Where do Eggs Fit in a Heart-healthy Diet?

Role of Eggs in the American Diet
From American Journal of Lifestyle Medicine
Joanne Curran Celentano PhD

Abstract
Eggs make up a relatively small contribution to the energy consumption of the average American, in part because of the perception that the cholesterol content makes them a for-bidden food for a heart-healthy diet. The relationship between egg cholesterol, blood cholesterol, and cardiovascular disease risk is complex and not clearly understood. In addition, eggs provide many valuable and bioavailable nutrients. Thus, the place of eggs in meal planning should be reconsidered while keeping in mind the diet and lifestyle recommendations of the American Heart Association.

Introduction
As far as single-component foods are concerned, there are few more nutrient dense than eggs. Yet this food category accounts for a very limited amount of energy consumption (1.3%) by the average American.[1] The perception of eggs as a forbidden food can be traced back to the 1970s recommendation by the American Heart Association to reduce the consumption of eggs and other sources of dietary cholesterol to lower the risk of cardiovascular disease (CVD).[2]

Since that time, compelling research suggests that the relationship between eggs and heart disease risk is not so simple, especially when examining free-living populations. There are responders and nonresponders to dietary cholesterol and, in some cases, paradoxical responses including lower cholesterol in egg consumers and/or elevated cholesterol with no change in the CVD risk ratio. The question of where eggs fit in a heart-healthy diet gets more interesting when considering the potential benefits of eggs in the diet, especially for infants and elderly persons. Eggs are a good or excellent source of many essential nutrients, are relatively low cost, and serve as an effective vehicle for functional ingredients in the diet.

Does Eating Eggs Increase Serum Cholesterol and Heart Disease Risk?
The relationship of dietary cholesterol to serum cholesterol has been demonstrated most clearly in experimental feeding studies. Such studies have shown that the consumption of an additional egg per day will lead to a modest increase (1%–3%) in serum cholesterol.[2] However, population studies examining the association between egg consumption and serum cholesterol concentration have not been so convincing.[3] Several large epidemiological studies have examined the association of egg consumption and serum cholesterol. The Framingham Heart Study[4] examined the serum cholesterol in high versus low egg consumption and found no significant difference in either men or women. The association between self-reported dietary intake of eggs and serum cholesterol was examined in a population of 12 000 men in the Multiple Risk Factor Intervention Trial. Paradoxically, those consuming more eggs had lower serum cholesterol than those men consuming fewer eggs.[5] Similarly, in the Third National Health and Nutrition Examination Survey (NHANES III), the diets of 20 000 participants were evaluated, and participants consuming less than 1 egg per week had a higher average serum cholesterol than those consuming more than 4 eggs per week.[6]

Recently, the association between egg consumption and the risk of CVD and mortality was investigated using data from the Physician's Health Study.[7] In this prospective cohort study, egg consumption was assessed using an abbreviated food-frequency questionnaire. In an average follow-up of 20 years, there was no association between egg consumption and myocardial infarct or stroke. However, egg consumption was positively related to all-cause mortality in a dose-dependent way, and this relationship was stronger among diabetic subjects. The researchers concluded from these data that infrequent egg consumption did not influence the risk of CVD in male physicians. The relationship of egg consumption with all-cause mortality required further investigation. A possible explanation for this finding was offered in a letter to the editor.[8] It was suggested that a nonadherer bias might be responsible for the association because there is no biological basis for an increased association of noncardio-vascular causes of death with high egg consumption. It was suggested that such bias is a limitation of observational studies. For example, high egg consumers tend to smoke more, and egg consumption is often associated with other high-fat foods such as bacon and butter. The "guilt by association" has to be parsed out in such studies to ensure that eggs are not inadvertently associated with risk.

