|
BACK
|
printed in: Taste for Life; April, 2001 |
|
FROM THE HUMBLE SOYBEAN By Marcia Zimmerman, M.Ed., C.N.
|
|
|
BEANS ARE GOOD FOR YOU! At least that's what medical researchers believe- and the humble little soybean has captured their interest more than any bean. What really intrigues scientists? Soybeans modify our body's defense against heart disease, cancer, osteoporosis and hormone-related problems. The interest in soybeans and disease prevention began when scientists started to question why people eating Asian and other traditional diets had lower rates of heart disease, several types of cancer, osteoporosis and hormonal problems. Epidemiological evidence suggested that low fat consumption, coupled with high fiber, legume based diets protected these people from the killer diseases we westerners commonly suffer. (1 Messina, 2 Persky) Today, enough evidence has accumulated from population based studies around the world, to support the theory that soy products and their constituents, are largely responsible for the lower rates of chronic diseases seen in many areas. (3 Moyad, 4 Tham) Researchers debate over which soy constituents are most responsible for the anti-cancer effects of this remarkable bean. Research teams from leading American universities, among them Harvard, University of Michigan, Stanford, UCLA, University of Pennsylvania, and the University of Alabama, have investigated soy's many constituents. Each contributed a piece to the puzzle of how soy helps prevent disease. (Note: these are discussed in detail & cited below) They all agree that there are several responsible for the diverse health benefits of soy. The soy constituents most studied are its protein, isoflavones, lignans, saponins and protease inhibitors. Soy Protein and Prostate Cancer The Harvard University group studied soy protein and prostate cancer. They compared the effects of soy protein, casein (milk protein) and concentrated soy isoflavones (phytoestrogens) upon transplanted prostate cancer cells in mice. While casein showed little inhibitory effect, soy protein reduced tumors by 11%, and soy isoflavones inhibited the tumors by 30 %. The greatest effects were seen with soy protein plus isoflavones at specified concentrations. Tumor growth was cut by up to 40%. The scientists noted that soy not only directly blocked growth of tumor cells, but cut off their blood supply which curtailed growth as well. (5 Zhou) The phytoestrogens are estimated to possess only 0.1 % the strength of estrogens, but it's enough to inhibit several enzymes involved in cell growth and regulation.(6 Messina) Low Methionine-Breast & Endomentrial Cancer Researchers at Wake University in Winston-Salem North Carolina looked at another way soy protein reduces tumor growth; in this case breast and endometrial cancer. Soy protein is low in the essential amino acid methionine, a fact that led to soy protein being maligned until recently as "incomplete" - therefore less desirable than animal proteins - in the minds of western nutritionists. Ironically, its low methionine levels may be one way soy protects against cancer. Tumors have an absolute growth requirement for methionine. It seems that healthy cells will grab whatever methionine is available leaving tumor cells wanting.(7 Cline) Genistein and Breast Cancer In New York, a research team at Rockefeller University studied how genistein, a soy isoflavone, exerts its anti-cancer effects. They chose breast tissue cultures for their investigation and induced cancer in the cells by exposing them to a well-known carcinogen. Untreated cells began the cancer process but those pre-treated with phytochemicals had up to a 65% reduction in cellular progression toward cancer. They found that phytochemicals halted cancer cell metabolism in two ways: by modifying cancer cell metabolism and increasing cell death, a process called apoptosis..(8 Katdare) The Harvard team also observed these actions. (5 Zhou) Genistein, Daidzein, Soy Fiber & Breast Cancer >Steve Barnes, PhD and colleagues at the University of Alabama have done several studies defining the mechanisms whereby the isoflavones genistein and daidzein reduce hormone dependent cancers. They found that genistein and daidzein alter the development of two kinds of breast tissue in mice. As the mice matured, they developed more tubules that are associated with a lower progression to cancer. Terminal end buds (TEBs) that are more susceptible to carcinogenesis were not as prevalent as in control animals. The scientists proposed that prepubertal exposure to soy foods was important for early gland differentiation and lower breast cancer risk. And this may be important in humans as well, according to the University of Alabama researchers.(9,10,11,12,21 Barnes, Brown, et al) Dave Heber, MD and associates at UCLA have investigated the effects of low fat diet enriched with soy foods in both normal and high breast cancer risk women. Their research, besides being on humans, expands our understanding of soy's effects on hormones. Soy fiber and a low-fat diet similar to those eaten by Asians, lowers estrogen levels and lengthens the menstrual cycle. The implications for this are less estrogen, less opportunity for hormone dependent cancers to begin or progress.(13,14 Heber) Protease Inhibitors and Other Factors At the University of Pennsylvania Medical School, researchers investigated protease inhibitors, phytic acid, beta-sitosterol, trypsin inhibitor and saponins. What is amazing about this group of substances, is that for some time they have been considered "anti-nutrients" by western nutritionists and a reason to avoid soy beans. Protease and trypsin inhibitors are proteins that block the enzymes that digest proteins and scientists have known for some time that protease inhibitors lock-up enzymes that activate specific cancer causing genes. Enough evidence has accumulated in the opinion of the Pennsylvania group, to warrant supplementation of the diet with soy foods. (15 Kennedy) While blocking the action of some enzymes may be good news for preventing cancer, what effect does this have on our enzymatic digestion of soy proteins? Must one choose between protection against cancer and gastro-intestinal discomfort from poorly digested soy protein? According to Mark Messina, Ph.D. a well-known soy researcher, very small amounts of protease inhibitors are effective - about the amount remaining after soy foods are cooked or processed and most of the proteases are deactivated. (16 Messina, 65) |
