Index

• Introduction
• Effects of Soyfoods on Circulating Lipid Levels
• Beyond Effects on Lipid Levels
• Practical Implications
• References

Beyond Effects on Lipid Levels
There is evidence from the epidemiologic literature that soyfoods have coronary benefits independent of their effect on cholesterol levels. For example, after controlling for a wide variety of CHD risk factors, a prospective study involving nearly 65,000 postmenopausal women from Shanghai found that soy protein intake was associated with an 86 percent reduction in the risk of non-fatal myocardial infarction.³¹

In agreement, a cross-sectional study involving 406 Chinese adults ages 40 to 65 years (134 males, 272 females) without confirmed relevant diseases found that soyfood intake was inversely related to bifurcation intima-media thickness, although the association was more apparent in men than women.³²

Also, a prospective study involving 40,462 Japanese participants (40 to 59 years old, without cardiovascular disease or cancer at baseline) found that, when comparing women with frequent (≥5x/week) versus infrequent (≤2x/week) soy consumption, the multivariable hazard ratios were 0.64, 0.55 and 0.31 for risk of the incidence of cerebral infarction, myocardial infarction and CHD mortality, respectively.³³

For two reasons, it is highly unlikely that the cholesterol-lowering effects of soyfoods were primarily responsible for the effects observed in these three epidemiologic studies. First, soy protein consumption in the upper intake categories was between 8 and 16 g/day, which, based on the results from the clinical studies, is far less than the amounts thought to be needed to lower cholesterol. Second, the protective effects were far greater than could be expected from a modest reduction in cholesterol.



In support of these epidemiologic studies are various clinical studies that show soyfoods, soy protein or soybean isoflavones favorably affect a number of biological measures that impact heart disease risk. The proposed hypotensive effects of soyfoods are particularly intriguing. A recent meta-analysis found that soyfoods reduced systolic and diastolic blood pressure by about 6 and 4 mm Hg, respectively, although these data were based on only five studies.³⁴

Heart disease is the leading cause of death for both men and women in the United States.

Other biological processes and measures that may be favorably affected by various soy components include endothelial function, systematic arterial compliance, LDLC-oxidation and LDL particle size.³⁵ It is likely that the isoflavones, which exert estrogen-like effects under some experimental conditions, are responsible for the effects on endothelial function. Soybeans are essentially the only nutritionally-relevant source of these diphenolic compounds.³⁶

However, because of the inconsistent findings, no firm conclusions can be made about the effects of soy on these lipid-independent risk factors. Recent studies, however, provide possible explanations for some of the inconsistency. For example, endothelial function, which is regarded as an independent CHD risk factor, was found to be improved by isoflavones primarily only among subjects whose endothelial function was impaired at baseline.³⁷

Similarly, the anti-inflammatory effects of isoflavones may be observed only in subjects at risk of CHD who have elevated levels of inflammatory markers.³⁸ The clinical studies, in which effects of different soy products on biological measures of CHD risk were examined, involved subjects at both normal and elevated risk. Thus, the differences in baseline subject characteristics may account for some of the inconsistent clinical results overall.

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