Eating tofu and foods that contain higher amounts of isoflavones is associated with a moderately lower risk of heart disease, especially for younger women and postmenopausal women who are not taking hormones, according to the results of an observational study in more than 200,000 people, carried out by researchers at Harvard Medical School and Brigham and Women’s Hospital. The study, reported in Circulation, found that consuming isoflavone-rich tofu more than once a week was linked with an 18% lower risk of heart disease, compared with a 12% lower risk for those who ate tofu less than once a month. There was no similar link between soy consumption and heart disease risk.
While the beneficial findings for tofu were more pronounced among young women and post-menopausal women who weren’t on hormone therapy (HT), the researchers acknowledged that the study doesn’t necessarily point to tofu as being a complete wonder-food. “Despite these findings, I don’t think tofu is by any means a magic bullet,” said lead study author Qi Sun, MD, ScD, a researcher at Harvard’s T.H. Chan School of Public Health in Boston. “Overall diet quality is still critical to consider, and tofu can be a very healthy component.” Sun and colleagues reported their findings in Circulation, in a paper titled, “Isoflavone Intake and the Risk of Coronary Heart Disease in U.S. Men and Women: Results From 3 Prospective Cohort Studies.”
The role of soy products and isoflavones in heart health is a “controversial topic,” the authors wrote. FDA approved a health claim for soy products being protective against coronary heart disease (CHD) back in 1999, but the agency is now reconsidering the specific health claim relating to soy intake and heart health, on the basis of mixed results from subsequent clinical trials and epidemiological studies. “In addition, updated nutritional guidelines by the American Heart Association concluded that the evidence for the cardiovascular health offered by isoflavones was minimal and that the health benefits of soy products may be attributed to its higher contents of polyunsaturated fats, fiber, vitamins and minerals, and lower contents of saturated fat,” Sun and colleagues noted. Even so, they pointed out, “multiple cardioprotective benefits have been attributed to isovlanones, including reducing the low-density lipoprotein cholesterol, inhibiting proinflammatory cytokines, cell adhesions proteins, and platelet aggregation, inducing nitric oxide production, and improving vascular reactivity.” And some prospective studies, though not all, have also reported inverse associations between isoflavones and cardiovascular risk factors and CHD incidence.
For their study described in Circulation, the researchers analyzed the health data of more than 74,000 women from the Nurses’ Health Study (NHS) from 1984 to 2012; approximately 94,000 women in the NHSII study between 1991 and 2013; and more than 42,000 men who participated in the Health Professionals Follow-Up Study from 1986 to 2012. All participants were free of cardiovascular disease and cancer at the beginning of each study. Dietary data was updated using patient surveys, conducted every two to four years. Data on heart disease was collected from medical records and other documents, while heart disease fatalities were identified from death certificates.
A total of 8,359 cases of heart disease were identified during 4,826,122 person-years of follow-up. “In these three prospective cohorts of U.S. men and women with more than two decades of follow-up, a higher isoflavone intake was associated with a moderately lower risk of CHD,” the authors reported. “Consumption of tofu was also significantly, inversely associated with the risk of CHD. In women, the inverse association of tofu was more pronounced in younger women before menopause and postmenopausal women receiving HT, whereas a null association was observed among postmenopausal women who used HT.”
The findings of significant, favorable associations between tofu intake and CHD risk were consistent with results for total isoflavones. In addition to isoflavones, tofu also contains several bioactive components such as polyunsaturated fats, fiber, and minerals, which the authors pointed out may act synergistically with isoflavones through various pathways to modulate CHD risk, although they added, “the inverse association for tofu persisted after adjustment for these nutrients in the present study.”
Sun emphasized that the team’s research results should be interpreted with caution, because while the findings do indicate a relationship between isoflavones and reduced CHD risk, they do not demonstrate causality. Many other factors can influence the development of heart disease, including physical exercise, family history, and a person’s lifestyle habits. “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. Although we have controlled for these factors, caution is recommended when interpreting these results,” said Sun.
Tofu, which is soybean curd, and whole soybeans such as edamame are rich sources of isoflavones. Chickpeas, fava beans, pistachios, peanuts, and other fruits and nuts are also high in isoflavones. Sun noted that populations that traditionally consume isoflavone-rich diets including tofu, such as in China and Japan, have lower heart disease risk compared to populations that follow a largely meat-rich and vegetable-poor diet. Further research will be needed to verify the potential benefits of tofu and isoflavones in relation to heart disease.
“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,” Sun stated. “If their diet is packed with unhealthy foods, such as red meat, sugary beverages, and refined carbohydrates, they should switch to healthier alternatives. Tofu and other isoflavone-rich, plant-based foods are excellent protein sources and alternatives to animal proteins.”
Interestingly, there was no significant association found between soymilk consumption and heart health, “potentially because of its lower consumption levels and the shorter duration of follow-up in the present analysis,” the team noted. It’s also possible that differences in the nutritional profiles of tofu and soymilk could also lead to differential associations with CHD risk. Soymilk does tend to be highly processed and is often sweetened with sugar, Sun noted. “Unlike tofu, soy milk may contain added sugar, emulsifiers, and other constituents, which may mask the protective effects of isoflavones on CHD risk to a certain extent,” the scientists commented.