Qin C, Lv J, Guo Y, et al; The China Kadoorie Biobank Collaborative Group. Associations of egg consumption with cardiovascular disease in a cohort study of 0.5 million Chinese adults [published online ahead of print May 21, 2018]. Heart.
Prospective cohort study
From 2004 to 2008, 2,512,891 men and women aged 30 to 79 years were recruited from 10 geographically diverse survey sites (5 city, 5 rural) across China to complete surveys about egg consumption. Participants were excluded if they had a history of cancer (n=2,577), heart disease (n=15,472), stroke (n=8,844), or diabetes (n=30,300). Individuals without BMI data were also excluded. Overall, 461,213 participants were eligible for analysis. The mean age was 50.7 years, 41% of the participants were men, and about 42% lived in urban areas.
Study Parameters Assessed
Frequency of egg consumption was assessed through questionnaires at local clinics over a 12-month period. The following were the possible categories of response to the question of consumption frequency: daily; 4 to 6 days per week; 1 to 3 days per week; 1 to 3 days per month; rarely; never. Accuracy and reproducibility of baseline data was confirmed in 926 participants by repeated interviews over the first year of the study.
Approximately 40 covariates were included in the data gathering. Participants’ diets were divided into 3 patterns: new affluence, traditional southern, and traditional northern patterns. Participants were followed to determine morbidity and mortality rates. Morbidity and mortality data were obtained from local disease and death registries.
Primary outcomes were incidence and mortality from cardiovascular disease (CVD), ischemic heart disease (IHD), hemorrhagic stroke, ischemic stroke, myocardial ischemic events (MCEs), including IHD, and nonfatal myocardial infarction (MI).
During a median follow-up of 8.9 years (total person-years=3.9 million) a total of 83,977 CVD cases, 9,985 CVD deaths, and 5,103 MCEs were documented. Multivariate analysis showed significant inverse associations between egg consumption and CVD. Compared with no consumption, daily egg consumption was associated with lower risk of CVD (hazard ratio [HR]: 0.89; 95% confidence interval [CI]: 0.87-0.92).
For each egg eaten per week there was an 8% reduction in risk for hemorrhagic stroke, CVD, and death from hemorrhagic stroke. Daily egg consumption vs no consumption was associated with a significant 18% reduction in death from CVD and 28% reduction in risk of stroke death.
Cardiovascular disease is the most common cause of death worldwide. Stroke is the most common cause of premature death in China, while the most common cause of premature death in Western countries is ischemic heart disease. Lifestyle influences that affect risk of CVD, even if the change is small, have large implications when viewed on a global scale.
This new study by Qin et al marks a tidal shift in how we view the impact of eggs on cardiovascular health. At one point in the past we assumed eating eggs would raise risk of CVD, then later we decided that eggs have no impact. Recently the data suggest consuming eggs may actually lower the risk.
This new information could shift the debate from “should patients at risk of heart disease avoid eggs?” to “should they eat eggs on purpose?” but it probably will not. The idea that eating eggs leads to heart disease has persisted for decades despite ample evidence to the contrary.1 If you doubt this, look at the breakfast menu in any American restaurant; you will find they offer egg-white omelets. People obviously do not order these omelets because they like the taste. They order them over whole eggs believing egg whites are a healthier choice because the cholesterol is found in the yolk.
The idea that eggs are bad was hatched in the 1950s, when high serum cholesterol was identified as a risk factor for heart disease.2 Egg yolks are high in cholesterol, so it made sense that eating eggs might increase serum cholesterol and increase CVD risk.
Few if any studies have associated egg consumption with CVD risk. Actually, this current Qin study reports an association but it is an inverse association: Greater egg consumption was associated with lower risk. This isn’t the first report of an inverse association. The 2003 Life Span Study in Japan reported that daily egg consumption was associated with a 30% decreased risk of death from stroke compared to rare egg consumption.3
Could the benefits of eggs on heart disease have been missed in earlier studies simply because researchers were looking for the opposite effect?
In a 1999 prospective study published in JAMA, Hu and colleagues at the Harvard School of Public Health analyzed data from 2 prospective cohort studies, the Health Professionals Follow-up Study and the Nurses' Health Study, creating a cohort of 37,851 men and 80,082 women. During follow-up they observed little change in risk with increasing egg consumption for healthy men or women. There was a trend of decreasing risk for CHD with an egg per day compared to less than an egg per week. However, in diabetic men there was a significant increase in CHD for those who consumed an egg per day compared to less than 1 egg per week. In diabetic men, risk doubled and in diabetic women risk increased by 49%.4 This association of increased risk in diabetics is seen in other studies.
