Cholesterol – is there a more controversial topic in the world of nutrition? For years, we were told that cholesterol is one of the most important biomarkers of health, particularly cardiovascular health. Prominent government- and health-related institutions have traditionally recommended upper limits on dietary cholesterol of 300 mg/day (effectively limiting egg consumption to 1 egg per day.) A recommendation based on the theory that dietary cholesterol – the cholesterol contained in food – negatively impacts cholesterol found in the blood, called serum cholesterol.
With the emergence of more and more scientific evidence, however, this theory has become increasingly untenable, causing many scientists to change their views on cholesterol. The counter theory – that dietary cholesterol has little impact on serum cholesterol – is actually nothing new. Even the progenitor of the misguided lipid theory of heart disease, Ancel Keys, acknowledged back in 1953 that dietary cholesterol doesn’t significantly impact serum cholesterol.1
Unfortunately, cholesterol came to be demonized due to its association with saturated fat and for decades both were thought to be unhealthy. New studies in the past several years have challenged this orthodox view of cholesterol.
The new research has also challenged traditional beliefs about eggs. Rather than limiting egg consumption, the available evidence suggests that eggs are actually cardio-protective.
In March 2017, for example, The Journal of Nutrition published a new study about egg consumption and its positive effects on both HDL function and plasma antioxidant levels.2 Previous studies had shown similar benefits, but those studies were largely conducted on unhealthy populations.3,4,5
The authors of this new study, therefore, decided to test how eating one, two, or three eggs daily would affect healthy young adults. This study was the latest in a series of scientific papers showing that foods rich in dietary cholesterol can actually decrease one’s risk for heart disease – a complete turnaround from the institutional forebodings of decades past.
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Important, But Not Essential
Cholesterol is not an essential nutrient. This means that although your body requires cholesterol, you’re not dependent on food to obtain it. In fact, your liver produces 90% of the cholesterol needed by your body. Some people use this fact to advance the hypothesis that dietary cholesterol is unnecessary or even unhealthy. This hypothesis is flawed for several reasons, including:
- The foods that contain significant amounts of cholesterol also contain many other important nutrients, particularly iron, and vitamin B12.
- Nearly all foods that contain bioavailable forms of high-quality protein also contain cholesterol.
The Origins of the Theory
So where does the idea that dietary cholesterol is unhealthy come from? Surprisingly, the early studies that inspired this idea were actually conducted on rabbits. While this may seem reasonable, it’s important to remember that rabbits are herbivores. And since their natural diets don’t contain cholesterol, it’s no surprise that it impacts them negatively.
Nikolay Anichkov was the scientist who originally conducted these rabbit studies back in the early 20th century. Interestingly, he fed rabbits a purified form of cholesterol.6 Obtaining cholesterol this way is completely different from obtaining cholesterol from healthy foods. In fact, most of Anichkov’s peers questioned the relevance to human health of his cholesterol experiments performed on rabbits.
Nearly 40 years later, John Gofman became the next major researcher to show interest in the topic. Gofman reported that higher levels of LDL were associated with an increased risk for coronary heart disease (CHD), whereas higher levels of HDL appeared to protect against CHD.7
After Gofman, Ancel Keys became the most prominent heart disease researcher. Through a series of experiments, Keys concluded that saturated fat consumption has the biggest impact on serum cholesterol levels. However, regarding dietary cholesterol, as mentioned above, Keys reported, “repeated careful dietary surveys on large numbers of persons in whom blood cholesterol was measured consistently fail to disclose a relationship between the cholesterol in the diet and in the serum.”8
Keys became known as the father of the lipid hypothesis – the theory that fat consumption, particularly saturated fat, drives heart disease. During the past 15 years or so, the lipid hypothesis has faced serious challenges. One of its core tenets is that saturated fat increase LDL cholesterol, which does in fact correlate with heart disease. However, LDL varies by particle size – small or large.
Small versus Large
Small particle LDL is more prone to oxidation and to the formation of subsequent arterial lesions and arterial plaque.9 Large particle LDL is less susceptible to such modifications and therefore carries little, if any, cardiovascular risk. In fact, a study that tracked women for 11 years found no significant association between large particle LDL and cardiovascular disease risk (CVD).10
So how does all this relate to eggs and their relatively high amounts of dietary cholesterol? Some interesting findings emerged from the recent Journal of Nutrition study.2 For example, as egg consumption increased, small LDL decreased and large LDL increased – a win-win with respect to reducing CVD risk. Additionally, concentrations of HDL and the antioxidants lutein and zeaxanthin also increased as daily egg consumption went up.2
One great benefit of HDL is its ability to remove cholesterol from macrophages. This is one of the key aspects of preventing the build-up of cholesterol inside the blood vessels. Additionally, this is one reason why low HDL is related to increased CVD risk.11
Conclusions
Eggs have gotten a bad rap, mostly due their high levels of cholesterol and due to our imprecise, yet ever-evolving, views on the relationship between cholesterol-rich foods and blood cholesterol levels.
