CARDIOVASCULAR DISEASE AND SOY FOODS

By Marcia Zimmerman, M.Ed., C.N.

Bob is one of the 20 million Americans who suffers from cardiovascular disease. Like most who have the disorder, Bob was unaware he had a problem - until his first heart attack. Bob was lucky because his heart attack was mild and quick medical attention prevented further complications. Each year 250,000 less fortunate Americans don't survive their first heart attack. However, these statistics are improving as quick life-saving techniques such as use of clot-busting drugs have become routine. (Margolis, 4)

For Bob and other survivors, a heart attack is a wake-up call to make lifestyle changes that will prevent future occlusive events. Accordingly, Bob has followed his doctor's advice by cutting down on his work load, and eliminated cigarettes and caffeine. However, he has not been diligent about exercising or following his doctor's recommendations for dietary change. Without these changes Bob may well suffer another and perhaps fatal heart attack. And what about the other 50 percent of the U.S. population that will develop cardiovascular disease, can dietary change also help them?

What is cardiovascular disease?

Cardiovascular disease (CVD) affects the interior walls of blood vessels in the heart, brain and extremities. A lesion develops on the arterial wall, usually caused by free-radical and enzymatic damage, but can also stem from an infection. A cascade of events follow that eventually block blood flow. The lesion that forms erodes the vessel's smooth lining and inflammatory debris, blood fats, cholesterol, dead cells, fibrous proteins, calcium and collagen collect within the lesion. The deposit filled lesion (plaque) becomes rough and cracked on the surface as it "matures." Blood platelets snag on the rough surface and form clots that finally cut off (occlude) blood flow through the vessel.

Depending on which vessels are affected, a heart attack, stroke or peripheral tissue damage results. (Margolis,4-6) For years, heart experts have thought occlusive events resulted primarily from a gradual build-up of plaque on artery walls, in the way just described. However, recent findings suggest a more likely cause is "unstable" plaques that rupture and quickly spew forth their contents - precipitating a blood clot. It has been estimated that 60 to 70 percent of heart attacks are caused by eruptions of unstable plaques.(HHL,4) Large "soft" centers packed with fats, cholesterol, immune cells and inflammatory mediators distinguish unstable plaques and make them extremely volatile. Thus, prevention and management of cardiovascular disease now centers more than ever on diet. Consumption of saturated fats and cholesterol should be kept to a minimum while free-radical fighters and fiber from fresh fruits and vegetables are liberally eaten. Cholesterol deserves special consideration in our discussion to clear up confusion over terms commonly associated with CVD. Then we'll see how adding one remarkable food to your diet - soy in its many forms - is the best dietary change you can make.

Cholesterol, the good, the bad and ugly

Dietary cholesterol is absorbed into the system, processed in the liver and then transported via the blood serum to body sites where it is converted into important bio-molecules including hormones. Excess cholesterol is bound to bile acids and eliminated in the feces. However, there is a by-pass route in the colon that kicks cholesterol back into circulation before it exits the body. Cholesterol is also produced within the liver and genetic factors determine how much of it will be produced. The combination of dietary intake, liver production of cholesterol and its recirculation contribute to total blood cholesterol levels. Other lifestyle factors determine which of several kinds of cholesterol predominate.

Not all cholesterol is bad - some kinds are better than others - and cholesterol is essential for many important bodily functions. It's a matter of density and whether the cholesterol will hang-up in arteries. Cholesterol is a fatty substance, insoluble in blood, that must be "piggy-backed" on proteins (apoproteins A,B,C,E) to make it more blood compatible. (Ganong, 275) The combined substance, a "lipoprotein," contains the denser protein and less dense cholesterol. The relative amounts of these two determine whether a lipoprotein is "high", "low" or"very low" density. High density lipoproteins (HDL) are "good" because they carry cholesterol to the liver for disposal. Low density (LDL) and very low density lipoproteins (VLDL) are bad because they accumulate on arterial walls and oxidize readily.(margolis) Oxidized LDL attracts immune cells that engulf them, swelling in the process to form huge foam cells. Foam cells further destabilize plaque by attracting other cells and inflammatory mediators. High levels of LDL and VLDL are the specific risk factors for CVD while HDL cholesterol is a positive indicator. Where does dietary cholesterol come from?

