This article reviews the egg cholesterol controversy over heart disease. The relationship between elevated blood cholesterol and cardiovascular disease (CVD) is well-established. Reducing low-density lipoprotein (LDL) cholesterol is the primary target of preventive therapy. Furthermore, studies report a lower risk for heart disease with greater high-density lipoprotein (HDL) cholesterol concentrations.
Cholesterol and Heart Disease
There is significant interest in exploring the relationship between dietary cholesterol intake and heart disease risk. Eggs are one of the richest sources of cholesterol in the diet. However, large-scale studies have found limited links between eating eggs and cardiovascular disease risk.
Clinical studies show highly variable outcomes of egg intake on dietary cholesterol, with two-thirds of the population having only minimal responses. Those with a significant impact increase both LDL and HDL cholesterol, thus maintaining their LDL/HDL ratio. Recent drug trials evaluating HDL cholesterol have been unsuccessful in reducing cardiovascular events, and therefore it is unclear if raising HDL cholesterol with regular egg intake is beneficial.
Other essential changes with egg intake include potentially favorable effects on lipoprotein particle profiles and enhancing HDL function. Overall, the increased HDL cholesterol commonly observed with dietary cholesterol intake also coincides with improvements in other markers of HDL function. Let’s now look deeper into the egg cholesterol controversy.
Phospholipids and Cardiovascular Disease
Another component of eggs, called phospholipids, may also lower the risk for heart disease by reducing serum lipids. Specifically, phospholipids modify the response to dietary cholesterol found in eggs. Continue reading to learn how eggs and dietary cholesterol affect serum cholesterol concentrations and other lipoprotein measures, including lipoprotein particle profiles and HDL metabolism.
Cardiovascular Disease and Atherosclerosis
Cardiovascular disease (CVD) impacts more than 17 million deaths annually around the globe, accounting for nearly half of all deaths from non-communicable diseases. Atherosclerosis is a chronic inflammatory disease of the arteries caused by plaque accumulation and is the primary cause of cardiovascular disease.
Key risk factors associated with atherosclerotic CVD events include the total concentration of cholesterol found in the blood and the cholesterol found in individual lipoprotein subclasses. LDL and HDL cholesterol have opposite effects on CVD risk, consistent with the role of LDL particles in promoting and HDL particles in protecting against atherosclerosis.
Chosesterol, Eggs, and Heart Disease
The Framingham Heart Study was the first to show a relationship between blood cholesterol levels and heart disease. Investigators hypothesized that dietary cholesterol influences heart disease by affecting blood lipids. This hypothesis was consistent with evidence from animal studies, such as the 1913 rabbit study showing that large doses of cholesterol in the diet significantly induced atherosclerotic plaques in the aortas.
However, even with these early studies, it was clear that there were differences among species in plaque build-up with large doses of dietary cholesterol. For example, rats were significantly more resistant than rabbits and guinea pigs.
U.S. adults’ average dietary cholesterol intake is typically between 200–350 mg/day, depending on gender and age group. Eggs are a major source of dietary cholesterol in the typical Western diet; one large egg yolk contains approximately 200 mg of cholesterol. The consumption of eggs and egg products contributes about a quarter of the daily cholesterol intake in the U.S. in both children and adults. Saturated fat strongly increases serum cholesterol, and eggs, which are relatively low in saturated fat, only contribute about 2.5% of total saturated fatty acid intake among U.S. adults.
Cholesterol and CVD Risk
Early studies showed a link between dietary cholesterol and the risk for CVD. However, they failed to account for many confounding variables, including dietary and lifestyle factors. More recent studies typically don’t find a strong association between egg intake and CVD risk in the general population, upending previous notions of the egg cholesterol controversy. However, there does appear to be a more consistent relationship between egg intake and CVD in people with diabetes.
Interestingly, this risk in diabetics may be related to the phosphatidylcholine content of eggs rather than cholesterol since dietary cholesterol is poorly absorbed in obese and insulin-resistant individuals. Phosphatidylcholine generates TMAO (trimethylamine N-oxide) in the gut, a metabolite that promotes atherosclerosis. However, to further complicate matters, eating 2–3 eggs a day did not increase TMAO concentrations in healthy young adults. Furthermore, TMAO levels in healthy men were significantly lower after eating egg intake than after eating fish, a direct source of dietary TMAO.
Research Finds No Smoking Gun
The impact of egg phospholipids on CVD and TMAO concentrations in humans is likely complex and requires further research. Berger et al. conducted a systematic review and meta-analysis of 17 research studies examining the relationship between dietary cholesterol and CVD. They did not find dietary cholesterol intake to be significantly associated with either heart disease, ischemic stroke, or hemorrhagic stroke.
In the past, the medical community believed that consuming cholesterol-rich foods would lead to increased cholesterol in the blood, promoting heart disease over several decades. However, this has proven to be an oversimplification as we now know that the serum cholesterol response to dietary cholesterol is much more nuanced.