Unfortunately, the nutrition establishment is still struggling to acknowledge the obvious – that eggs and other cholesterol-rich foods are healthy. Back in 2015, the USDA was in the process of updating its official Dietary Guidelines for Americans. In its Preliminary Report, published in February 2015, the group’s Advisory Committee recommended dropping the decades old 300 mg/day limit on dietary cholesterol. The available evidence, they reported, “shows no appreciable relationship between consumption of dietary cholesterol and serum cholesterol.”12 Accordingly the committee concluded, “cholesterol is not a nutrient of concern for over-consumption.”
Unfortunately, as soon as the clouds of bad conclusions dissipated, they quickly reemerged, again blocking the light of reason and evidence. For in their final report, published in January 2016, the USDA backed away from the Preliminary Report’s encouraging conclusions. Instead, they settled on “individuals should eat as little dietary cholesterol as possible while consuming a healthy eating pattern.”13
So we’re back to square one. The usual suspects are still warning against dietary cholesterol, whereas the scientific evidence draws other conclusions. Egg-white omelets are still served in many restaurants, whereas the nutrient-rich yolks are often discarded. Are eggs healthy? The science says yes, but you can decide.
References
[1] Keys, A. (1953). Prediction and Possible Prevention of Coronary Disease. Am J Public Health Nations Health., 43(11). Retrieved from (link).
[2] DiMarco DM, et al. (2017). Intake of up to 3 Eggs per Day Is Associated with Changes in HDL Function and Increased Plasma Antioxidants in Healthy, Young Adults. Journal of Nutrition, 147(3). Retrieved from (link).
[3] Herron KL, et al. (2004). High intake of cholesterol results in less atherogenic low-density lipoprotein particles in men and women independent of response classification. Metabolism, 53(6). Retrieved from (link).
[4] Mutungi G, et al. (2008). Dietary Cholesterol from Eggs Increases Plasma HDL Cholesterol in Overweight Men Consuming a Carbohydrate-Restricted Diet. Journal of Nutrition, 138(2). Retrieved from (link).
[5] Blesso CN, et al. (2013). Whole egg consumption improves lipoprotein profiles and insulin sensitivity to a greater extent than yolk-free egg substitute in individuals with metabolic syndrome. Metabolism, 62(3). Retrieved from (link).
[6] Finking G, et al. (1997). Nikolaj Nikolajewitsch Anitschkow (1885-1964) established the cholesterol-fed rabbit as a model for atherosclerosis research. Atherosclerosis, 135(1). Retrieved from (link).
[7] Gotto AM, et al. (2011). Jeremiah Metzger Lecture: Cholesterol, Inflammation and Atherosclerotic Cardiovascular Disease: Is It All LDL? Trans Am Clin Climatol Assoc., 122. Retrieved from (link).
[8] Keys, A. (1953). Prediction and Possible Prevention of Coronary Disease. Am J Public Health Nations Health., 43(11). Retrieved from (link).
[9] Ross R. (1999). Atherosclerosis – an inflammatory disease. New England Journal of Medicine, 340(2). Retrieved from (link).
[10] Mora S, et al. (Feb 2009). Lipoprotein particle profiles by nuclear magnetic resonance compared with standard lipids and apolipoproteins in predicting incident cardiovascular disease in women. Circulation, 119(7). Retrieved from (link).
[11] Assmann G, et al. (1996). High-density lipoprotein cholesterol as a predictor of coronary heart disease risk. The PROCAM experience and pathophysiological implications for reverse cholesterol transport. Atherosclerosis, 124. Retrieved from (link).
[12] 2015 Dietary Guidelines Advisory Committee. (Feb 2015). Scientific Report of the 2015 Dietary Guidelines Advisory Committee. US Department of Health and Human Services and US Department of Agriculture. Retrieved from (link).
[13] USDA. (2016). Dietary Guidelines for Americans, Final Report. Retrieved from (link).