Animal versus soy foods in effects on blood lipids

Animal products contain cholesterol and most are also high in saturated fats, another risk factor for CVD. Therefore, a diet high in animal foods including meat, poultry, dairy products and butter, raises blood fats (triglycerides), LDL and VLDL blood cholesterol levels. On the other hand, a diet rich in non-animal foods and specifically one that includes soy protein (25 grams or more per day) lowers total blood lipids specifically total cholesterol, LDL, VLDL and triglycerides. Dr. Clare Hasler, an expert on soy foods and heart disease from the University of Illinois, Chicago emphasizes that soy protein is what lowers cholesterol, although soy isoflavones appear to have other benefits.

A comparative analysis of thirty-eight controlled clinical trials confirmed that consumption of diets rich in soy protein as opposed to those high in animal protein, significantly decreased blood levels of total cholesterol, LDL cholesterol and triglycerides without lowering helpful HDL cholesterol. (Anderson) Numerous investigations have been done to find out if soy protein will also lower cholesterol in those who do not have high cholesterol. Although some benefit has been found among groups of "normal" men and women, the greatest improvement has been seen among the high cholesterol groups. (Wong) Among the low cholesterol subjects, those who had high triglycerides experienced lower levels of these blood lipids when they ate soy protein enriched diets.(Grundy) It is important to note that those who ate a diet low in saturated fats and cholesterol, in addition to using soy protein, experienced the greatest benefits.(Jenkins)

Besides its protein, soy has other components that appear to have significant cardiovascular benefits. Isoflavones are naturally contained in soy foods and these seem to aid the proteins in lowering cholesterol. (Potter) Isoflavones are also excellent free-radical scavengers and some researchers propose these protect vessels. Additionally, soy is rich in soluble fiber. Some studies have shown that soy fiber is also important in lowering LDL cholesterol and triglycerides.(Lo)

How does soy protein lower cholesterol?

Several mechanisms have been proposed for the way soy protein selectively lowers total cholesterol, LDL cholesterol and triglycerides. The body has specific mechanisms for ridding itself of excess cholesterol and it has been proposed that soy protein enhances these mechanisms. The first group are involved with clearing cholesterol from the body. Cellular receptors that pluck excess cholesterol from the blood are prompted to pick more of it up, thus speeding its elimination. (Baum) Soy also appears to increase the conversion of cholesterol into bile acids, a necessary step for its excretion. (Potter) Still yet, resorption of cholesterol in the colon appears to be blocked by soy fiber, its phytic acid and saponins.(Lo, Potter)

Soy also acts on hormonal systems that are involved with regulation of blood lipids. Thyroid, insulin to glucagon ratio and the estrogenic effects of soy proteins/isoflavones have all been studied. (Crouse, Potter) Thyroid hormones regulate turnover of lipids to produce energy and body heat. Both thyroid hormones and estrogens decrease circulating cholesterol by up-regulating cellular LDL receptors. (Guyton, 279- Ganong, 297) Insulin promotes fatty acid metabolism and keeps blood levels of triglycerides within normal range. In conditions of low insulin concentration (ie diabetes) fatty acid metabolism is disrupted and blood levels of triglycerides rise. This is a reason diabetics are more prone to CVD. Glucagon is a liver hormone that opposes insulin's effects, among them increasing blood lipids. Thus, the ratio between these two hormones has a dramatic effect on fat metabolism.

Soy isoflavones are phytoestrogens with weak estrogenic activity that appear to enhance the effects of soy protein by the means already mentioned. Isoflavones are antioxidants that according to some researchers prevent lesion formation and oxidation of LDL cholesterol. (Lichtenstein) The medical community has recognized the importance of adding soy protein to diets, and while doctors may be aware many forms are available in stores, they may not have information on brands and ways to prepare soy for their patients. (Goldberg)

History of use and kinds of soy foods available today

Use of vegetable proteins to reduce cardiovascular disease was first proposed by Russian scientists in 1909.(Sitori) And, since 1940 scientists have studied the effect of soy protein on cholesterol. (Liechtenstein) Today the blood lipid lowering effects of soy protein along with its naturally occurring components are well established. This led the Food and Drug Administration in 1999 to approve the claim that foods containing soy protein may reduce the risk of heart disease when included in a diet low in both saturated fat and cholesterol. In order to make the claim, a food must contain at least 6.25 grams of soy protein per serving and be low in saturated fat and cholesterol. Natural soy products such as soy beverages, protein powders, bars, tofu, tempeh and some soy-based meat and dairy alternatives meet the claim. To get the maximum benefit from soy based foods, be sure to select those with a high isoflavone content. You can avoid soy foods made from genetically altered seeds by choosing organic.

