Coronary heart disease (CHD) is the major cause of death in most developed countries and is rapidly increasing in prevalence in developing countries. In the US, for example, death rates from cardiovascular disease exceed 1 million annually and the total cost is estimated to exceed US$120 billion, the largest disease-related cost to health. While many modifiable risk factors, such as cigarette smoking and hypertension, contribute to risk for CHD, lipid abnormalities are the most important factors. Low-density lipoproteins (LDL) have a central role in the atherosclerotic process. LDL penetrate the walls of blood vessels where they are oxidized by free radicals and accumulate as a gruel-like material that blocks the blood vessel lumen; this material also can leak into the blood vessel to cause a thrombosis. High-density lipoproteins (HDL) cholesterol have a protective effect and act to prevent LDL oxidation and remove cholesterol that accumulates in the blood vessel wall.

Soy foods contain protein and isoflavone components that have specific effects on reducing risk for CHD. The isoflavones or soy estrogens contribute an estimated three-fourths of the protective effect while the soy protein may be responsible for the remaining one-quarter of the protection. Soy protein and its isoflavones exert at least four anti-atherogenic effects. These effects are itemized and then reviewed in greater detail.

1. Soy protein has favorable effects on all the blood lipid levels;
2. Soy isoflavones are important antioxidants that prevent oxidation of LDL;
3. Soy foods decrease tendency to form blood clots or thromboses; and
4. Soy isoflavones have health promoting effects on blood vessels.

Correction of blood lipid levels
Our recent critical statistical analysis of 38 careful human clinical trials noted that soy protein intake decreased serum concentrations of total cholesterol, LDL-cholesterol, and triglycerides while increasing HDL-cholesterol concentrations. Soy protein intake was associated with a 9.3% reduction in serum cholesterol, a 12.9% reduction in serum LDL-cholesterol, and a 10.5% reduction in serum triglycerides. All of these decreases were statistically significant. Serum HDL-cholesterol levels increased by 2.4%, a non-significant increase. These findings had a strong consistency because 34 of 38 studies reported that soy protein intake decreased serum cholesterol levels. Because this study has not been seriously questioned in the literature or in public debate, it appears to have resolved questions about whether soy protein significantly affects serum cholesterol concentrations. The mechanisms responsible for this effect are still under investigation but it appears likely that the soy isoflavones account for >70% of the cholesterol-lowering effects.

Antioxidant effects
Many laboratory studies document the strong antioxidant properties of soy isoflavones. In a recent study using rats, we reported that soy isoflavones appear to be incorporated into lipoprotein particles, such as LDL, and protect from oxidation. This antioxidant effect would get at the heart of the atherosclerosis problem by decreasing LDL accumulation in blood vessel walls.

Anti-thrombotic effects
Soy protein with its isoflavones has a number of effects in the blood, which reduce the risk for blood clot formation. Soy protein intake decreases platelet aggregation or "clumping" an early step in blood clot formation. Soy isoflavones also appear to decrease the tendency of the blood to form thromboses, thus further reducing the risk for thrombotic occlusion of the blood vessel.

Blood vessel protection
Blood vessels damaged by atherosclerotic plaque tend to develop spasm or "clamping down" when exposed to stress. This further decreases blood flow though the partially occluded vessel and can aggravate angina or brain symptoms. Soy proteins and their isoflavones restore normal reactivity to damaged blood vessels, protecting them from abnormal spasm or inappropriate "clamping down."

Conclusion
Regular intake of soy protein and its isoflavones has important protective effects against CHD. Soy foods improve the serum lipid profile, provide antioxidants, decrease blood clotting and promote healthy blood vessel function.

References