Soyfoods, Blood Cholesterol and Heart Health

Coronary heart disease (CHD) looms large over the United States, with almost 17 million Americans diagnosed.¹ Approximately one-half of men and one-third of women in America will develop CHD after age 40.²

While individual genetics play a role, four decades of rigorous investigation have proven the relationship between CHD and high blood cholesterol, high blood pressure, a sedentary lifestyle, smoking and obesity.^3,4 In fact, research has shown that proper diet along with other heart-healthy lifestyle changes can actually reverse severe coronary atherosclerosis.⁵

Soyfoods can play an important role in a healthy diet designed to decrease dietary intake of cholesterol and saturated fat and reduce blood cholesterol levels.

The medical and nutrition communities have long emphasized reducing serum cholesterol levels, since elevated blood cholesterol is a well-established CHD risk factor. Nearly 100 million Americans age 20 or older — or almost half of this population — have cholesterol levels above the recommended target of 200 mg/dl. In addition, over 34 million Americans have cholesterol levels above 240 mg/dl, which puts them at high risk of developing heart disease.¹

Furthermore, over the past decade, opinions about what constitutes a desirable cholesterol level have shown a downward trend. Although the current target goal for low-density-lipoprotein cholesterol (LDLC, bad cholesterol) is <100 mg/dl, some data suggest that the ideal LDLC level may be 50 to 70 mg/dl.^6,7

Elevated cholesterol is just one CHD risk factor. Evidence suggests that proper diet and soyfoods may favorably affect several other factors as well, including elevated blood pressure⁸ and damaged arteries.⁹

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Soy Protein and Cholesterol

The first human study to show that soy protein lowered blood cholesterol was published in 1967.¹⁰ But it was not for another 30 years that the hypocholesterolemic effects of soy protein came to the attention of the medical community. In 1995, a comprehensive statistical analysis that included 34 studies showed conclusively that soy protein lowered LDLC.¹¹ The decrease was a direct effect of the protein, and not because soyfoods are low in saturated fat.

In 1999, the Food and Drug Administration issued the following health claim in recognition of the cholesterol-lowering effects of soy protein: 25 grams of soy protein per day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease.¹²

Since the health claim was issued, many more clinical trials have been conducted. Recent analyses confirm that soy protein lowers LDLC, but suggest that the magnitude of the effect is less than initially reported. Current estimates suggest that soy protein lowers LDLC by 3 to 5 percent.^13,14 Although this impact appears modest, it is important to note that each 1 percent drop in LDLC is associated with as much as a 2 to 4 percent decrease in heart disease risk. As a result, soy protein could be associated with a CHD risk decrease of 10 to 15% over time.^15,16

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Fatty Acid Content of Soyfoods

Many soyfoods have relatively high polyunsaturated and low saturated fat content. In addition to offering the direct cholesterol-lowering effect of soy protein, these foods can help lower serum cholesterol by displacing higher-saturated fats from the diet.

In their recent review of the scientific literature, the American Heart Association (AHA) emphasized the important role that polyunsaturated fat can play in reducing risk of CHD.¹⁷ Not surprisingly therefore, the AHA recently stated that using soy protein as a replacement for animal protein products that are high in saturated fat may provide cholesterol-lowering benefits.¹⁸ Given the many heart-healthy attributes of soyfoods, it is also not surprising that they have played a key role in research diets that have been shown to lower cholesterol by as much as 30 percent.¹⁹

Furthermore, the soybean is one of the few good plant sources of the essential fatty acid, α-linolenic acid (ALA), an omega-3 fatty acid. Although the omega-3 fatty acid in soy differs from those found in certain types of fish, ALA is thought to have independent coronary benefits.²⁰ A recent analysis found that subjects who consumed the most ALA were approximately 20 percent less likely to die from CHD than those who ate the least α-linolenic acid.²⁰ 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 soymilk.