Heart failure (HF) represents a subtype of CVD and may be underrepresented in studies investigating associations of diet and CVD, especially in younger cohorts with lower incidence of this subtype.[9] Little dietary-related research specific to HF is available; however, as the population ages, HF is increasingly adding to the health care burden and affecting the quality of life.[10] A recent study examined the association between egg consumption and risk of HF in a prospective cohort study of 21 275 male physicians.[11] Egg consumption up to 6 times per week was not associated with incident HF; however, consumption equal to or greater than 7 times per week was associated with increased risk of HF in male physicians. The Atherosclerosis Risk in Communities study evaluated incident HF and diet in a longitudinal cohort study including 14 153 African American and white men and women aged 45 to 64 years. During the 13-year follow-up, there were 1140 cases of HF. Whole-grain intake was associated with lower HF risk, whereas intake of eggs and high-fat dairy was associated with a greater risk of incident HF.[9]

Responders versus Nonresponders

Despite extensive research, the link between egg consumption and risk for coronary heart disease (CHD) is not clearly established. The relationship is likely influenced by variability in individual responses to dietary cholesterol. This variability was examined in relation-ship to the blood cholesterol response to egg consumption.[12,13] For some individuals, consuming eggs did cause a rise in blood cholesterol. These individuals are considered hyperresponders to a cholesterol challenge. However, 70% of the population experience little to no change in blood cholesterol following consumption of dietary cholesterol (hyporesponders). In addition, the egg cholesterol increased both circulating low-density lipoprotein (LDL-C) and high-density lipoprotein (HDL-C) in those individuals who experience changes in blood cholesterol following egg consumption. Moreover, eggs have been shown to promote the formation of large LDL particles, shifting individuals to a less atherogenic lipid profile. Such individual variations in the response to dietary cholesterol can be attributed to a variety of factors, including ethnicity, body mass index, and hormone status.[14] Genetics certainly has a role. Recently, it was reported that polymorphism in the ATP-binding cassette G (ABCG5) is thought to influence the response to dietary cholesterol.[15] In this study, 40 men and 51 postmenopausal women were randomly assigned to consume eggs (equivalent to 640 mg/d additional dietary cholesterol) or placebo for 30 days in a crossover design. It was found that genotype influenced the response to cholesterol, with one specific group experiencing a greater increase in LDL-C in response to the cholesterol challenge. Although this study was small, the results suggest that genetic makeup influences an individual response to cholesterol and may explain some of the controversy surrounding the questions and recommendations about dietary cholesterol in relationship to blood cholesterol and disease risk.

The data from the NHANES III (1988–1994) were examined to assess the nutritional contribution of eggs in the American diet and to estimate the strength of the association between egg consumption and serum cholesterol. Nutrient intake from 24-hour recall and egg intake from food-frequency questionnaires were used to group 27 378 participants into egg consumers and nonconsumers. Egg consumers had considerably greater nutrient density contributing vitamin A, E, folate, and B12. The results also indicated that egg consumption was negatively correlated with serum cholesterol. In this population, those who reported eating 4 or more eggs per week had significantly lower mean serum cholesterol than those who reported eating less than or equal to 1 egg per week. In this cross-sectional and population-based study, egg consumption made important nutritional contributions to the American diet and was not associated with high serum cholesterol concentrations.[6]

Thus, it is important to look at eggs as more than a cholesterol-delivery sys-tem. Eggs are an inexpensive and low-calorie source of high-quality protein and other nutrients, including folate, riboflavin, selenium, choline, and vitamins B12 and A, D, and K.[15] In addition, the lipid matrix of the egg yolk enhances the bio-availability of valuable carotenoid pigments, including lutein and zeaxanthin.[16] Thus, the positive contribution of eggs to a healthy diet should be considered in the risk–benefit analysis.[17]

This may be particularly true for elderly persons. The incidence of age-related diseases will continue as our population ages. By the year 2020, the number of people older than 60 years is expected to top 1 billion.[18] The burden of treating chronic disease is significant both in dollars spent and lost productivity. The need to identify risk factors for disease must be evaluated along with diet and lifestyle factors that promote healthy aging.