In 2001, Weggemans, Zock, and Katan reported the results of a meta-analysis of data on the effect of egg consumption on the ratio between total cholesterol and high-density lipoprotein (HDL), which at the time what was thought to be the key predictor of CVD risk. They identified 17 studies and determined that adding 100 mg dietary cholesterol to the diet each day, which is equivalent to about half an egg, increased the total cholesterol to HDL cholesterol ratio by 0.02 units. Based on these estimates, eating 6 eggs per week might increase the ratio of total cholesterol to HDL by 0.2 units.5
In a short-term study, blood lipids increased in participants who ate 3 eggs per day, but the ratios between “good and bad cholesterol” did not change. The authors concluded that cholesterol from eggs regulates endogenous synthesis of cholesterol in a way that maintains the low-density lipoprotein (LDL)-cholesterol to HDL-cholesterol ratio.6
A 2013 study by Rong et al analyzed data from 8 articles and 17 reports, for a total of 3,081,269 person-years and 5,847 incident cases for CHD, and 4,148,095 person-years and 7,579 incident cases for stroke. The authors saw no evidence of an association between egg consumption and risk of CHD or stroke. However, diabetics were at increased risk in this review as well. When the authors compared the highest vs the lowest egg consumption, they found the relative risk of CHD was 1.54 (95% CI: 1.14-2.09; P=0.01). They concluded that, overall, their data suggests that egg consumption confers a significant benefit. Moreover, they determined that people who ate more eggs had a 25% lower risk of developing hemorrhagic stroke (95% CI: 0.57-0.99; P=0.04).7
However, a 2015 Cochrane Review on dietary cholesterol and CVD risk concluded that the 40 studies selected for review, “lacked the methodologic rigor to draw any conclusions regarding the effects of dietary cholesterol on CVD risk.”8
An exception to this general “no effect” pattern comes from a 2015 paper by Larsson et al. They reported that while daily egg consumption was not associated with heart failure in women nor with risk of MI in men or women, risk of heart failure was nevertheless higher in men who ate eggs once a day or more often, but not less often.9
A 2016 meta-analysis by Alexander et al, however, failed to draw any associations to suggest risk or benefit. The authors analyzed 7 prospective studies and found that, compared with consuming 2 eggs per week, up to 1 egg per day had no significant association with CHD (HR: 0.97; 95% CI: 0.88-1.07), concluding that there was no clear association between egg consumption and either increased or decreased risk of CHD.10
An April 2018 report by Xu et al, another meta-analysis, showed that 7+ eggs per week was not associated with all-cause mortality (HR: 1.09; 95% CI: 0.997-1.200) or IHD (HR: 0.97; 95% CI: 0.90-1.05), but there was a small significant reduction in risk of stroke (HR: 0.91; 95% CI: 0.85-0.98).11
Again, several studies do report a positive association between egg consumption and type 2 diabetes. In their 2013 meta-analysis, Shin et al echoed others’ findings that egg consumption is not associated with the risk of CVD and cardiac mortality in the general population. However, they note that egg consumption may be associated with increased incidence of type 2 diabetes among the general population and CVD comorbidity among patients with diabetes.12
This association with diabetes was also suggested in the present (Qin et al) study. A 2018 study among Korean adults reported no association between eggs and CVD in healthy individuals but the study did find a link for participants with diabetes; those with diabetes who ate slightly more than 4 eggs per week had almost triple the incidence of CVD compared to those who ate few eggs.13 The Qin study was designed to assess only healthy individuals (ie, those without a personal history of heart disease or diabetes) so the findings may not be relevant to a broader, high-risk population.
The positive association between eggs and diabetes is curious. Higher egg consumption is associated with higher blood sugar levels,14 and it may be associated with an increased incidence of type 2 diabetes among the general population and CVD comorbidity among diabetic patients.11 The odd thing is that higher egg consumption is at the same time inversely associated with metabolic syndrome.15 Perhaps these findings are being confounded by the simple fact that people with diabetes often choose to eat eggs because they make a convenient noncarbohydrate meal.
The current findings from Qin et al stand out for having both statistically significant and clinically relevant findings. This was a large, carefully done study so the findings are hard to ignore. Could the benefits of eggs on heart disease have been missed in earlier studies simply because researchers were looking for the opposite effect? The results of the present study suggest that increasing egg consumption in the general population might reduce risk of dying from heart disease.
The caveat of course is that this data comes from a Chinese population and it is not clear whether eggs will have the same impact on a Western population following a Western-style diet. The potential difference is more likely related to diet than genetics. The obvious difference is that the Chinese diet has lower protein. Comparing Chinese to American diets, among men aged 30 to 39 years, Americans consume about 20% more protein than the Chinese.16 Could the extra protein provided by eggs provide a benefit for those who follow a traditional Chinese diet but not for Americans who follow a Western diet?
These researchers were diligent about analyzing multiple possible confounders that might explain this shift toward benefit, and failed to identify any. Still, many possibilities were left unexamined. Could some change in chicken rearing, genetics of the chickens, or feed account for this change? Patients frequently ask me about the relative benefits of the various choices in eggs now available in grocery stores. There are free range, vegan feed, and other variations in egg production, each claiming to have health advantages. Could one of these production shifts—for example, enriching chicken feed to produce eggs that are high in omega-3 fatty acids—make a difference?
At a minimum, though, these studies clearly suggest that it is time for most of us to abandon egg-white omelets or cholesterol-lowered egg white mixes, with the possible exception of those with type 2 diabetes.