Other applications of soy proteins

While investigating the cholesterol lowering effects of soy protein, one group of researchers also checked on the protective effects of soy against cancer. They found that soy proteins prevent oxidative damage to DNA. Cancers are caused in part by damage to the cellular reproductive apparatus or DNA. §

References: ♦ "Coronary Heart Disease," by Simeon Margolis, M.D. Ph.D.Stephen Achuff, M.D. (Johns Hopkins White Papers 1998 pp 4-34).  ♦ "A New Theory of Heart Attack," by Anthony L. Komaroff, M.D., ed. (Harvard Health Letter, vol. 25, no. 2 December, 1999)  ♦ "The Thyroid Hormones" & "Insulin, Glucagon, and Diabetes Mellitus" by Arthur Guyton M.D. (Human Physiology and Mechanisms of Disease, pp 583-585 & 602-606)  ♦ "Energy Balance, Metabolism and Nutrition" & The Thyroid Gland" by William Ganong, M.D. (Medical Physiology, 1993 $32.00)  ♦ "FDA Allows Soy Health Claims on Labels" & "Soy Good for You! Disguises Are Made for Simple Bean" by Nancy Clark, M.S., R.D. ed. (Environmental Nutrition, September, December, 1999)  ♦ Anderson, J.W., et al. "Meta-Analysis of the Effects of Soy Protein Intake on Serum Lipids." New England Journal of Medicine 1995;333(5):276-82.  ♦ Crouse, J.R., et al., "A Randomized Trial Comparing the Effect of Casein with That of Soy Protein Containing Varying Amounts of Isoflavones on Plasma Concentrations of Lipids and Lipoproteins." Archives of Internal Medicine 1999; 159(17):2070-6.  ♦ Carroll, K.K., Kurowska, E.M., "Soy Consumption and Cholesterol Reduction: Review of Animal and Human Studies." Journal of Nutrition 1995 125(3 Suppl):594S-597S.  ♦ Wong, W.W. et al., "Cholesterol-Lowering Effect of Soy Protein in Normocholesterolemic and Hypercholesterolemic Men." American Journal of Clinical Nutrition 1998; 68(6 Suppl):138rS-1389S.  ♦ Gooderham M.H., et al., "A Soy Protein Isolate Rich in Genistein and Daidzein and its Effects on Plasma Isoflavone Concentrations, Platelet Aggregation, Blood Lipids and Fatty Acid Composition of Plasma Phospholipid in Normal Men." Journal of Nutrition 1996;126(8):2000-6.  ♦ Jenkins, D.J., et al., "Combined Effect of Vegetable Protein (soy) and Soluble Fiber Added to a Standard Cholesterol-Lowering Diet." Metabolism 1999;48(6):809-16.  ♦ Baum J.A., et al., "Long-Term Intake of Soy Protein Improves Blood Lipid Profiles and Increases Mononuclear Cell Low-Density-Lipoprotein Receptor Messenger RNA in Hypercholesterolemic, Postmenopausal Women." American Journal of Clinical Nutrition 1998;68(3):545-51.  ♦ Potter, S.M., et al., "Depression of Plasma Cholesterol in Men by Consumption of Bake3d Products Containing Soy Protein." American Journal of Clinical Nutrition 1993; 58(4):501-6.  ♦ Lichtenstein, A.H., "Soy Protein, Isoflavones and Cardiovascular Disease Risk." Journal of Nutrition 1998;128(10):1589-92.  ♦ Potter, S.M., "Overview of Proposed Mechanisms for the Hypocholesterolemic Effect of Soy." Journal of Nutrition 1995;125(3 Suppl):606S-611S.  ♦ Sirtori, C.R., et al., "Soy and Cholesterol Reduction: Clinical Experience." Journal of Nutrition 1995; 125(3 Suppl):598S-605S.  ♦ Grundy, S.M., Abrams, J.J., "Comparison of Actions of Soy Protein and Casein on Metabolism of Plasma Lipoproteins and Cholesterol in Humans." American Journal of Clinical Nutrition 1983;38(2):245-52.  ♦ Potter, S.M., et al., "Soy Protein and Isoflavones: Their Effects on Blood Lipids and Bone Density in Postmenopausal Women." American Journal of Clinical Nutrition 1998;68(6 Suppl):1375S-1379S.  ♦ Mitchell, J.H., Collins, A.R., "Effects of a Soy Milk Supplement on Plasma Cholesterol Levels and Oxidative DNA Damage in Men - A Pilot Study." European Journal of Nutrition 1999;38(3):143-8. 

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