For elderly persons, it has been suggested that the widely accepted risk factors for CHD may not be applicable.[19] Whereas elevated total cholesterol and LDL-C values are considered predictive of CHD risk in the middle years, this may not be relevant for the elderly population. In this population, a low-fat diet prescription may actually lead to a diet pattern that increases CHD risk. A higher carbohy-drate, especially simple carbohydrate, diet is associated with elevated triglycerides, low HDL-C, and the production of small, dense LDL particles. In cases in which fat/ cholesterol restriction is practiced over energy restriction, a high-carbohydrate diet may have the net effect of promoting insulin resistance.[14] To promote energy restriction instead, eggs may provide a low-calorie and nutrient-dense option for meal planning. In addition, the protein quality of eggs reflects its balanced amino acid profile, and the high-quality egg protein may be helpful in avoiding the loss of muscle mass associated with aging.[20]

While not typically recommended, carbohydrate-restricted diets (CRD) have been popular for rapid weight loss and are associated with lower glucose and insulin levels and improve dyslipidemia. Studies have shown that the improved lipid profile (elevated HDL-C and lower triglycerides) are associated with a higher intake of dietary cholesterol.[21] Eggs have been shown to modulate the inflammatory response to CRD. Daily egg consumption along with CRD in overweight men was associated with decrease in C-reactive protein and increased adiponectin compared with CRD without eggs.[20]

Egg Carotenoids and Eye Health
The leading cause of irreversible blind-ness in the United States is age-related macular degeneration (AMD).[22] Although the causes of AMD are complex and multifaceted, a lifetime exposure to oxidative damage is clearly implicated. Although both light and oxygen are essential for normal vision, the accumulated by-products from oxidative metabolism in the retina over time can cause damage. Lutein and zeaxanthin (L/Z) accumulate in the macular region of the retina and are collectively referred to as macular pigment (MP). Because of its antioxidant and light-filtering properties, the MP may protect the retina and reduce the risk of developing AMD. Studies have shown that individuals who consume foods rich in L/Z have a lower risk for AMD,[23] higher blood levels of L/Z,[24,25] and higher MP density.[26–28]

Several studies have shown that eggs provide a highly bioavailable source of L/Z.[16,24,29] While the average content of L/Z in the yolk is ~200 to 300 μg, the lipid matrix allows for efficient uptake of these pigments. As little as 1 egg per day or 6 eggs per week have been shown to increase serum L/Z and MP, respectively.[16,29] The concentration of L/Z in the egg yolk can be easily modified by alterations in the animal diet, and the difference in the L/Z content of grocery store eggs varies widely depending on the type and care of the animal.[16] Many local egg producers allow their animals to graze in appropriate weather, and their egg yolks may range from pale yellow to deep orange depending on the L/Z avail-able in the diet. Consumers can identify lutein as associated with eye health, reflecting the media's attention in magazine articles and advertisements. For example, a recent article titled "Eat for Your Eyes"[30] discussed 5 food strategies to help you "see more clearly." The inclusion of eggs in the diet was one of the recommendations, citing a study in which consuming 2 eggs per day was associated with increased circulating L/Z with no change in blood LDL-C. The recommendation was to "go ahead and enjoy eggs regularly unless advised otherwise by your doctor." The article did include the caveat to stay within the American Heart Association guidelines, as noted below.

Current Recommendations
The American Heart Association[31,32] Diet and Lifestyle Recommendations for Cardiovascular Disease Risk Reduction are listed in Table 1. The current recommendations reflect a shift toward looking at dietary patterns and overall diet quality rather than focusing on specific nutrients and a diet based more on inclusion of a wide range of foods and less on avoidance of particular foods or ingredients. Whereas there was previously a stated recommendation to limit eggs to 4 yolks per week, this food-specific recommendation is not in the current guidelines. The American Heart Association recommendation is to limit dietary cholesterol to <300 mg/d. The average egg yolk contains on average 213 mg of cholesterol.

If eggs are judged on their nutritional content, convenience, and cost, the positive contributions of eggs may outweigh the potential risk associated with the cholesterol. This may be particularly true for healthy elderly individuals whose cholesterol risk and nutritional needs differ from earlier years. For individuals with diabetes or major risk factors for CHD, the recommendation to limit dietary cholesterol to <200 mg/d[33] allows for less frequent use of unmodified egg products. Along with the numerous modified egg products on the market today (eg, Egg Beaters), there will likely be additional egg and egg products with higher lutein and lower cholesterol in the future.[34,35]