Although soy products are a major source of plant proteins, particularly for vegans and vegetarians, health benefit of soy products on coronary heart disease (CHD) and associated risk factors is an association that remains debatable due to limited epidemiological evidence. However, an analysis of three U.S. cohorts of men and women, conducted by Harvard’s T.H. Chan School of Public Health in Boston, has revealed promising results.
According to a study published in the journal Circulation, consuming tofu and other foods with high amounts of isoflavones (plant-based phytoestrogens) might possibly lower the risk of heart disease, primarily in women before their menopause and in postmenopausal women who are not taking any hormones. Substantial evidence in this regard could play a significant role in formulating food-based dietary guidelines.
Link Between Soy Consumption and Cardiovascular Health
In 1991, the U.S. Food and Drug Administration (FDA) sanctioned a health claim which stated that soy products were protective against coronary heart disease (CHD). Tofu is soybean curd and whole soybeans (e.g., edamame) are rich sources of isoflavones. Other foods with high amounts of isoflavones include fava beans, chickpeas, peanuts, pistachios, and various fruits and nuts.
However, due to mixed evidence from epidemiological studies and clinical trials, the FDA is now reconsidering its prerogative. Moreover, the latest nutritional guidelines given by the American Heart Association (AHA) determined that the health benefits of isoflavone-based foods on cardiovascular health were minimal, and the overall benefits of soy products may instead be attributed to their high content of polyunsaturated fats, vitamins, fiber and minerals, along with their low content of saturated fat. Despite the contradictory evidence available, isoflavones are believed to have various cardio-protective functions, such as inhibiting pro-inflammatory cytokines, lowering the amount of low-density lipoprotein (LDL) or ‘bad’ cholesterol, inducing nitric oxide production, promoting cell adhesion proteins and platelet aggregation, and enhancing vascular reactivity.
Dr. Satjit Bhusri, a cardiologist at Lenox Hill Hospital in New York City, says:
Although the impact of specific isoflavone-rich food is still not clear, what is known is that increase in plant-based protein and lean, low carbohydrate meals reduce overall cardiovascular risk. This simple understanding and mindfulness of what to eat can make a major impact on overall cardiovascular health and outcomes.
Many significant observational studies have been performed to evaluate the effect of isoflavone and soy intake on the incidence of coronary heart disease (CHD). Some of these examinations have reported an inverse association, meaning that an increase in isoflavone and soy consumption decreases the incidence of CHD and related risk factors. However, due to limitations such as small sample size, relatively short follow-up duration, diversity of food sources considered, and most significantly the lack of repeated evaluations of diet during the follow-up, the findings of such studies may be inconsistent. Moreover, due to heterogeneity in constituents present in individual isoflavone-rich sources, it is important to examine specific foods containing isoflavones.
Another crucial aspect involving isoflavone research is the possibility of their interaction with estrogens (female sex hormones). Isoflavones are similar in structure to estradiol (estrogen steroid hormone and major female sex hormone), and could potentially express estrogenic effects via estrogen-receptor binding. This opens a new debate on the plausible association between isoflavones and soy foods and menopausal status and hormone therapy (HT).
Methodology and Outcomes
Three cohorts were investigated for the study. The NHS (Nurses’ Health Study) originated in 1976 and comprised of 121,700 female registered nurses from 11 states, aged 30 to 55 years. The NHSII (Nurses’ Health Study II) was a parallel cohort of 116,671 relatively younger female registered nurses, aged 25 to 42 years, and was initiated in 1989. Lastly, the HPFS (Health Professionals Follow-Up Study) cohort was established in 1986 and consisted of 51,529 US male health professionals between the ages of 40 and 75 from all 50 states. Information on lifestyle and medical history was updated every two years via a self-administered questionnaire.
From all three cohorts, individuals free of cancer and cardiovascular disease were included in the research, as well as those with complete information on dietary data and isoflavone intake. After fulfilling the inclusion criteria, participants for the study comprised of 74,241 women from the NHS, 94,233 women from the NHSII, and 42,226 men from the HPFS. Dietary data was updated every two to four years via a validated food frequency questionnaire. Records of nonfatal myocardial infarction episodes and deaths attributed to coronary heart disease were obtained by reviewing medical archives, death certificates and other relevant documents.
After an evaluation and complete follow-up of all the cohorts, a total of 8,359 incidents of coronary heart disease (CHD) were documented. Increased isoflavone intake was associated with a moderately lower incidence of CHD. Also, tofu consumption, but not soymilk, was associated with a lower risk of CHD. As for its estrogenic effects, an inverse relationship was observed among young women before menopause, and menopausal women who had not undergone hormone replacement therapy (HRT).
Lead author, Dr. Qi Sun of the Harvard’s T.H. Chan School of Public Health in Boston, says:
Other human trials and animal studies of isoflavones, tofu and cardiovascular risk markers have also indicated positive effects, so people with an elevated risk of developing heart disease should evaluate their diets.
Clinical Implications and Suggestions
After adjusting for other factors contributing to heart disease, researchers concluded that having tofu more than once a week lowered the risk of heart disease by 18%, as compared to people who ate tofu less than once a month (12% reduction in risk of heart disease). Soy products may be integrated into healthy plant-based dietary regime to provide essential plant proteins and help prevent coronary heart disease.
In a news release, Dr. Sun explains what inferences should be made from the findings:
Despite these findings, I don’t think tofu is by any means a magic bullet. For example, younger women who are more physically active and get more exercise tend to follow healthier, plant-based diets that may include more isoflavone-rich foods like tofu. Overall, diet quality is still critical to consider and tofu can be a very healthy component. If the diet is packed with unhealthy foods, such as red meat, sugary beverages and refined carbohydrates, (people) should switch to healthier alternatives.
Dr. Benjamin Hirsch, a health expert who directs Preventive Cardiology at Northwell Health’s Sandra Atlas Bass Heart Hospital in Manhasset, New York, notes:
Isoflavones are thought to support heart health by their association with improvements in vascular function, reduction in total cholesterol, and reduction in inflammation. What is interesting is that premenopausal women derived the greatest benefits, which may suggest that it is the estrogen and hormonal component of soy-based food that leads to better outcomes in women.