More ways that soy protects the heart
Research suggests that soy foods may also protect the heart in other ways, including lowering blood pressure, reducing the oxidation of cholesterol and keeping blood vessels flexible.
Soy helps lower blood pressure:
Several short-term studies have found that blood pressure decreases when soy is included in the diet. The addition of 40g of soy protein (providing 118mg isoflavones) to the daily diet significantly reduced blood pressure in a study of more than 200 healthy adults
19 . In a second short term study of men with high blood pressure, the consumption of 20g of soy protein (providing 80mg isoflavones) mixed into bread, cereal and biscuits led to significant drops in blood pressure compared to receiving no soy protein (i.e. plain bread, cereal and biscuits)
20 .
The beneficial effects of soy foods on blood pressure have also been demonstrated in healthy post-menopausal women. Results from the Shanghai Women's Health Study, a long-term study involving over 45 000 women aged between 40 and 70 years, show that a regular soy protein intake (greater than 25g/day) is linked with lower systolic and diastolic blood pressure. This effect is observed even after adjustment for a range of factors that affect blood pressure, including age and body weight
21 . This study suggests that women who regularly consume more than 25g soy protein per day (from a range of food sources) are less likely to develop high blood pressure.
While there are now several studies showing that soy foods can lower blood pressure, more research is still required.
Soy isoflavones act as antioxidants
Soy isoflavones can act as antioxidants in the body, which may contribute to the heart health benefits of soy foods. Studies have shown that oxidation of LDL cholesterol (the bad type) plays a role in the development of atherosclerotic plaques. Dr. Jonathon Hodgson and colleagues from the University of Western Australia were one of the first in 1996 to show that the isoflavones in soy can inhibit lipoprotein oxidation
22 . Later in 2000 and 2002, Professor David Jenkins from the University of Toronto showed in three separate studies that diets containing soy foods (high in soy protein and isoflavones) reduced the levels of oxidised LDL cholesterol when compared to control diets
23 ,
24 ,
25 . A more recent study in post-menopausal women who had metabolic syndrome found a reduction in lipid peroxidation and an increase in total antioxidant capacity with the consumption of soy nuts or soy protein compared to a control diet
26 . This means that soy foods can be a valuable source of dietary antioxidants to protect the body from oxidation reactions that damage the heart and have been linked with premature ageing.
Soy foods help blood vessels remain flexible
Recent research also indicates that soy protein and isoflavones can help keep blood vessels flexible by improving endothelial function
26 ,
27 (endothelial function is a measure of the ability of blood vessels to dilate and contract normally). In a study of post-menopausal women with the metabolic syndrome, the consumption of 30g of soy nuts in place of one serve of red meat per day increased nitric oxide levels, a marker of good endothelial function and relaxed blood vessels
28 . In people at increased risk of heart attack and stroke, a greater isoflavone intake was linked with improved endothelial function and a reduction in the build up of plaques in their arteries, which tends to make the arteries stiffen up with age
29 .
While the evidence to date is positive, ongoing research is required to determine the effectiveness of soy foods in improving blood vessel health.
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