High serum cholesterol levels are a major risk factor for cardiovascular disease, but unlike other major risk factors such as age, race, and gender, they can be modified to some extent. Only about one-fourth of low-density lipoproteins in the body are associated with diet and the remainder is produced by the liver or other cells in the body 1. The two major strategies for managing and/or reducing cholesterol levels are a) pharmacological therapy, and b) therapeutic lifestyle changes. Pharmacologic therapies can include HMG CoA reductase inhibitors (commonly known as statins), selective cholesterol absorption inhibitors, renin inhibitors, fibrates, and niacin. These medications can also be used in combination. Therapeutic lifestyle changes include smoking cessation, increasing physical activity, and modifying one's diet, including keeping daily cholesterol intake less than 200 mg 2.

Dietary changes, while at first glance unequivocally positive, are not without possible detriments. Eliminating a source of cholesterol from the diet may create an opportunity for another, possibly more deleterious food. Also, foods containing cholesterol also contain other components, some of which may be beneficial. Balancing a reduction in cholesterol intake with complete nutritional needs is difficult and should consider maximizing other potential benefits.

One food that has been scrutinized in particular in terms of its nutritional value given its cholesterol content is the egg. Eggs are a good source of high quality protein as well as carotenoids such as lutein and zeaxanthin, and choline. Consumers of eggs are more likely to meet their recommended daily allowances of dietary folate and vitamins A, E, and B₁₂ than non-egg consumers 3. There is a large body of literature evaluating the impact of both these nutrients of eggs and their dietary cholesterol content on health conditions, including ophthalmic conditions, coronary heart disease, and neural tube defects, for example. There is also literature on the costs of each condition associated with egg consumption. The goal of the present study is to synthesize what is known about the risks and benefits of eggs and the associated costs. The basic framework is the findings of the Health Professionals' Follow-up Study (HPFS) and the Nurses' Health Study (NHS), which evaluated the health effects of egg consumption 4. Using available data on the contribution of eggs to various conditions and existing estimates of the costs of each condition, this study estimates the economic impact of consumption of one additional egg daily.

The analysis involved a multi-stage process. First, it was necessary to identify the conditions associated with egg consumption. Second, estimates of the contribution of egg consumption to each health risk or benefit were needed. Third, the economic impact of each of these conditions was required. Finally, the cost estimates were adjusted to reflect the contribution of eggs to each condition.

A literature search in PubMed was conducted to identify the conditions associated with the nutrients in eggs for which there was quantitative support. Based on the literature review, the proportion of each condition that would be influenced by the addition of one egg daily was estimated. A second PubMed search was conducted to identify publications estimating the costs of the conditions of interest. Abstracts were reviewed to identify publications with original data that assessed the cost of illness; reviews and comparative cost studies were excluded. The estimates were reviewed to determine the type of costs presented (e.g., direct medical vs. lost productivity), the population considered (e.g., Medicare-eligible vs. younger patients), and the year in which costs were estimated (in order to inflate to a common year).

For each condition, a "base case" was developed and a sensitivity analysis was conducted, assuming the minimum and maximum values as identified in the literature. If there were no minimum or maximum values to use for sensitivity analysis, the base case estimate was decreased and increased by 25% in order to arrive at extreme values. Similarly, if minimum and maximum values were available but not a specific base case, the midpoint of these extreme values was used as the base case input parameter.

The literature search identified a wide range of estimates for the costs of the conditions of interest, with various methods used to develop these estimates. A listing of the studies used to estimate costs and attribution in each area is provided in Table 1. Additional file 1 provides details on the methods for attribution.

The results presented in this study must be interpreted carefully. The two sources of data on which they are based, economic studies and data on the contribution of cholesterol to various conditions, are both subject to variation. However, each of the estimates provided here includes not only a base case value, but also a high and low value, based on the literature where possible, so that the full range of possibilities can be considered.

A number of limitations should be considered in terms of the cost estimates used in this analysis. While the cost estimates identified and used in this analysis were published, that does not imply that they are comprehensive. For example, no single economic study on any of the conditions of interest captured all the costs one might attribute. In most cases, direct medical costs are likely to be the primary source of disease-related costs, but lost productivity cannot be overlooked. In the case of AMD, a series of studies captured a variety of costs: direct medical 14, caregiving 19 and use of assistive devices and services 18. The differences across studies make it difficult to compare literature-based values directly. Also, in the case of CHD, the annual cost estimates provided by the American Heart Association 11 address a wide variety of costs. The estimate for NTD, while well-constructed, is based on hospital costs 30. The conversion from costs to charges reduces the precision of the estimate and the fact that the estimate is limited to hospitalizations is another limitation. It is also important to recognize that these factors mean that the estimates presented here are necessarily wide and still contain uncertainty. Future studies could replicate the format of this analysis, eventually narrowing the estimates. In the absence of such studies, this analysis represents a reasonable approximation of the economic impact of adding one egg daily to the diet.

