Certain plasma lipoproteins are associated with an increased risk of cardiovascular disease. In particular, elevated levels of very low density lipoprotein (VLDL) and the small dense fraction of low density lipoprotein (LDL) are biomarkers used to attribute risk of cardiovascular disease. The LDL and VLDL particles carry cholesterol and triglycerides from the liver to the peripheral tissues, respectively. Because elevated levels of these lipoproteins are risk factors for cardiovascular disease, it had been assumed that they are the causative factors. From this assumption has flourished the theory that dietary saturated fat and cholesterol are the direct cause of elevated levels of plasma VLDL and LDL, respectively. The result of this theory, the diet heart hypothesis, has been an aggressive attempt to dissuade the public from consumption of foods that contain high levels of saturated fat and cholesterol. One such food that has been targeted is eggs.
Advice to limit egg consumption dates back decades, and can be traced to a number of papers published using data from the Seven Countries Study. This longitudinal study followed cohorts of men in seven countries and researchers used the data to investigate the associations between cardiovascular disease and dietary lipids. A researcher called Ancel Keys used this data to publish a number of papers that reported associations between dietary lipids and cardiovascular disease. However, this research has since been questioned regarding bias, due to the fact that the author excluded data that did not fit the positive correlation. Subsequently, researchers have included this missing data and shown that no association exists. At the time however, dietary guidelines were altered to reflect the conclusions from this study, which included reductions in foods containing saturated fat and cholesterol, particularly eggs.
More recently, epidemiological evidence suggests that free living populations eating eggs do not have elevated levels of plasma cholesterol. Further, similar studies investigating associations between diet and cardiovascular disease show no correlation for eggs. Data generally shows that dietary lipids do not affect plasma levels of LDL or VLDL, although some studies have reported a positive correlation between dietary cholesterol and blood lipids. Many such positive studies have been criticised for poor methodological protocols because they have not made adjustments for intakes dietary fibre. Those with the highest dietary cholesterol intakes tend to have the lowest fibre intakes. Therefore studies showing an association between dietary cholesterol and plasma cholesterol, which have not controlled for dietary fibre, should be treated with caution. The role played by dietary fibre in influencing plasma lipoprotein levels is increasingly being recognised by the nutritional science.
The evidence that eggs are a contributory cause of cardiovascular disease has been extensively reviewd1. Generally, the evidence from epidemiology and clinical trials suggests that consumption of eggs does not increase the risk of cardiovascular disease. In fact more recent data suggests overall diet quality is more indicative of the subsequent risk of cardiovascular disease. A high quality diet containing whole grains, fresh fruit, essential fatty acids in the correct ratios, vegetables and nutrient dense foods may be protective of cardiovascular disease. Eggs are a good source of protein and should be included in such a diet, without fear of detrimental health effects. In fact the most recent advice from the American Heart Association no longer recommends avoidance of eggs. It is therefore becoming clear that the diet heart hypothesis was a research dead end that yielded little benefit, and that the dangers of eggs have been overstated.
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