Regarding associations of egg consumption with cardiovascular disease in a cohort study of 0.5 million Chinese adults

To the Editor Thank you for your comments. Our recently published research found that a moderate level of egg consumption was associated with cardiovascular disease (CVD) among the non-diabetic Chinese population, after adjusting for a series of established and potential confounders, including socioeconomic factors (such as education level and household income), lifestyle behaviours, other dietary factors and medical history as detailed as possible. Although we only adjusted for dietary patterns with three groups, further adjustment for the other 11 food groups (including fresh vegetable, fresh fruit and dairy product consumption) in the sensitivity analyses showed the robustness of the results. Such beneficial effects of egg consumption may be due to the substitution of red meat, which is one of the alternative sources of dietary protein. Yet, we observed that the frequencies of meat and egg consumption positively correlated in our study population (crude Spearman coefficient: 0.13; energy-adjusted Spearman coefficient: 0.03). Furthermore, results of the substitutional analyses were hardly changed. So the healthy effect of egg on CVD is unlikely due to the substitution of red meat. It is worth reminding that our results need to be carefully interpreted and extrapolated in the context of unique dietary habits and other lifestyle behaviours among the Chinese population. In our study, participants who had the lowest risk of CVD consumed an average of 0.76 egg daily, lower than the highest level in the Western and Japanese population. But this average level was close to the amount (40 – 50 g per day) recommended by the Chinese Dietary Guideline. One egg weighs about 50 g and contains 292.5 mg according to the guidelines. In our study, daily consumers had about 222.3 mg cholesterol per day, which is below the 300 mg daily limit recommended for the general population in a number of other guidelines. The relationship between eggs and diabetes warrants further exploration in the CKB study. Two recent meta-analyses found positive association between egg consumption and diabetes in the US population. 7 In the present paper, we did not find positive associations between egg consumption and CVD among patients with diabetes in the sensitivity analyses. We supposed that one possible mechanism might be that egg consumption elevated the level of high-density lipoprotein cholesterol (HDL-C). A recent meta-analysis reported that egg consumption was associated with increased HDL-C, as well as total cholesterol and low-density lipoprotein cholesterol (LDL-C). Previous studies have observed that egg consumption is associated with a greater LDL-C diameter which has less harm to vascular health, and greater HDL-C particles, which are more protective against CVD. Furthermore, this meta-analysis found that the LDL-C/HDL-C ratio, a major predictor of coronary heart disease, remained unchanged. Therefore, the favourable effect of HDL-C may counteract the detrimental effect of LDL-C. We lack information about lipid profiles and are unable to confirm this. Other components such as carotenoids and protein may likewise explain the beneficial effects of egg consumption. In addition, about 14% phosphatidylcholine from egg yolk can be transformed into trimethylamine N-oxide (TMAO) through gut microbiota and liver. Consuming ≥2 egg yolks per day results in an increased level of TMAO in healthy individuals. A moderate level of egg consumption (0.76 egg per day) might not deteriorate vascular health through TMAO. We currently have not tested the plasma TMAO level and will look into it in the future if possible. We acknowledge that lacking information about cooking methods and other food groups such as nuts is a limitation of our study. Compared with the popularity of fried eggs in the Western countries, boiled eggs were more common in China, which may lead to the distinct results. Cooking methods may affect the nutrients in eggs, for instance, boiled eggs have higher bioaccessibility of carotenoids than fried eggs. In conclusion, consuming a moderate level of eggs is associated with lower risk of CVD among the non-diabetic Chinese population.