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Cardiovascular diseases kill more than 16 million people worldwide and account for some 30 percent of all deaths each year. In the United States, heart disease and stroke, life-threatening conditions associated with cardiovascular disease, account for nearly 40 percent of all adult deaths. Physicians have known for many years that lifestyle change helps prevent heart attack and stroke. Only recently, however, have scientists studied how a diet low in fat and rich in soy protein helps lower cholesterol and may help reduce one's risk of heart disease.
Recent controversy has surfaced regarding the efficacy of soy protein consumption in reducing serum cholesterol. Of primary concern is whether a 1995 meta-analysis1 (which generated considerable excitement in the medical community) accurately estimated the impact of soy consumption on cholesterol lowering.
Since publication of this landmark meta-analysis in 1995, six additional meta-analyses and two significant reviews have been published (2003 through 2006). The preponderance of peer-reviewed data supports the conclusion that soy protein consumption " on its own or as a package with other dietary modifications " leads to improved lipid profiled and a reduced risk of coronary heart disease. In particular, these scientific examinations demonstrate the LDL ("bad" ) cholesterol-lowering effect of daily soy protein consumption is moderate but additive, contributing to a cumulative heart disease risk reduction of approximately 20 percent.
A 2003 meta-analysis published in the European Journal of Clinical Nutrition examined the impact of soy protein consumption on lipid lowering compared to a non-soy protein control product2. Ten studies were identified as meeting the inclusion criteria, resulting in 21 comparisons. A total of 959 subjects were included in the analysis with mean ages between 41 and 63 years and mean baseline cholesterol levels between 209 and 255 mg/dl. Eighteen of the 21 comparisons reported a decrease in LDL cholesterol, with a mean LDL cholesterol reduction of four percent (6.6 mg/dl) and a mean increase in HDL ("good" ) cholesterol of three percent (1.2 mg/dl). The authors concluded that although the use of soy may exert a smaller risk reduction at the individual level than a pharmaceutical intervention, its risk reduction is considerable at a population level.
In 2005, Zhan and Ho published results of a meta-analysis aimed at quantifying the effects of soy protein on lipid lowering.3 Twenty-two studies were identified, resulting in 52 comparisons. A total of 1,381 subjects consumed a diet rich in either soy protein or a control protein source (dairy or meat). The authors concluded that consuming soy protein resulted in a significant decrease in total cholesterol, LDL cholesterol and triglycerides, and a significant increase in HDL ("good" ) cholesterol. Additionally, the researchers suggested that the sustained increase in HDL cholesterol is encouraging from a public health perspective because it provides evidence that soy protein improves the protective component of the lipid profile.
A meta-analysis published in 2006 reported on the effect of isolated soy protein feeding on serum lipids.4 This analysis included only randomized controlled studies using isolated soy protein as the treatment product. Soy protein intake of 1,756 subjects ranged from 20 to 106/g day, with a total of 41 comparisons. The researchers found a net decrease in total cholesterol (5.26 mg/dl), LDL cholesterol (4.25 mg/dl) and triglycerides (6.26 mg/dl), and a net increase in HDL ("good" ) cholesterol (0.77 mg/dl). Effects were statistically significant for all variables, leading the authors to conclude that replacing foods high in saturated fat, trans-saturated fat and cholesterol with soy protein may have a beneficial effect on coronary heart disease risk factors. Moreover, the researchers suggested that inclusion of other components of soy, such as fiber, may reduce lipids even further.
Balk et al. (AHRQ) appears to be the most comprehensive meta-analysis to date5. Researchers assigned grades to studies based on a variety of criteria, including the design of the study, power of the study, reporting of information important to the study's outcome and appropriateness of the control group. With respect to the outcomes reported by these researchers, soy protein consumption resulted in a statistically significant net reduction in LDL cholesterol (5 mg/dl) and triglycerides (8 mg/dl). In their conclusion, Balk et al. stated that soy products appear to exert a small benefit on LDL cholesterol and triglycerides, and that such effects may be clinically-modest in individuals but possibly large enough to have a significant population-wide effect.
It is also noted that research results concerning soy and cholesterol often depend on the risk profile of those individuals who participate in the studies, as dietary interventions have little effect in lowering the cholesterol levels of already-healthy individuals. Participants' initial (starting) cholesterol levels materially influence the degree to which soy protein reduces cholesterol.
In 1999, the U.S. Food and Drug Administration (FDA) approved an unqualified health claim in support of soy's heart health benefits after reviewing scientific evidence from more than 50 independent studies. More recent and equally strong research into soy protein and heart disease prevention continues to support the FDA claim, and has prompted many health experts to endorse the value of soy protein within a low fat, low cholesterol diet. Since the FDA-approved heart health claim in 1999, seven additional government health agencies around the world have approved similar claims, including the United Kingdom and Japan.
To be sure, soy protein is a food, not a prescription drug. Soy foods have the unique ability to lower both LDL ("bad" ) cholesterol and triglycerides, but not to the degree expected from cholesterol-lowering medications. That soy products have been found to lower blood cholesterol even a modest amount, however, has the potential to dramatically impact public health. One may argue whether a three to eight percent reduction is "clinically significant," but the bottom line is that heart disease prevention depends on small life changes. Neither pharmaceuticals nor soy foods are by themselves panaceas for a healthy heart.
For more information about the heart health benefits of soy protein, please contact the Soyfoods Association of North America (SANA) at +1 202/659-3520.
1 Anderson JW, BM Johnstone, & ME Cook-Newell. Meta-Analysis of the Effects of Soy Protein Intake on Serum Lipids. New England Journal of Medicine 1995; 333:276-282.
2 Weggemans RM & EA Trautwein. Relation Between Soy-Associated Isoflavones and LDL and HDL Cholesterol Concentrations in Humans: A Meta-Analysis. European Journal of Clinical Nutrition 2003; 57:940-946.
3 Zhan S & SC Ho. Meta-Analysis of the Effects of Soy Protein Containing Isoflavones on the Lipid Profile. American Journal of Clinical Nutrition 2005; 81:397-408.
4 Reynolds, K, A Chin, KA Lees, A Nguyen, D Bujnowski, & J He. A Meta-Analysis of the Effect of Soy Protein Supplementation on Serum Lipids. American Journal of Cardiology 2006; 98:633-640.
5 Balk E, M Chung, P Chew, S Ip, G Raman, B Kupelnick, A Tatsioni, Y Sun, B Wolk, D DeVine, & J Lau. Effects of Soy on Health Outcomes. Agency for Healthcare Research and Quality: Evidence Report/Technology Assessment (no.126).
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