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Egg Consumption and Cardiovascular Risk: Limitations of Observational Data

  • Michael R. Bubb
    Correspondence
    Requests for reprints should be addressed to Michael R. Bubb, MD, 1600 SW Archer Rd, UF Division of Rheumatology and Clinical Immunology, Department of Medicine, University of Florida, Gainesville, FL, 32610-0221.
    Affiliations
    The Research Service, Malcom Randall Department of Veterans Affairs Medical Center, Gainesville, Fla

    Department of Medicine, University of Florida College of Medicine, Gainesville, Fla
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      To the Editor:
      Krittanawong et al
      • Krittanawong C
      • Narasimhan B
      • Wang Z
      • et al.
      Association between egg consumption and risk of cardiovascular outcomes: a systematic review and meta-analysis.
      address the association of egg consumption and cardiovascular disease with a meta-analysis of observational data. Overall, diverse results reported in the primary data require cautious interpretation, and there are methodological concerns. Observational data for egg consumption are enormously complicated depending on the prevailing dietary recommendations for cholesterol at the time of the study because these recommendations have had a major influence on egg consumption for the past 40 years. For those subjects at high risk of cardiovascular disease who were told to avoid cholesterol and eggs, it is likely that low egg consumption will be associated with higher than otherwise expected rates of cardiovascular disease due to reverse causality. High compliance rates in certain populations, such as studies of physicians and the highly educated
      • Djoussé L
      • Gaziano JM
      Egg consumption in relation to cardiovascular disease and mortality: the Physicians' Health Study.
      ,
      • Zazpe I
      • Beunza J
      • Bes-Rastrollo M
      • et al.
      Egg consumption and risk of cardiovascular disease in the SUN project.
      will exaggerate this effect. In contrast, low compliance rates in a subset of subjects may be associated with unhealthy behaviors that confound these data, which could explain, for example, the association of egg consumption with adverse cardiovascular outcomes in diabetes.
      • Qureshi AI
      • Suri FK
      • Ahmed S
      • et al.
      Regular egg consumption does not increase the risk of stroke and cardiovascular diseases.
      Lack of understanding of pathophysiology makes adjustment for confounders problematic. If the pathway of causality is through impaired glucose tolerance or improved blood pressure control, as previously postulated,
      • Qureshi AI
      • Suri FK
      • Ahmed S
      • et al.
      Regular egg consumption does not increase the risk of stroke and cardiovascular diseases.
      then adjustment for these causal factors will incorrectly force the conclusion to favor the null hypothesis.
      A fundamental principle of the scientific method is independent sampling. Here, the outcomes for individual subjects are counted multiple times. First, some of the studies use the same database. For example, Djoussé et al
      • Djoussé L
      • Gaziano JM
      Egg consumption in relation to cardiovascular disease and mortality: the Physicians' Health Study.
      is a study of the Physicians’ Health Study (PHS I) and Djoussé et al
      • Djoussé L
      • Gaziano JM
      • Buring JE
      • Lee IM
      Egg consumption and risk of type 2 diabetes in men and women.
      is a study of Physicians’ Health Study and the Women's Health Study; both papers extract data from the same pool of 22,071 subjects. Secondly, the question addressed in Figure 2 is the hazard ratio of cardiovascular disease associated with consumption of more than 1 egg per day. The forest plot includes separate data from Qin et al
      • Qin C
      • Lv J
      • Guo Y
      • et al.
      Associations of egg consumption with cardiovascular disease in a cohort study of 0.5 million Chinese adults.
      for coronary artery disease, ischemic stroke, and hemorrhagic stroke. These data are actually for cardiovascular disease in the same subjects.

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        Association between egg consumption and risk of cardiovascular outcomes: a systematic review and meta-analysis.
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