According to a new study, the consumption of tofu reduces the risk of developing coronary heart disease. The isoflavones contained in tofu seem to mediate part of the protective effect.
Vegetarian and vegan diets are becoming increasingly popular. The consumption of foods based on the soya bean has increased rapidly. These include tofu as a meat substitute and soy milk as an alternative to conventional milk. Soy products are considered to be beneficial to health, possibly because of their content of isoflavones, a secondary plant substance similar to the female sex hormone estrogen. In fact, some studies have linked the consumption of tofu and similars to a lower risk of cardiovascular disease. But most of the trials had too few people and were not optimally designed to prove a positive effect. A research team led by Le Ma from Harvard University in Boston (USA) looked at three long-term trials in the USA with an observation period of 22 to 28 years.
The researchers were able to draw on data from more than 210,000 test persons aged between 25 and 75 years. All participants had no cardiovascular disease or cancer at the beginning of the study. The consumption of soy products was systematically recorded every two to four years using nutritional questionnaires. To ensure that the data were accurate, the researchers measured the isoflavone content in the urine (as a biological marker for soy consumption) of 47 test subjects in a random sample and found a significant correlation between the information provided by the test subjects and the values measured.
In a second step, hospital databases and death certificates were used to determine which study participants developed coronary heart disease (CHD) or died during the course of the study. They then used a survival analysis (time-dependent Cox regression) to link the occurrence of CHD to the consumption of soy products. The statistical models were adjusted for possible influencing variables. For this purpose, age, sex, ethnicity, overweight, alcohol and nicotine consumption, socioeconomic status, physical activity, drug intake, lipid levels, family history, menopause and hormone therapy (only women), as well as the consumption of other healthy or unhealthy foods and vitamins, were examined.
Most test persons consumed around 0.1 to 4.2 mg of isoflavones per day - 100 g of tofu contains around 2 mg. The researchers divided the test subjects into five groups according to their isoflavone consumption. In the group with the lowest consumption, the participants consumed about 0.16 mg of isoflavones per day. In the group with the highest consumption, on the other hand, about nine times as many isoflavones were consumed: 1.43 mg per day. The first result was not surprising: higher consumption of isoflavones and tofu was associated with a generally healthier lifestyle (more physical activity, plant-based diet). The results were much more interesting in terms of cardiovascular risk.
Overall, about 4% of the participants developed CHD during the follow-up phase. The statistical analysis showed: After consideration of all known influencing factors, higher consumption of isoflavones reduced the risk of developing CHD. The participants with the highest isoflavone consumption showed a 13% lower cardiovascular risk. In trend analysis, the research team was able to show that there was a dose-response relationship (p-trend = 0.008). The higher the isoflavone consumption, the lower the cardiovascular risk.
As isoflavones are similar to the hormone estrogen, the researchers also investigated the influence of menopause and hormone replacement therapy on the cardioprotective effect of isoflavones. It was found that only premenopausal women benefited from the protective effect of isoflavones. In women who had already gone through menopause or were receiving hormone replacement therapy, no protective effect of the isoflavones could be detected.
In the next step, the researchers analyzed the tofu consumption of the test persons and came to a comparable result. Test persons who ate at least one tofu meal a week (4% of the participants) had a 9% lower risk of developing CHD compared to participants who consumed tofu less than once a month. A dose-response relationship was also found here (p-trend = 0.005). In contrast, the positive effect could not be demonstrated for soy milk. In contrast to isoflavones, tofu consumption also seemed to protect women beyond menopause from cardiovascular disease. Only women with hormone replacement therapy did not benefit from additional tofu consumption.
In summary, the results of the study suggest that consumption of tofu and isoflavones could protect against the development of CHD. However, this study is also only an observational study and therefore cannot prove causality. Another weakness of this (and any other nutritional study) is the use of nutritional questionnaires. It is well known that it is difficult for anyone to summarise their eating habits in a concise way within a year. This also applies to the study presented here and can lead to a significant bias in the results.
A well-designed intervention study is therefore needed to conclusively clarify the effect of soy products on heart health. The only question is whether funding can ever be found for this. In any case, the soy industry does not need such a study, and its products sell so well.
Source:
Ma L et al. Isoflavone Intake and the Risk of Coronary Heart Disease in US Men and Women. Circulation. 2020;141:1127-1137. Originally published 23 Mar 2020. DOI: https://doi.org/10.1161/CIRCULATIONAHA.119.041306