Every day, an increasing number of new soy products appear on store shelves. Some tout soy protein content, while others claim a hefty dose of isoflavones. To help your customers avoid confusion and better understand the health benefits of their soy choices, a review of recent research is in order.
Soybeans have the highest concentration of isoflavones in the legume family. Isoflavones are phytochemicals, or plant compounds, that can help lower cholesterol; reduce the risk of heart disease, stroke, certain cancers and osteoporosis; and relieve menopause symptoms.1 Isoflavones are also phytoestrogens, or plant hormones, that resemble human estrogen in chemical structure, but with weaker effects. Scientists have identified 12 soy isoflavones; the primary two are genistein and daidzein.2 They work in various ways against many common health problems.
Heart Disease And Stroke
There is strong evidence that soy isoflavones reduce coronary heart disease and stroke risk by lowering total and low-density lipoprotein cholesterol levels and by preventing arterial plaque buildup. Isoflavones, particularly genistein, provide this antioxidant plaque-fighting protection,3,4 and when taken with soy protein, can increase high-density lipoprotein cholesterol.5
Statistics from a meta-analysis of 38 human clinical trials show soy causes a decrease in LDL and an increase in HDL cholesterol. An average daily intake of 47 g soy protein from either isolated soy protein, textured soy protein or a combination of the two reduced total cholesterol 9.3 percent, LDL cholesterol 12.9 percent, triglycerides 10.5 percent and increased HDL cholesterol 2.4 percent. Researchers noted the soy protein in these studies contained isoflavones, which most likely contributed to these improved cholesterol profiles.6
However, it seems isoflavones alone do not improve cholesterol profiles. An eight-week study of healthy men and women between the ages of 35 and 69 resulted in no significant differences in cholesterol levels between groups taking a placebo or 55 mg isoflavone tablets.7
The synergistic effect of isoflavones and soy protein was demonstrated in studies involving men and pre- and postmenopausal women. In one nine-week study, researchers found that when men and women with elevated cholesterol levels took isolated soy protein containing 62 mg isoflavones, their total and LDL cholesterol dropped 10 percent. Those given 37 mg isoflavones saw an 8 percent drop, while those given 3 mg had no significant changes.8
In a trial of healthy premenopausal women ages 18 to 35, researchers used isolated soy protein powder containing three levels of isoflavones (10 mg, 64 mg or 128 mg). The women took one of the protein powders, respectively, for each of three menstrual cycles plus nine days. The preparation with the most isoflavones lowered LDL cholesterol by 10 percent and total cholesterol by 6.5 percent, increased HDL cholesterol by 3.4 percent, and reduced the ratios of total to HDL cholesterol by 10.2 percent and LDL to HDL cholesterol by 13.8 percent.9
FDA's Soy Health Claim
In 1999, the FDA concluded that eating at least 25 g of soy protein daily can help lower total cholesterol and LDL cholesterol. They gave food manufacturers permission to claim a link between soy and reduced heart disease risk on soy-based food labels. However, to get soy's full heart-health effect, soy consumption must coincide with a diet low in saturated fat, trans fat and cholesterol.
To qualify for this health claim, a food must contain at least 6.25 g soy protein per serving (one-fourth the effective level of 25 g per day) and have less than 3 g total fat, 1 g saturated fat, 20 mg cholesterol and 480 mg sodium. Foods eligible include soy beverages (soymilk), tofu, tempeh, soy-based meat analogs and some baked goods. Foods made with whole soybeans may also qualify if they don't contain any added fat.27
—M.N.G.In another trial of healthy 45- to 70-year-old postmenopausal women, researchers again used isolated protein powders with varying amounts of isoflavones—7 mg, 65 mg and 132 mg respectively—for each of three 93-day periods. Their results were similar to those of the previous study. Compared with the control diet, the high-isoflavone diet reduced LDL cholesterol 6.5 percent and total cholesterol 3.1 percent, increased HDL cholesterol 1.5 percent, and lowered the ratios of total to HDL cholesterol 4.1 percent and LDL to HDL 7.7 percent. The low-isoflavone diet reduced LDL cholesterol 5.3 percent and total cholesterol 2 percent, increased HDL cholesterol 3 percent, and lowered the ratios of total to HDL cholesterol 5 percent and LDL to HDL 8.5 percent. 10 Results of these studies suggest soy protein alone cannot lower cholesterol but must be paired with isoflavones.