Where Does Cholesterol Come From?
Most of the cholesterol in the human body is produced internally, with only 25% coming from the foods we eat. The average 150 lb adult generates about 850 mg of cholesterol a day. If they consumed 400 mg of dietary cholesterol in a day, absorbing 60% of it, that’s only 22% of cholesterol from the diet (240 mg from the diet out of a total of 1090 mg).
As early as the 1960s, it was clear that dietary cholesterol was not a significant factor in regulating serum cholesterol. Dr. Ancel Keys, a pioneer in studying diet-CVD relationships, stated in 1965, “For the purpose of controlling the serum level, dietary cholesterol should not be completely ignored but attention to this factor alone accomplishes little”.
Most people have a minimal change in serum cholesterol in response to dietary cholesterol due to feedback regulation of whole-body cholesterol stores. For example, an 88-year-old man compulsively ate 25 eggs a day yet still had normal serum cholesterol (~200 mg/dL). He absorbed only a small fraction (18%) of the dietary cholesterol he consumed and had twice the mean rate of bile acid synthesis as compared to control study volunteers.
Eggs and Cholesterol
Similarly, numerous clinical trials have demonstrated differences in serum cholesterol responses when consuming an additional 500–650 mg of dietary cholesterol from eggs for at least four weeks. Most participants had no or only a mild increase in serum cholesterol when they consumed a large amount of dietary cholesterol. These individuals are classified as hypo-responders because they can compensate by reducing cholesterol biosynthesis, absorption, and excretion. In comparison, a small proportion of the group were hyper-responders, having a more significant increase in serum cholesterol.
Berger et al. also examined the blood lipid responses to dietary cholesterol across 19 clinical trials. Cholesterol intake, mainly from eggs, significantly increased serum LDL and HDL. However, these increases only marginally increased the LDL/HDL ratio. The LDL/HDL ratio estimates how much arterial plaque develops via LDL and how much is being removed by HDL. An LDL/HDL ratio <2.5 is optimal. This ratio plays an important role in resolving the egg cholesterol controversy.
It’s All About The Cholesterol Ratio
In children and adults with normal cholesterol levels, 2–4 eggs per day significantly increased both LDL and HDL while having no change in the LDL/HDL ratio. Healthy men classified as hyper-responders showed a significant increase in the LDL/HDL ratio with three eggs per day for 30 days. However, the mean ratio (2.33) was still within the optimal range of <2.5. Investigators observed similar responses in adults with high blood cholesterol levels. Consuming two eggs a day resulted in elevated HDL without a change in LDL. At the same time, there was an increase in LDL and HDL in people with high cholesterol and triglycerides.
Size and Concentration Matter
Although LDL and HDL indicate CVD risk, lipoprotein particle characteristics like particle diameter and concentration may also influence disease risk. For example, having a greater plasma concentration of particles of the large HDL subclass is strongly associated with a lower risk for CVD. In contrast, the concentration of smaller HDL particles is less protective. Conversely, the concentration of large LDL particles is only weakly associated with CVD risk, while there is a strong positive link to CVD with small LDL concentrations.
The impact of smaller LDL particles on CVD is related to their greater susceptibility to oxidation compared with larger LDL particles. Oxidized LDL significantly drives atherosclerosis development and CVD. In addition, clinical studies have examined the effects of cholesterol from eggs during weight maintenance and weight loss conditions. During weight maintenance, increases in LDL size and large LDL concentration are seen, sometimes at the expense of small LDL.
Researchers see similar responses in LDL particle profiles during weight loss. Furthermore, several studies examining the effects of added cholesterol from eggs under weight maintenance and weight loss conditions have shown increases in the size of HDL and the concentration of large HDL particles.
The Egg Cholesterol Controversy Resolved
Recent clinical studies evaluated the effects of consuming cholesterol from eggs on LDL and HDL. Chronic daily egg intake does increase LDL to a certain extent in individuals classified as hyper-responders. However, LDL responses are typically minimal when consuming eggs during weight loss conditions. Egg intake shifts LDL particles to the less detrimental, large LDL subclass and does not appear to affect the levels of oxidized LDL.
Furthermore, egg intake typically increases HDL and the concentration of large HDL. And the effect of egg intake on the LDL/HDL ratio is insignificant during weight maintenance and weight loss conditions. In conclusion, research confirms that dietary cholesterol from eggs does not increase the risk of cardiovascular disease.
Next Steps
Other important ways to manage cholesterol include getting regular exercise, avoiding sweets, and managing stress. Acupuncture aids the process by improving metabolism, strengthening the digestive tract, reducing stress, and increasing Qi and blood circulation throughout the body.
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Reference
Blesso, C.N.; Fernandez, M.L. Dietary Cholesterol, Serum Lipids, and Heart Disease: Are Eggs Working for or Against You? Nutrients 2018, 10, 426. https://doi.org/10.3390/nu10040426
Focus keyphrase: The Egg Cholesterol Controversy
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