Chinese researchers recently reported that soy consumption was associated with a reduced risk of mortality from acute myocardial infarction (heart attack).1 This finding comes from the China Kadoorie Biobank study, which may be the largest prospective epidemiologic study to examine health outcomes related to soy intake.
The China Kadoorie Biobank study involved 487,034 individuals free of cardiovascular disease (CVD) and 22,923 individuals with a history of CVD at study baseline.1 Among those free of CVD at baseline, during the median follow-up period of up of 8.2 years, there were 12,582 cardiovascular deaths (including 3,764 coronary heart disease deaths and 5,916 stroke deaths). Among those with a history of CVD, during the median follow-up period of up of 7.8 years, there were 2,860 total CVD cardiovascular deaths (including 1,123 coronary heart disease deaths and 1,473 stroke deaths).
Before going over the results, it is instructive to consider the amount of soy consumed within this Chinese cohort.1 Participants came from 10 geographically defined regions of China. As can be seen from the table, most people (>85%) did not consume soy daily. The highest intake category was ≥4x/week, which suggests that few people consumed soy more than 1x/day. Although soyfoods are often identified with China, many areas within China consume relatively little soy.2 An exception is Shanghai; Shanghainese women3 and men4 consume about 9g/day and 9-13g/day soy protein, respectively.
|Baseline status of participants||Soy intake frequency (n/%)|
|Free of CVD||53,795/11%||142,436/29.2%||243,674/50%||47,129/9.7%|
|History of CVD||2,044/8.9%||4,928/21.5%||12,843/56%||3,108/13.6%|
The results showed that among those individuals without CVD at baseline, soy intake was unrelated to overall CVD mortality.1 However, for cause-specific mortality, soy product consumption was inversely associated with mortality from acute myocardial infarction (hazard ratio, 0.75; 95% confidence interval, 0.61, 0.92). Among people with a history of CVD, higher soy product consumption was not associated with cardiovascular mortality.
Observational studies are not designed to establish causal relationships, although they tend to form much of the basis for dietary guidelines. Because intake is so low, except for studies involving a large percentage of vegetarians, such as the Adventist Health Study 2 and the Oxford arm of the European Prospective Investigation into Cancer, Western observational studies are unable to provide meaningful insight about the health effects of soy.5 Therefore, most useful data about soy comes from Asian observational studies. However, as shown in the table, even in China, soy intake can be low.
In the China Kadoorie Biobank study, soy intake was associated with a lower risk of dying from a heart attack.1 However, it is difficult to understand the mechanism by which soy could exert this beneficial effect. The soy protein intake threshold for cholesterol reduction established by the U.S. FDA is 25g/day. In this Chinese cohort, very few individuals would have come close to consuming this amount of soy protein.1 Isoflavones may also reduce risk of heart attack in postmenopausal women by improving endothelial function, but most studies showing such benefits intervene with at least 50 mg of isoflavones daily.6,7 As in the case of soy protein, few individuals in the China Kadoorie Biobank study would have consumed anything close to this amount.
While it is theoretically possible that the consumption of lower amounts of isoflavones or soy protein over many years may be needed to exert benefits compared to the amounts used in shorter-term intervention studies, this contention is highly speculative. Therefore, the lack of effect of soy on overall CVD in people with or without a history of CVD should not be viewed as evidence of lack of efficacy because intake was so low in this Chinese cohort.1 However, the associated lower risk of soy against mortality from acute myocardial infarction in people without a history of CVD that was observed should also be viewed skeptically because no mechanism has been proposed to explain how the low amount of soy consumed could exert benefits.
- Wang X, Yu C, Lv J, et al. Consumption of soy products and cardiovascular mortality in people with and without cardiovascular disease: a prospective cohort study of 0.5 million individuals. Eur J Nutr. 2021.
- Liu Z, Li W, Sun J, et al. Intake of soy foods and soy isoflavones by rural adult women in China. Asia Pacific journal of clinical nutrition. 2004;13:204-9.
- Yang G, Shu XO, Jin F, et al. Longitudinal study of soy food intake and blood pressure among middle-aged and elderly Chinese women. Am J Clin Nutr. 2005;81:1012-7.
- Lee SA, Wen W, Xiang YB, et al. Assessment of dietary isoflavone intake among middle-aged Chinese men. J Nutr. 2007;137:1011-6.
- Messina M. Western soy intake is too low to produce health effects. Am J Clin Nutr. 2004;80:528-9.
- Abshirini M, Omidian M, Kord-Varkaneh H. Effect of soy protein containing isoflavones on endothelial and vascular function in postmenopausal women: a systematic review and meta-analysis of randomized controlled trials. Menopause. 2020;27:1425-33.
- Li SH, Liu XX, Bai YY, et al. Effect of oral isoflavone supplementation on vascular endothelial function in postmenopausal women: a meta-analysis of randomized placebo-controlled trials. Am J Clin Nutr. 2010;91:480-6.