by Mark Messina, PhD
Extract from Vegetarian Nutrition & Health Letter Vol.1 n.5, published by Loma Linda University (Traduzione in Italiano).
Dr. Mark Messina has organized and chaired two international symposia on soy and disease prevention. He is the co-author of The Simple Soybean and Your Health and is a senior editor of Vegetarian Nutrition and Health Letter.
Most women agree: The decision about whether to use hormone replacement therapy is not an easy one, despite research showing clear advantages to doing so. Women who take estrogen after menopause have a lower risk of heart disease and are less likely to fracture bones. Estrogen can also help to minimize the discomfort of the "hot flashes" that some women experience during menopause. But because estrogen therapy has acute side effects and may also raise the risk for breast cancer, many women are wondering if the risk is worth the benefits. So it is not surprising that there is much interest among both consumers and researchers in finding alternative therapies with estrogen-like benefits. That search has led many researchers to a food that has long been a mainstay in the diets of vegetarians: the soybean.
Soybeans and Phytoestrogens
Soybeans are rich in plant estrogens and are one of the few food sources of a particular type of plant estrogen called isoflavones. These are compounds with a structure that is similar to estradiol, the main estrogen produced by a woman's ovaries. However, isoflavones are weak estrogens and are anywhere from one ten-thousandth to one onethousandth as potent as estradiol. Despite their lower potency they are likely to produce physiological effects because blood levels of isoflavones are so high in women (and men) who eat soyfoods. Drinking just two cups of soymilk or eating one cup of tofu produces blood levels of isoflavones that can be 500 to 1,000 times higher than typical estrogen levels in women.
Soybean isoflavones also possess some attributes that are separate from their estrogenic activity. For example, they inhibit the activity of enzymes that control cell growth and regulation. And one of the soybean isoflavones is believed to inhibit the growth of blood vessels that support tumor growth. Finally, isoflavones act as antioxidants. For these reasons and others, isoflavones may play a role in reducing the risk of certain cancers.
Initially, interest focused on the weakestrogenic potential of isoflavones, particularly as it relates to breast cancer. Some researchers suggested that weak estrogens like isoflavones actually function as anti-estrogens; that is, they inhibit effects of estrogen. Since estrogen raises the risk for breast cancer, it has been hypothesized that isoflavones may lower risk and that soyfood consumption contributes to the lower breast cancer mortality rates in Asian countries where soyfood consumption is high. Although the evidence that this is true is conflicting, there is some very encouraging research suggesting that soy isoflavones reduce risk for prostate cancer.
Are Soybeans the New Hormone Replacement Therapy?
As women go through menopause, the ovaries stop producing estrogen. This has several effects, some immediate and some long term. Changes in estrogen levels produce changes in temperature regulation that can result in the "hot flashes" and "night sweats" that many women experience. Also, decreases in blood estrogen levels can result in significant bone loss. Estrogen also protects against heart disease and is believed to be a major reason that young women rarely have heart attacks. Research points to possible roles for soy isoflavones in all of these areas.
Hot Flashes and Night Sweats: Japanese women are about two-thirds less likely than North American women to report that they have hot flashes [1]. One reason might be that they eat more soyfoods. So far, though, the results of studies are mixed. In one study, women who consumed soy experienced a 45% decrease in hot flashes, whereas the study group that didn't get soy had a 30% decrease in hot flashes [2]. That is, women who believed that they were consuming soy were likely to have fewer hot flashes. This suggests that soy is effective and also that there was a strong placebo effect.
Another study found that consuming soy didn't affect the number of hot flashes but the hot flashes were less severe [3]. Overall, results suggest that the effect of soy on hot flashes is modest. Since effects probably vary among individuals, it makes sense for menopausal women to try soyfoods.
Osteoporosis: Women can lose as much as 15 percent of their bone mass in the years surrounding menopause, an effect that is directly attributed to loss of estrogen. Hormone replacement therapy reduces the risk of bone fractures. Also, an experimental drug that has been used successfully for years to treat osteoporosis is remarkably similar to soybean isoflavones [4]. So it isn't surprising that isoflavones are being studied for their enhancing effects on bone density. Results from animal studies are largely favorable [5]. Although there hasn't been much research yet in humans, two preliminary studies indicate that soy isoflavones benefit bone health in postmenopausal women [6, 7]. An additional benefit is that many soy products including calcium-set tofu, calcium-fortified soymilk, tempeh, and textured vegetable protein, are rich in calcium.
Heart Disease: More than 30 years of research has shown that soy protein reduces blood cholesterol levels in people who have elevated cholesterol levels, but it isn't clear whether this effect is due to the protein alone or to protein plus isoflavones [8]. Hormone replacement therapy also reduces blood cholesterol levels, and it has other beneficial effects on heart disease risk as well. Similarly, recent research suggests that isoflavones may reduce heart disease risk in a number of different ways. For example, in an Australian study, isoflavone consumption improved artery elasticity in postmenopausal women, an effect also seen with estrogen therapy [9]. This increased elasticity reduces risk for heart disease.