This analysis also did not consider the cost of eggs. Compared with other protein sources, they are reasonably priced; we assumed zero additional cost for egg purchase. As with any other food, proper cooking is essential to limiting food-borne illness, particularly salmonella. The cooking process may increase the digestibility of eggs as well as reduce the possibility of illness. This model does not assume any additional cost associated with cooking eggs. Again, since other proteins that could be consumed in place of eggs would likely require cooking, this omission is appropriate.

It should be noted that the estimate of the fraction of CHD risk that can be attributed to eggs was based on an apportionment analysis that included a number of modifiable risk factors. One of the major considerations for applying the risk apportionment approach is the selection of appropriate risk factors and estimates of relative risks. The apportionment model included only multivariate-adjusted relative risk estimates derived from the same population, mainly from the HPFS and NHS. Both were chosen because they are large studies with long-follow-up periods and carefully collected information, and because the relative risk estimates were multivariate adjusted. It is possible that different estimates of the share of eggs to total CHD risk may have been derived if other assumptions were made about the potential interaction between the various risk factors, if other factors were included, or if risk estimates from other cohorts, e.g., population with different education levels or without the health background the participants in the HPFS and NHS had. Further the apportionment model used only considered the cholesterol effects of an egg, without considering other potentially beneficial components such as those described in this paper. Another important limitation to this analysis is the fact that considering any single food to the exclusion of others is inherently problematic. Kritchevsky and Kritchevsky 8 evaluated the studies estimating the relationship between egg consumption and coronary heart disease and found that very few had adjusted for other dietary intake (particularly total calories, fiber, and fat) and advise caution in interpreting these types of studies. Finally, it should be noted, that the published studies did not find any association between CHD risk and egg consumption, and there no statistically significant association between dietary cholesterol and CHD was found in the NHS and HPFS. The share of CHD attributed to eggs was modeled based on the cholesterol content of eggs and modeled association between dietary cholesterol, serum cholesterol and CHD 3132333435363738394041. This approach is likely to have resulted in an overestimate of the CHD risk estimate associated with egg consumption.

The implications of these findings for public health are two-fold. First, they echo those of several clinical studies that suggest that moderate egg consumption may not be harmful, and are indeed beneficial, among non-diabetic and non-hypercholesterolemic individuals. Well meaning public health campaigns designed to avoid eggs as a mean to lower serum LDL cholesterol and reducing CHD risk may be out of date and deserving of an update. If a cost-effectiveness study were conducted to evaluate public health campaigns limiting egg consumption as a means to reduce CHD risk, it may very well be discovered that these campaigns are not cost-effective from a societal perspective, particularly if the benefits of egg consumption are considered. Second, the human diet is a complex mixture of nutrients, antinutrients, functional components of varying degree of biological activities. Public health intervention that focuses on a single dietary constituent may lead to unintended consequences of removing other beneficial constituents and the net effect may not be a desirable public health outcome. The development of other examples would be a useful and interesting test of this hypothesis.

These findings suggest that public health campaigns promoting limiting egg consumption as a means to reduce CHD risk are not cost-effective from a societal perspective when other benefits are considered. Public health interventions that focus on a single dietary constituent and foods that are high in that constituent may lead to unintended and undesirable consequences. As newer data become available, the model presented here should be updated. In particular, as there are conflicting data about CHD, including data suggesting that there may not be an increased risk, these findings may underestimate savings.

The authors have no competing interests. All authors are employed at Exponent, Inc. Neither Exponent nor the authors have any financial ties or interests to the organization that provided funds to develop this manuscript, The Egg Nutrition Center, nor is the authors' employment dependent on the preparation or success of this project.

NT and LB were involved in the initial conception of the project; JS helped refine the design. JS conducted the initial literature review and analysis; LB reviewed the calculations. JS drafted the manuscript; LB and NT provided critical review and comment. All authors read and approved the final manuscript.