A meta-analysis is a statistical technique whereby results from several studies are combined to derive a single estimate on the association or effect of a particular treatment on an outcome. The advantage to this approach is that by combining multiple studies, the estimate is statistically stronger than the results of any one given study. This technique is frequently used in nutrition research and can be applied to results generated from observational or clinical intervention studies.
Over the past 15 years, several prospective cohort studies (a type of observational study) have evaluated associations between dietary patterns and risk of chronic disease. Egg consumption, in particular, has been of interest given the cholesterol content of eggs (186 mg of cholesterol per large egg). Shin et al. recently conducted a meta-analysis on results from these prospective cohort studies, specifically focusing on associations between egg consumption and cardiovascular disease (CVD), mortality, and type 2 diabetes mellitus (T2DM)1. A total of 16 studies, the majority of which focused on CVD, were included in the analysis and each study tracked subjects over 6 to 20 years of follow up.
Results showed that compared to those consuming less than one egg per week, individuals in the highest intake category (≥1 egg/day) did not have a higher risk of CVD, stroke, or overall mortality. This is consistent with findings from controlled intervention studies showing that egg consumption does not adversely affect blood cholesterol levels or other biomarkers for CVD risk, such as endothelial function2-4.
Results were more surprising with respect to T2DM, particularly in light of the body of scientific evidence on eggs and health. Individuals in the highest egg consumption category were more likely to develop T2DM compared to those consuming eggs infrequently. Further, individuals with T2DM were more likely to develop CVD than those who never ate eggs or consumed them less than once per week. While there were several limitations of the study that may have influenced these results, one possible explanation for such findings may relate to how eggs are consumed in an overall diet. Research presented at the 2013 Experimental Biology meeting showed that egg intake was positively correlated with T2DM risk factors (i.e., waist circumference and body mass index) only when consumed as part of dietary patterns low in vegetables, legumes, and grains5. Other dietary patterns containing eggs were not associated with these risk factors. This calls into question the results of prior observational studies looking at egg intake and disease risk and suggests that future studies should consider not only frequency of egg consumption, but the dietary context under which they are consumed.
As is so frequently the case in nutrition, additional studies are warranted to better understand the relationships between egg consumption and chronic disease risk.
1Shin JY, Xun P, Nakamura Y, He K. Egg consumption in relation to risk of cardiovascular disease and diabetes: a systematic review and meta-analysis. Am J Clin Nutr. 2013;98:146-59.
2Goodrow EF, Wilson TA, Houde SC, Vishwanathan R, Scollin PA, Handelman G, Nicolosi RJ. Consumption of one egg per day increases serum lutein and zeaxanthin concentrations in older adults without altering serum lipid and lipoprotein cholesterol concentrations. J Nutr. 2006;136:2519-24.
3Wenzel AJ, Gerweck C, Barbato D, Nicolosi RJ, Handelman GJ, Curran-Celentano J. A 12-wk egg intervention increases serum zeaxanthin and macular pigment optical density in women. J Nutr. 2006;136):2568-73.
4Katz DL, Evans MA, Nawaz H, Njike VY, Chan W, Comerford BP, Hoxley ML. Egg consumption and endothelial function: a randomized controlled crossover trial. Int J Cardiol. 2005;99:65-70.
5Nicklas TA, O’Neil CE, Fulgoni VL. Relationship between egg consumption patterns and nutrient intake, diet quality, weight measures, and cardiovascular risk factors (CVRF): 2001-2008 NHANES.Experimental Biology, 2013, Boston, MA.