Soy is a traditional food of many countries from the Asian continent, where it is made into a number of foods including tofu, miso, soy sauce and tempeh. Soy has recently become more popular in the West because of reports that it may be beneficial to the cardiovascular and skeletal systems. As well as the traditional Asian soy foods, a number of other soy products including soy milk and soy yoghurts are now widely available. Soy may be beneficial to the cardiovascular system because consumption has been shown to affect blood lipids. There is some evidence that soy may be able to lower total cholesterol, LDL cholesterol and triglyceride plasma levels, although the amount of benefit is dependent on the initial levels. Those with higher levels tend to benefit more from addition of soy to the diet.
Figure 1. The chemical structures of isoflavones and oestradiol1.
The mechanism by which soy is able to affect blood lipids is though to be via up‑regulation of LDL receptors, an increase in the excretion of bile acids (which contain cholesterol), as well as a suppression of cholesterol absorption from the intestine. Soy contains a number of phytooestrogen chemicals which are thought to be responsible for the physiological effects of soy consumption. Genistein and daidzein are two phytooestrogens present in soy that are structurally related to mammalian oestrogen and oestradiol. Methylation of genistein and daidzein produces the isoflavones biochanin A and formononetin respectively (figure 1). Because these isoflavones resemble oestrogen, it has been theorised that they may interact with the oestrogen receptor and induce mild oestrogenic activity. In support of this, some evidence indicates that oestrogen is able to lower LDL cholesterol when administered to postmenopausal women.
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