Isoflavones also promote healthy blood vessels and improve blood pressure.11 In a three-month study of healthy men and women ages 50 to 75, researchers showed soy protein containing daidzein and genistein reduced hypertension. Participants received either isolated soy protein powder containing 40 g soy protein and 118 mg isoflavones or a placebo. On average, those receiving soy reduced their systolic blood pressure by 7.5mmHg and diastolic blood pressure by 4.3mmHg.12
In another three-month study involving mild to moderately hypertensive men and women, researchers found a greater reduction in blood pressure with soymilk than cow's milk. Participants who drank soymilk instead of cow's milk reduced systolic blood pressure by 18.4 mmHg and diastolic blood pressure by 15.9 mmHg.13
Isoflavones may hinder hormone-related cancers, such as breast and prostate, by inhibiting the growth and spread of both estrogen-dependent and -independent cancer cells.2 Genistein can restrain certain enzymes that convert normal cells to cancer cells and activate the body's natural killer cells. Thus, the immune system can fight cancer by preventing tumor development and progression.14 Genistein also disrupts the way cancerous cells and tumors receive nutrients and oxygen. They block new blood cell and blood vessel growth needed to support tumor proliferation.15
Isoflavones may offer protection against the detrimental effects of human estrogen. Before the body can use estrogen, it must bind to proteins called estrogen receptors. Isoflavones can serve as anticarcinogenic estrogen-receptor blockers. Their similar chemical makeup enables the isoflavones to fit into receptor sites where the endogenous estrogen usually attaches. This prevents the more powerful human estrogen, which can promote cancerous tumors, from binding to these receptors and exerting its full effect.2 Isoflavones with soy protein can lengthen the menstrual cycle, which also reduces estrogen exposure. Reducing estrogen exposure can, over the years, decrease breast cancer risk.16
There is hypothetical evidence that large amounts of isoflavones may stimulate tumor development, but there is no evidence that eating soy products is a problem for women with breast cancer or those with an increased risk for the disease. Until more is understood, these women should be cautious with pure isoflavone supplementation.17
Women on the estrogen-dependent breast cancer drug tamoxifen may want to limit their isoflavone consumption. Tamoxifen is similar to genistein—both combat tumors containing estrogen receptors, work on estrogen-positive and -negative cells and attach to estrogen-receptor sites. To minimize interference, researchers recommend restricting soy intake while on this medication.18
Soy also shows promise against prostate cancer. Researchers found that diets rich in animal-based foods and saturated fat increase prostate cancer risk, and other hormone-sensitive cancers, by raising sex hormone levels (androgens such as testosterone in men and estrogens in women). Prostate cancer, which is associated with testosterone levels, is often treated by eliminating its production either surgically or chemically. Clinical evidence indicates soy can reduce testosterone levels with no ill effects.19 Isoflavones also concentrate in prostate tissue, inhibiting cancer growth and spread by preventing blood vessel growth, which starves tumors to death.15
Soy isoflavones may also ward off osteoporosis. They help build and maintain strong bones by increasing bone mineral content, density and mass. Bones are living tissue in a continuous state of reformation. Osteoblasts build bone, while osteoclasts break down and resorb bone. When osteoclasts dominate, bone breakdown starts outpacing bone formation, causing bone density to decline. Daidzein and genistein can protect bones by suppressing this osteoclastic action.20