Interestingly, one recent study found that soy oil was almost as effective as fish oil in increasing heart rate variability, which decreases risk of cardiac arrhythmia.²¹ This is likely due to the ALA content of the soy.

Substantially reducing the risk of heart disease through lifestyle changes requires commitment and effort in many areas. But it is clear that soyfoods play an important lead role in a heart-healthy diet.

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Beyond Cholesterol Reduction

As noted previously, elevated cholesterol is one just one CHD risk factor. Happily, evidence suggests that the coronary benefits soyfoods extend beyond the effects of cholesterol reduction alone.

Several recently conducted Asian epidemiologic studies provide indirect support for the multiple coronary benefits of soyfoods. For example, in a study involving nearly 65,000 women in Shanghai, China, even after controlling for a wide variety of risk factors, soy protein intake was associated with a reduction of more than 80% in the risk of having a heart attack.²² Similar benefits were noted in a study of Japanese women.²³ A study in southern China found that higher soyfood intake was associated with a 44 to 82% reduction in the risk of stroke among both men and women.²⁴ Generally, the high-soy consumers in these studies were eating about two servings of soy each day. Such protective effects can't be attributed solely to a reduction in cholesterol.

These observations are intriguing, but only the results from strict clinical studies can be used as a basis for firm conclusions. Many such studies, although not all, have shown that soyfoods may lower blood pressure,⁸ directly improve the health of the arteries,⁹ and make LDLC less harmful.²⁵ Research into these effects continues.

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Take Home Message

Soyfoods have a lot to offer toward the goal of substantially reducing the risk of CHD. They provide high-quality protein but are low in saturated and high in polyunsaturated fat. They are also one of the few plant sources of omega-3 fatty acids. Furthermore, soy protein directly lowers blood cholesterol. There is also suggestive evidence that soyfoods exert coronary benefits independent of their effects on blood cholesterol levels. For all of these reasons, soyfoods can play an important role in a heart-healthy diet.

