trans fat. Saturated fats are fats that are solid at room temperature that you get from animal products like fatty meats.Trans fats are those found in products like hard margarines and store baked goods like cakes and pastries (because they are made from vegetable shortening which are high in trans fat). And while not a fat, added sugars have been observed to also increase blood cholesterol levels. This is possible because your body can produce cholesterol from excess carbohydrates.
Given this background, the American heart association (AHA) after many years of warning the public against consuming more than 3 eggs a week, softened their stand in 2014. The Dietary guidelines for Americans in 2015 also removed the dietary requirement for cholesterol and lifted the restrictions on egg consumption. The Ghana Dietetics association (GDA) which is the professional association of dietitians in Ghana only this year informed the general public that for a healthy person, consuming an egg a day as part of a balanced diet was safe so long as you did not have any underlying conditions like high lipid levels that required you to reduce your total fat intake.
Apart from cholesterol, do eggs supply any other nutrients?
Eggs are a relatively affordable source of protein. If you were getting your protein from beef and if you bought directly from the local market, you will spend GHC 2.25 on a serving of beef (about 3 matchboxes of beef). The price of eggs in Accra, in order to get the same amount of protein (about 3 eggs), is GHC 1.70 (this is if you buy per crate). Besides being cheaper, they are easier to prepare and easily digested and absorbed by the body. An environmental enthusiast could also argue that egg production is friendlier to the environment than beef production because of the high contribution by cows to greenhouse emissions.
Apart from protein, eggs are also an important source of many other nutrients that your body needs to function properly, specifically vitamins like biotin, Vitamin B5, and the B-vitamin-like compound cholin, as well as minerals like selenium and iodine.
What are the details of the JAMA paper?
The JAMA paper was a pooled analysis of 6 cohort studies that observed that for each additional half egg consumed a day, there was a 1.11% higher risk of CVD as well as a 1.93% increased risk of mortality. In other words, the more eggs you eat each day, the higher your risk of getting a heart disease or dying. A pooled analysis means that the authors put together data from different observational studies so that they could have a large data set, in this case a total of almost 30,000 adults. Cohort studies, a type of observational study, are useful because it is not always ethical, or even possible, to conduct experiments on human beings. So researchers collect information about people's behaviors and then observe them over a period of time to determine who develops a disease or otherwise. The universal catch to these studies however is that we can never be too sure if the observations made are the true causes of the diseases in question.There are always so many other factors that we generally cannot control for when people are being people.
While this particular paper is strong because it observed a large number of people, it had some limitations that are worth noting. First and most importantly, diet was assessed in this population once throughout the follow-up period which was an average of 17 years (up to 31 years for some participants). The authors use dietary information that was collected at the beginning of each of the cohort studies. This goes without saying that this one time dietary assessment may not be representative of the participant's long term dietary intakes. People's diets change throughout the course of their lives; indeed some people's diets fluctuate over the course of a single week, and to fully understanding a diet-disease relationship, it is more useful to have several time points of data collection to get a holistic picture of a person's overall dietary exposure.Using how someone ate 17 years ago to assess their lifetime risk of a disease may not give a truly accurate picture.
Secondly, in the statistical analysis (the part that many of us skip when reading scientific papers), the authors discover that when they adjust for egg consumption (as well as processed and unprocessed meat intake), the association observed between dietary cholesterol and CVD risk is no longer significant, which may suggest that eggs (and other high fat foods), and not necessarily dietary cholesterol is what is driving the association. However, in their analysis of the association between egg consumption and CVD risk, the association was no longer statistically significant when they adjusted for dietary cholesterol, something I consider contradictory to the former observation. The authors in their discussion interpret this to mean that the cholesterol from egg yolks have an independent association with CVD risk and mortality, but this rather raised many questions for me: are eggs on their own really the problem? Is it cholesterol intake in its totality? Just the cholesterol from egg yolks on it's own? Or is it a combination of both? Most importantly, are we just simply talking about a person who is consuming not only many eggs a day, but also high amounts of fatty and unprocessed meat like the regular American does?
Finally, and this applies more to non-US populations, the results may not be generalizable to populations outside of the US. The authors specifically mention this in their discussion and I quote: “Findings of the current study suggest that cholesterol from egg yolk may be harmful in the context of the current US diet, in which overnutrition and overweight/obesity are more common than malnutrition and underweight…..generalizing our results to non-US populations requires caution due to different nutrition and food environments and chronic disease epidemiology”
Within the current global setting, it may be argued that trade liberalization has caused the Westernization of diets and transformation of food systems across the world and especially in low middle income countries like Ghana. This notwithstanding, undernutrition, particularly protein energy malnutrition, continues to plague the country in a phenomenon popularly known as the double burden of malnutrition. That said, and following the caution of the authors, health professionals in Ghana may benefit from contextualizing these findings to the Ghanaian situation.
The authors recommend that their research findings be used as a basis to update the current American dietary guidelines, but given that their findings are observational, and the strength of evidence is modest, this may be a long shot. This is because observational studies, as I alluded to earlier, cannot establish causality. They can supply information that some exposure may be associated with some outcome. As a result, unless they are supplying overwhelmingly convincing evidence, they usually serve as a basis for subsequent more controlled studies.