A daily breakfast treat I relished as a child was half- boiled eggs. Eggs have become part and parcel of our diets.
For egg-lovers, a common question is how many eggs one can eat a week without increasing the risk of heart disease?
Eggs are an excellent source of high- quality protein with low saturated fat and no trans fatty acids. In addition , it is a natural source of fat-soluble vitamins (A, D, E, and K), folate, riboflavin, vitamin B12, choline, and selenium.
The main concern about consuming eggs is their high cholesterol content. Elevated cholesterol levels, especially "bad" cholesterol or LDL cholesterol, have been shown to be a major risk factor for heart attack and stroke, whereas elevated "good" cholesterol or HDL cholesterol level has been shown to be protective.
A large egg, on the average, contains about 200mg of cholesterol in the egg yolk and this accounts for more than 50 per cent of your recommended daily requirement of 300mg of cholesterol for healthy individuals and 100 per cent of the daily cholesterol requirement for those with heart disease.
Egg and heart disease
Data from various studies comparing egg consumption with heart disease can be confusing. In the prospective Harvard Medical School Physicians' Health Study on more than 20,000 US male physicians published in 2008 in the American Journal of Nutrition, the data suggested that egg consumption of up to six per week had no major effect on the risk of heart disease, stroke, and death, and that consumption of seven-plus per week is associated with a modest but significant increased risk of total mortality in US male physicians.
Another publication on the Physicians' Health Study in Circulation journal in 2008 suggested that infrequent egg consumption was not associated with the risk of heart failure but daily egg consumption was related to an increased risk of heart failure among US male physicians.
In a 13-year follow-up study on a younger group of more than 10,000 healthy men and women by Mann in Heart journal in 1997, egg consumption was associated with increased death, with a higher death rate being associated with higher egg consumption.
Data from the Framingham study published in 1999 in the Journal of the American Medical Association showed no association between egg consumption and heart disease among more than 900 men and women after 16 years of follow up.
However, the average weekly egg consumption in the Framingham study was relatively low with six per week for men and four per week for women. Another two large studies involving about 38,000 men and 80,000 women from the Health Professionals Follow-Up Study and the Nurses' Health Study showed that overall there was no increase in the risk of heart disease or stroke in healthy men or women when comparing those eating more than seven eggs per week with those taking less than one egg per week.
Response to cholesterol
Response to cholesterol
The difference in findings between the large studies may be due to the significant variation in response by individuals to dietary cholesterol. While about 75 per cent of the population are normal responders or hypo-responders who show little or no increase in blood cholesterol following a high intake of dietary cholesterol, about 25 per cent are hyper-responders who show elevation of serum LDL and HDL cholesterol with dietary cholesterol.
Even with hyper-responders, the LDL and HDL cholesterol elevation by dietary cholesterol was modest compared to the impact of saturated and trans fatty acids on LDL cholesterol elevation and the cholesterol elevation did not alter the LDL/HDL ratio. Dietary cholesterol intake of 100mg/dL may potentially result in 1.9mg/dL increase in LDL and 0.4mg/dL increase in HDL.
Effect on cholesterol
Egg consumption has been shown to change the LDL pattern to a favourable profile containing benign large LDL cholesterol particles rather than disease causing small dense LDL cholesterol particles which are linked to narrowing of the arteries.
This may be due to the presence of strong antioxidants, such as lutein and zeaxanthin, in the eggs which may prevent oxidation of the cholesterol particles which will otherwise increase the likelihood of artery narrowing.
Eggs and diabetes
While the data on egg consumption in healthy individuals is conflicting, the effect of egg consumption on diabetics is more consistent. In the Physicians' Health Study, when compared to those who took less than one egg per week, for those who took seven-plus eggs per week, the risk of death increased 22 per cent for non-diabetics and two-fold for diabetics.
Similarly, the Health Professionals' Follow-up Study and Nurses' Health Study showed a two-fold and 49 per cent increased risk of heart disease for type 2 diabetics respectively, when comparing those eating more than seven eggs per week with those taking less than one egg per week.
Data from the Physicians' Health Study and the Women's Health Study (more than 36,000 women) published in Diabetes Care journal in 2009 suggests that high levels of egg consumption daily are associated with an increased risk of type 2 diabetes in men and women.
The current data suggests that if you have heart disease or diabetes mellitus, consuming one egg or more per day can be detrimental to your health.
For healthy individuals with no major risk factors for heart disease or stroke or for those whose cholesterol levels are well controlled with cholesterol lowering drugs, consuming one or more eggs per day as part of a low-fat diet does not appear to increase the risk of heart disease significantly.
While this may sound like good news for egg-lovers, before rushing off to feast on eggs, it is good to be aware that research has also shown that egg substitutes, which contain 99 per cent egg whites and multiple vitamins, may be more beneficial than eggs.
A study by Njike published in Nutrition Journal in 2010 showed that while egg consumption was found to be non-detrimental to the function of arteries and serum cholesterol in adults with high cholesterol levels, consumption of egg substitutes was beneficial both in terms of resulting in a decrease in LDL cholesterol levels and in significantly improved arterial function. You can still have your "egg" and eat it!
Dr Lim is medical director at the Singapore Heart, Stroke & Cancer Centre. He is also editor-in-chief, Heart Asia (a journal of the British Medical Journal Publishing Group); chairman, scientific advisory board, Asia Pacific Heart Association; and honorary professor and senior medical adviser, Peking University Heart Centre
This series is brought to you by the Heart, Stroke and Cancer Centre. It is produced on alternate Saturdays.