Phytic acid, Colon, Breast & Other Cancers Phytic acid is another soy component that has shared the bad reputation of an anti-nutrient because it binds minerals, primarily calcium and iron. Yet,iron-binding may be another soy plus against colon, breast and perhaps other cancers. Cancer cells rely on iron for their growth which is slowed if not enough iron is available. Too much iron is also bad news for the cardiovascular system. Iron can act as a free-radical - attacking vascular tissue and oxidizing LDL cholesterol - generally creating havoc in the circulatory system. Phytic acid is found in all legumes, including soy, in addition to whole grain wheat, rye, barley, oats, wild rice and corn. (16 Messina, 66-68) Eating high complex carbohydrate foods along with soy offer even more protective benefits. (for more on the effects of soy and cardiovascular disease, see the March Issue of Taste for Life) Soy Saponins & Colon Cancer have not been studied with the intensity of the other phytochemicals, in part because they were difficult to extract. A novel extraction technique has now allowed researchers at the University of Illinois at Urbana-Champaign to isolate soy saponins, thus enabling study of their biological activity. The scientists were happy to find that soy saponins suppressed the activity of the major human dietary carcinogen that we all are exposed to when we cook meat. Soy saponins may well be a key factor in reversing the high rate of colon cancer in the US, according to the researchers. They also found that soy saponins could arrest tumor growth following chemotherapy making them useful as adjuvant therapy.(17 Plewa) A word of caution What does all of this laboratory and animal research mean for you? Should you rush out and start adding soy foods to your diet and begin taking soy supplements to protect against cancer? Maybe not! We should consider what some scientists have reported about the growth enhancing effects of genistein on a particular breast cancer cell line. Research teams from the Medical University of South Carolina, Charleston (18 Willard), Michigan State University, East Lansing (19 Helferich), and the University of Minnesota, St Paul have shown that in some animal models, genistein can actually increase growth of breast cancer cells. When asked about this, Dr. Mindy Kurzer from the University of Minnesota (20 Kurzer), summed up the current thinking within the scientific community; that the data both for and against genistein are "theoretical" in basis, and lack good human clinical evidence. According to Dr. Kurzer and Dr. Bill Helferich from Michigan State, both authors of some of the research, there is little to be concerned about in consuming soy foods. The benefits of eating soy far outweigh any risk to women with breast cancer. As Dr, Helferich puts it, you can't eat enough soy to get to the levels it took to produce adverse effects in mice. All told, the safest bet if you are a cancer survivor, is to stick to soy foods and perhaps avoid the genistein supplements, unless your doctor recommends them. (21 interviews) So how much soy should you eat to protect against cancer? Soy expert Mark Messina offers some sensible guidelines: eat foods you enjoy by learning to make easy substitutions with soy in your diet, add supplements if and when you can't fit two to three daily servings of soy into your diet. All the researchers interviewed agree, soy as part of a diet high in fruits and vegetables and low in saturated fats offers the best protection against breast, prostate and colon cancers. And, the sooner in life you begin to eat this way, the better chance you have of cancer protection.(21) Why Scientists Recommend You Add Soy foods to Your Diet Research groups from the University of Michigan, Ann Arbor and Stanford University, emphasize that population based studies from around the world warrant increased consumption of soy foods by those eating a western diet.(3 Moyad, 4 Tham) Researchers at Children's Hospital Medical Center in Cincinnati offer evidence that adding soy protein (60 g /day) with isoflavones may help protect against cancer by altering the hormone balance and length of the menstrual cycle. (22 Setchell) Furthermore, ethnic groups within the US population who stick to their traditional diets, both Asian and Latino, have reduced incidence of breast and prostate cancer, even when they are overweight - a risk factor for hormone-related cancers such as these. (23 Horn-Ross) Are some soy foods better than others Some questions have been raised among nutritionists about the digestability of soy products and whether you can obtain high enough levels of isoflavones to be of benefit. To check this, a team from the Cancer Research Center in Hawaii measured levels of isoflavones circulating in blood serum following ingestion of soymilk. If isoflavones in food are not absorbed into the system or the liver isn't able to covert them into usable forms, the evidence for cancer prevention seen in the laboratory isn't useful. They found that although there was a wide variance in plasma levels of isoflavones among individuals, all had increased levels of the converted, biologically active forms after drinking soy beverages. This means that drinking and cooking with soy beverages is one easy way of increasing your protective isoflavones.(24 Franke) As for individual differences in absorbing isoflavones from soy, some may absorb daidzein better than genistein - at least that's what a research team at Iowa State University reported. They noted that although 85% of isoflavones are degraded in the intestines, the uptake is great enough to offer health protective effects, regardless of individual differences. (25 Xu) What all this means? Beans are indeed good for you and soy heads the top of the list in providing many important health benefits. § |
|
References: |
|
Copyright©, 2001 by The Zimmerman Group, Inc. All rights reserved. Except as permitted under the Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written permission of the publisher. |
|
BACK
|