In a three-year study of 19- to 86-year-old women, researchers discovered diets containing high levels of isoflavones and soy protein reduced bone loss and increased bone mineral density in postmenopausal women, but not premenopausal women. Researchers suspect the differences in outcomes may result from premenopausal women's naturally higher estrogen levels, which influence osteoblastic and osteoclastic activity. They noted that phytoestrogen protects bones similarly to the way human estrogen does. Because postmenopausal women have reduced estrogen levels, isoflavones can help stimulate bone mineral density, minimize bone loss and increase bone strength, especially in the lumbar spine and hip region.20
Other studies also confirm soy's cardiovascular and bone-protecting benefits for women with estrogen deficiency. In a 12-week study, daily intakes of soyfoods containing 60 mg to 70 mg isoflavones and 40 g soy protein had positive effects on LDL oxidation, HDL cholesterol and bone turnover in healthy postmenopausal women.21
In a 24-week study, soy protein isolate containing 80.4 mg isoflavones per day minimized bone loss in perimenopausal women. These women experienced significant increases in both bone mineral density and content in their lumbar spines, whereas the women taking the low isoflavone dose (4.4 mg per day) or placebo (whey protein) saw no effect.22
Concern about hormone-replacement therapy's health risks has many women exploring natural ways to cope with hot flashes, night sweats, insomnia, irritability, mood swings, headaches, anxiety and depression. Isoflavones are a safe alternative to HRT for relieving mild menopausal symptoms. Results of recent studies show women can reduce their hot flashes 40 percent to 55 percent by adding soy to their daily diets for 12 weeks.16,23 Results of other studies suggest that although 30 mg isoflavones with soy protein per day reduces hot flashes 30 percent to 50 percent, only 10 percent to 20 percent is attributable to the isoflavones. The rest, researchers theorize, is the placebo effect.17
In a three-month study, researchers found a daily dose of 114 mg isoflavones (containing 6 percent genistein and 36 percent daidzein) without soy protein did not successfully relieve hot flashes, depression, anxiety or fatigue or improve self-confidence in postmenopausal women with a history of breast cancer.24
However, in another study, when soy protein was included, isoflavone supplementation effectively improved hot flashes, insomnia, nervousness, melancholia, headache and other menopausal symptoms. In a four-month study, researchers used a 100 mg daily isoflavone supplement consisting of 60 percent soy protein and 40 percent isoflavones—23.3 mg genistein, 6.2 mg daizein and 3.8 mg glycitein. The placebo contained glucose and purified soy protein without isoflavones. The women taking the isoflavone-rich formula experienced a significant reduction in their menopausal symptoms. Compared with the placebo group and the pretreatment baseline, their overall symptoms were reduced by 40 percent and 44 percent, respectively. Researchers speculated the soy protein and higher contents of genistein and daidzein contributed to the outcome.25 Effects on hot flashes are quite variable, however. There have been both positive and negative results in other studies, with the positive effects showing no more than a 50 percent reduction.
These findings demonstrate that isoflavones appear to be most effective when taken with soy protein. Together they may help lower cholesterol, prevent heart attack and stroke, reduce high blood pressure, protect against hormone-related cancers, fight osteoporosis and relieve mild menopausal symptoms. Because isoflavones and soy protein work best in combination,17 the daily recommendation is between 30 mg and 50 mg isoflavones26 with 25 g soy protein.27 Influencing bone health and easing menopausal symptoms may require higher daily amounts of isoflavones.26,28
Monique N. Gilbert, B.Sc., is a health advocate, freelance writer and author of Virtues of Soy: A Practical Health Guide and Cookbook (Universal Publishers, 2001). E-mail her at [email protected].
1. Tham DM, et al. Potential health benefits of dietary phytoestrogens: a review of the clinical, epidemiological, and mechanistic evidence. J Clin Endocrinol Metab 1998 Jul;83(7):2223-35.
2. