Soy, Heart Health & Cholesterol

Coronary heart disease looms large over the U.S. with 13 million Americans having been diagnosed with coronary heart disease (CHD). Approximately one-half of men and one-third of women will develop CHD after age 40.¹

While individual genetics play a role, four decades of rigorous investigation have proven the relationship between CHD and high cholesterol, a sedentary lifestyle, a high-fat diet, smoking and
obesity.^{2, 3} In fact, research published in 1990 showed that the proper diet along with other heart-healthy lifestyle changes can reduce severe coronary atherosclerosis.⁴

Soy foods can play an important role of the healthy diet that helps reduce cholesterol and fat in particular.⁵

There has been emphasis on reducing serum cholesterol levels, since elevated blood cholesterol is a well-established CHD risk factor. More than 100 million people age 20 or older — or more than half of this population — have cholesterol levels above the recommended target of 200 mg/dl. In fact, nearly 38 million Americans have cholesterol levels above 240 mg/dl, which puts them at high risk of developing heart disease.

Furthermore, over the past decade there has been a downward trend in the estimates of what is considered the ideal cholesterol level.

Although the current target goal for low-density-lipoprotein cholesterol (LDL) is <100 mg/dl, some data suggest that the ideal LDL level may be 50 to 70 mg/dl.^6,7

Beyond elevated cholesterol there is evidence that diet favorably affects a number of risk factors for CHD, including elevated homocysteine levels,⁸ impaired vascular reactivity,⁹ decreased LDL particle size¹⁰ and hypertension.¹¹

Soy Protein & Lowering Cholesterol

A substantial body of research – more than 80 human trials over the past 30 years – has been conducted investigating the effects of soy intake on cardiovascular health, with the general finding being that soy protein can help lower LDL (bad) cholesterol between at least 3 and 5 percent. Even these modest reductions have considerable relevance, since each 1 percent drop in LDL is associated with a 2 to 4 percent decrease in heart disease risk.^{12 13}

The American Heart Association’s nutrition guidelines state that using soy protein as a replacement for animal protein products that are high in saturated fat may provide cholesterol-lowering benefits and that “the consumption of soy protein containing isoflavones, along with other heart-healthy diet modifications, is recommended for those high-risk populations with elevated total and LDL cholesterol.”¹⁴

In fact, in 1999 the Food and Drug Administration (FDA) issued the health claim that consuming 25 grams of soy protein per day, as part of a diet that is low in saturated fat and cholesterol, may reduce the risk of heart disease.

While soy is not medicine, it is a nutrient-rich food that can be an important part of a balanced diet. The degree of cholesterol reduction generally found with soy is comparable to that of other wholesome foods that carry an FDA heart-health claim, such as oatmeal. They are both foods that the FDA continues to advocate for use as part of a balanced diet to promote heart health. The hypocholesterolemic effects of soy protein began gaining widespread attention¹⁵ in 1995, when a meta-analysis by Anderson et al. proved that soy protein helped reduce serum cholesterol levels in 34 of 38 trials.¹⁶

Studies have shown that the reduction in cholesterol due to soy consumption is greater than that achievable on a low-saturated fat diet alone, since in most trials, subjects were on low-fat diets prior to participation in the trials. And, while soy protein alone may not lower cholesterol to a target goal in overtly hypercholesterolemic individuals, it can provide effects similar to that of soluble fiber.¹⁷

Most recently, the American Journal of Clinical Nutrition (AJCN) published a study¹⁸ that measured the effect of several soy protein sources vs. animal protein on many cardiovascular disease risk factors, including LDL cholesterol levels, by examining the effect of four diets (i.e., animal protein, whole soybeans, soy flour and soymilk). The study concluded that a diet of soymilk showed a significant reduction (4 percent) in LDL cholesterol compared to all other groups. The soymilk used in this experiment was Silk.

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Eating Soy for Heart Health

In 1999, the U.S. Food and Drug Administration (FDA) approved a health claim for soy protein and coronary heart disease,¹⁹ and soon after the American Heart Association endorsed the use of soy foods for people with elevated cholesterol.²⁰ Similar claims have been approved in the United Kingdom and the Philippines.

The FDA established 25 g/day of soy protein as the threshold amount required to lower cholesterol. Because the FDA assumed that it was reasonable to consume four servings of soy foods per day, products that provide at least 6.25 g soy protein per serving may carry the FDA health claim on their label. Some research shows that even smaller amounts of soy foods can also be efficacious.^21-24

Anderson et al., in a new meta-analysis, recently concluded that dividing soy protein servings into multiple small servings is more effective in LDL reduction than the consumption of a single large serving.²⁵

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The Role of Soy Isoflavones

Science has not fully explained why general measures of heart health seem to improve when people eat soy foods. However, the isoflavones found in soy protein are thought to explain some of its impact.

Indirect support of this theory comes from a prospective epidemiologic study involving nearly 65,000 women in Shanghai, China. Even after controlling for a wide variety of risk factors, the study found that soy protein intake was associated with a marked reduction in the risk of non-fatal myocardial infarction (relative risk = 0.14 for the highest vs. the lowest quartile of intake; P for trend = 0.001), a protective effect far beyond that which could be expected from a modest reduction in cholesterol. Also of note: The fourth quartile soy protein intake cutoff in this study was only >11.19 g/day, far less the 25 g/day supported by the FDA.²⁶

Soy may also favorably affect endothelial function.²⁷ The endothelium, the thin layer of cells lining blood vessels, secretes several biologically active molecules (such as nitric oxide), which influences the health of the coronary vessels and, as a result, risk of heart disease.

Several studies, but by no means all, have shown that isoflavone-rich soy protein and isolated isoflavones increase arterial dilation in postmenopausal women – indicating improved enthothelial health.^28-31

More speculative research suggests that soy lowers blood pressure,^{32, 33} improves systemic arterial compliance,³⁴ inhibits LDL oxidation,^{35, 36} and reduces LDL particle size,³⁷ making LDL less atherogenic.

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Soy and Fatty Acid Content

Many soy foods, because of its relatively high polyunsaturated fat content (~25 percent of total energy), help lower serum cholesterol by displacing higher-saturated fats in the diet. ^{38, 39}

The soy bean is one of the few good plant sources of the essential fatty acid, α-linolenic acid,⁴⁰ an omega-3 fatty acid that has coronary benefits.⁴¹

A recent meta-analysis found that those subjects who consumed the most α-linolenic acid were approximately 20 percent less likely to die from CHD than those who ate the least α-linolenic acid.⁴¹ Importantly, the difference between the high- and low-consumers was only 1.2 g/day, the amount found in about 3 cups of full-fat soy milk.

Interestingly, one recent study found that soy oil, likely because of its α-linolenic acid content, was almost as effective as fish oil in increasing heart rate variability, which decreases risk of cardiac arrhythmia.⁴²

Substantially reducing the risk of heart disease through lifestyle changes requires commitment. But it is clear that soy foods take an important lead in a heart-healthy diet.

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Soy Fact

The cholesterol-lowering effects of soy protein were first demonstrated in animals in the 1940s and in humans in 1967. In 1977, researchers proved that soy protein accompanied major reductions in serum cholesterol in hypercholesterolemic patients.^43-46

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References

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