In the present study, dietary choline and betaine intake did not associate with the risk of CVD events and mortality in the general population. After sensitivity analysis, no significant association between per one SD increased in choline and betaine and cardiovascular events and death was found among healthy adults and high-risk patients of CVD. Besides, there was no relationship between food sources of choline and betaine and outcomes of interest.
This study was not the first to report that there is no significant link between dietary choline and betaine and CVD outcomes in the general population. Data of 16165 women free from CVD who participated in the PROSPECT-EPIC cohort, revealed no significant relationship between dietary choline and betaine and risk of total CVD, CHD, and CAD over eight years of follow-up (20). Results of the Atherosclerosis Risk in Communities (ARIC) study, in 14430 adults with no CHD at baseline, indicated that 14-year CHD incident in the highest quartile of choline (HR: 1.22, 95% CI: 0.91 to 1.64) and choline with betaine (HR: 1.14, 95% CI: 0.85 to 1.53) is comparable with the lowest quartile of intake (16). In the Jackson Heart Study, choline consumption is inversely associated with stroke incidents (β=-0.33). But after post hoc analysis, the incidence of stroke across quartiles of dietary choline was not significant (14). In another related study conducted by Nagata et al. (21), there was no significant relationship between choline and CVD mortality among Japanese males and females.
On contrary, Mazidi et al. (22) in a prospective study (6.5-year of follow-up) reported that subject in the quartile of choline had 23% (95% CI: 1.09 to 1.38) higher risk of all cause-mortality, 33% (95% CI: 1.19 to 1.48) CVD mortality and 30% (95% CI: 1.02 to 1.66) stroke mortality than those in the first quartile. Millar et al. (14) also showed betaine intake increased the risk of CHD by 89% in the third quartile of intake compared with the lowest quartile. While in the Takayama study, dietary intake of betaine was inversely linked to the CHD mortality among Japanese males (21).
Although all these studies had prospective designs; but their conflicting results may be related to different sample sizes, various follow-up periods, and disparity in subjects’ geographical location and race or residual confounders. Besides, inconsistency in findings may be attributed to diversity in food sources of choline and betaine among various populations and distinct bioavailability of choline and betaine from separate foods. In the present study, the same as Bidulescu et al. (16) who reported no link between choline and betaine and CVD events, meat and eggs were major food sources of choline, and grains and bakery products were for betaine. In Nagata et al. (21) also eggs were the most contributor of choline. But the seafood was the main food supplying betaine that may explain an inverse association between betaine and mortality from CHD. Seafood is the major source of omega-3 polyunsaturated fatty acid (PUFA), which has been shown to protect against CVD in many studies (23–25). A recent meta-analysis reported an inverse association between fish consumption and risk of CHD mortality (RR: 0.85, 95% CI: 0.77 to 0.94), results of this study revealed each 20 g of fish consumed per day reducing CHD mortality by 4% (RR: 0.96, 95% CI: 0.95 to 0.97) (26).
In addition, we evaluated the association between each food source of choline with the risk of CVD events and mortality, with null findings. As motioned above, in our study, meat and eggs were major food sources of choline. In the past studies, the link between food sources of choline and outcomes of interest did not investigate. Mei et al. (27) in a cross-sectional study on 3973 adults indicated fish, red meat and eggs were the most dietary factors that significantly increased TMAO production. However, they noted that the association between red meat and TMAO is due to TMA production from carnitine. Their findings were in agreement with the results of a clinical trial that showed carnitine content of red meat increased production of TMAO but not choline (28). These surveys supported a null association between meat and eggs with CVD in the current study.
Among dietary contributors betaine, we observed a positive link between fruits and vegetables and oils and fats intake and death from CVD. It is unclear why the betaine content of fruits and vegetables is responsible for the increased CVD mortality, but in our study fruits and vegetables and oils and fats are insignificant food sources of betaine.
Furthermore, differences in results of studies may be due to variations in gut microbiota composition across populations. TMAO is formed by the gut microbiota and is positively associated with CVD. Results of a new study indicated a significant relationship between nine gut bacterial species and TMAO (27). Therefore, differences in the gut microbiota of subjects may have an impact on TMAO development. Several factors such as dietary habits, lifestyle, and health status have an impact on gut microbiota composition. In the present study, we stratified subjects into two groups: healthy subjects and patients at high-risk of CVD then re-analyzed data, but the results of the study remained unchanged. Previous studies did not evaluate the relationship between dietary choline and betaine and CVD outcomes among high-risk patients. Hence further investigations to assess whether gut microbiota plays a role in the association between dietary choline and CVD events are warranted.
There are several strengths and limitations. The follow-up period was extended. Outcomes were not self-reported and collected based on medical records. FFQ applied in our study was also validated to assess dietary intake of foods that are the source of choline and betaine. In addition, we assessed the association between each food source of choline and betaine with outcomes of interest. However, the main limitation was that dietary intake of choline and betaine were collected at baseline and they might be changed over the follow-up. Furthermore, residual confounders cannot be ruled out even after adjusting for possible confounders in the studies.