Eating More Soyfoods is Easy and Delicious
Because the soybean is such an amazingly versatile food, there are countless possibilities for adding more soy to your diet. This makes eating more soyfoods one of the easier healthpromoting changes you can make. Research suggests that adding two servings of soyfoods to your daily menu might help to reduce risk for osteoporosis and heart disease and perhaps can help to case some of the side effects of menopause. One serving is equal to one-hall cup cooked soybeans, tofu, tempeh, or rehydrated TVPtm, one-fourth cup roasted soynuts, or one cup soymilk.
Here are some easy ways to incorporate more soyfoods into your diet.:
Soymilk is the liquid expressed from whole soaked soybeans. It can be used to replace cow's milk as a beverage or in most recipes. Soymilk is available unflavored or flavored with chocolate, vanilla, or carob. Look for brands that are fortified with calcium and vitamin D.
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Pour soymilk over hot or cold breakfast cereal.
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Add soymilk to pancake or waffle mix.
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Blend vanilla soymilk with frozen bananas and strawberries to make a frosty shake.
Tofu is a delicate soybean curd made by curdling fresh hot soymilk with a coagulant. It is available as a firm or soft product and also as silken tofu, a creamy, custard-like product.
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Marinate chunks of firm tofu in barbecue sauce and grill over hot coals or in your oven's broiler.
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Mash firm tofu with cottage cheese and herbs to make a savory sandwich spread.
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Create homemade tofu burgers by mixing mashed tofu with bread crumbs, chopped onions, and favorite seasonings. Form into patties and sauté in oil.
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Add taco seasoning to crumbled firm tofu, sauté with onions and serve in a taco shell with chopped tomatoes and lettuce.
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Purée soft tofu in a food processor and season with fresh lemon juice, parsley and salt for a cholesterolfree topping for baked potatoes.
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Blend 10 ounces of soft tofu with 2 cups of melted chocolate or carob chips, pour into a graham cracker pie crust and chill.
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Purée silken tofu with cooked spinach. Add to sautéed onions with vegetable broth and seasonings for a fast cream of spinach soup.
Tempeh is a chunky, tender cake of soybeans that is traditional to the cuisine of Indonesia.
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Crumble and pan fry tempeh and add to your favorite chili recipe.
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Marinate chunks of tempeh in olive oil and fresh lemon juice and grill or broil; serve on French rolls.
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Marinate tempeh in soy sauce and add to soups and stews.
Textured Soy Protein is also called textured vegetable protein or TVP. Pour 7-8 cups boiling water over 1 cup dried TVP to produce a replacement for ground beef in many recipes.
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Add rehydrated TVP to spaghetti sauce, chili, tacos, or sloppy joes.
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Make homemade veggie burgers by combining TVP with cooked beans, chopped onions and celery and favorite herbs. Add f tour, oatmeal, or bread crumbs if necessary to help hold the burgers together, then form into patties and fry.
Whole soybeans can be used in soups and stews. Soak the uncooked beans for eight hours (or overnight). Then simmer for two hours or until tender. Add the cooked beans to a barbecue-flavored tomato sauce and serve over rice.
Whole roasted soynuts are a crunchy topping for salads.
Conclusion: Estrogen or Soyfoods?
From a public health viewpoint, hormone replacement therapy has been a failure because most women do not use it long enough to reap the most important clinical benefits. The acute side effects and concerns about breast cancer make long-term use of estrogen therapy a subject of debate. Are soyfoods a suitable alternative? We don't have the answer to that question yet, but the evidence suggests that they might offer some of the same benefits as estrogen. More evidence is needed before we know for certain. However, soyfoods are certainly an important addition to the diets of women who decide against hormone replacement therapy.
How much soy should you eat? It's not easy to establish a specific recommendation. It appears that it takes at least 25 grams per day of soy protein to lower cholesterol-this translates to two to three servings of traditional soyfoods a day. For cancer, osteoporosis, and hot flashes, studies have used doses of between 45 and 90 mg of soy isoflavones, which also translates to about two to three servings of soyfoods per day. For comparison, people in Japan typically consume about 30 mg of isoflavones per day. Good sources of isoflàvones include whole soybeans, textured vegetable protein, soy flour, soy protein isolate, tempeh and miso. The isoflavone content of soymilk varies quite a bit but many brands are reasonably good sources. Soy sauce and soy oil are devoid of isoflavones, however.
References
- Experimental Gerontology 29:307, 1994.
- Obstet Gynecol 91; 6, 1998
- Second International Symposium on the Role of Soy in Preventing and Treating Chronic Disease, Brussels, Belgium, 1996, page 40.
- Calcified Tissue International 61; S5, 1997
- J Nutr 126;161, 1996
- Second International Symposium on the Role of Soy in Preventing and Treating Chronic Disease, Brussels, Belgium, 1996, page 21.
- Second International Symposium on the Role of Soy in Preventing and Treating Chronic Disease, Brussels, Belgium, 1996, page 44.
- N Engl J Med 333:276, 1995
- Arteriosder Throm Vasc Biol 17; 3392, 1997