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References

1. Lloyd-Jones D, Adams R, Carnethon M, De Simone G, Ferguson TB, Flegal K, et al. Heart disease and stroke statistics--2009 update: a report from the American Heart Association Statistics Committee and Stroke Statistics Subcommittee. Circulation 2009;119:e21-181.
2. Lloyd-Jones DM, Larson MG, Beiser A, Levy D. Lifetime risk of developing coronary heart disease. Lancet 1999;353:89-92.
3. Blackburn H, Taylor HL, Keys A. Coronary heart disease in seven countries. XVI. The electrocardiogram in prediction of five-year coronary heart disease incidence among men aged forty through fifty-nine. Circulation 1970;41:I154-61.
4. Keys A, Kimura N. Diets of middle-aged farmers in Japan. Am J Clin Nutr 1970;23:212-23.
5. Ornish D, Brown SE, Scherwitz LW, Billings JH, Armstrong WT, Ports TA, et al. Can lifestyle changes reverse coronary heart disease? The Lifestyle Heart Trial. Lancet 1990;336:129-33.
6. O'Keefe JH, Jr., Cordain L, Harris WH, Moe RM, Vogel R. Optimal low-density lipoprotein is 50 to 70 mg/dl: lower is better and physiologically normal. J Am Coll Cardiol 2004;43:2142-6.
7. Cannon CP. The IDEAL cholesterol: lower is better. JAMA 2005;294:2492-4.
8. Hooper L, Kroon PA, Rimm EB, Cohn JS, Harvey I, Le Cornu KA, et al. Flavonoids, flavonoid-rich foods, and cardiovascular risk: a meta-analysis of randomized controlled trials. Am J Clin Nutr 2008;88:38-50.
9. Cuevas AM, Irribarra VL, Castillo OA, Yanez MD, Germain AM. Isolated soy protein improves endothelial function in postmenopausal hypercholesterolemic women. Eur J Clin Nutr 2003;57:889-94.
10. Hodges RE, Krehl WA, Stone DB, Lopez A. Dietary carbohydrates and low cholesterol diets: effects on serum lipids on man. Am J Clin Nutr 1967;20:198-208.
11. Anderson JW, Johnstone BM, Cook-Newell ME. Meta-analysis of the effects of soy protein intake on serum lipids. N Engl J Med 1995;333:276-82.
12. Food Labeling: Health Claims; Soy Protein and Coronary Heart Disease. Federal Register: (Volume 64, Number 206)]; 1999. p. 57699-733.
13. Balk E, Chung M, Chew P, Ip S, Raman G, Kuplenick B, et al. Effects of soy on health outcomes. Evidence report/technology assessment No. 126 (prepared by Tufts-New England Medical Center Evidence-based Practice Center under Contract No. 290-02-0022.) AHRQ Publication No. 05-E024-2. Rockville, MD Agency for Healthcare Research and Quality; July 2005.
14. Zhan S, Ho SC. Meta-analysis of the effects of soy protein containing isoflavones on the lipid profile. Am J Clin Nutr 2005;81:397-408.
15. Law MR, Wald NJ, Thompson SG. By how much and how quickly does reduction in serum cholesterol concentration lower risk of ischaemic heart disease? BMJ 1994;308:367-72.
16. Law MR, Wald NJ, Wu T, Hackshaw A, Bailey A. Systematic underestimation of association between serum cholesterol concentration and ischaemic heart disease in observational studies: data from the BUPA study. BMJ 1994;308:363-6.
17. Harris WS, Mozaffarian D, Rimm E, Kris-Etherton P, Rudel LL, Appel LJ, et al. Omega-6 fatty acids and risk for cardiovascular disease: a science advisory from the American Heart Association Nutrition Subcommittee of the Council on Nutrition, Physical Activity, and Metabolism; Council on Cardiovascular Nursing; and Council on Epidemiology and Prevention. Circulation 2009;119:902-7.
18. Sacks FM, Lichtenstein A, Van Horn L, Harris W, Kris-Etherton P, Winston M. Soy protein, isoflavones, and cardiovascular health: an American Heart Association Science Advisory for professionals from the Nutrition Committee. Circulation 2006;113:1034-44.
19. Jenkins DJ, Kendall CW, Faulkner D, Vidgen E, Trautwein EA, Parker TL, et al. A dietary portfolio approach to cholesterol reduction: combined effects of plant sterols, vegetable proteins, and viscous fibers in hypercholesterolemia. Metabolism 2002;51:1596-604.
20. Brouwer IA, Katan MB, Zock PL. Dietary alpha-linolenic acid is associated with reduced risk of fatal coronary heart disease, but increased prostate cancer risk: a meta-analysis. J Nutr 2004;134:919-22.
21. Holguin F, Tellez-Rojo MM, Lazo M, Mannino D, Schwartz J, Hernandez M, et al. Cardiac autonomic changes associated with fish oil vs soy oil supplementation in the elderly. Chest 2005;127:1102-7.
22. Zhang X, Shu XO, Gao YT, Yang G, Li Q, Li H, et al. Soy food consumption is associated with lower risk of coronary heart disease in Chinese women. J Nutr 2003;133:2874-8.
23. Kokubo Y, Iso H, Ishihara J, Okada K, Inoue M, Tsugane S. Association of dietary intake of soy, beans, and isoflavones with risk of cerebral and myocardial infarctions in Japanese populations: the Japan Public Health Center-based (JPHC) study cohort I. Circulation 2007;116:2553-62.
24. Liang W, Lee AH, Binns CW, Huang R, Hu D, Shao H. Soy consumption reduces risk of ischemic stroke: a case-control study in southern china. Neuroepidemiology 2009;33:111-6.
25. Desroches S, Mauger JF, Ausman LM, Lichtenstein AH, Lamarche B. Soy protein favorably affects LDL size independently of isoflavones in hypercholesterolemic men and women. J Nutr 2004;134:574-9.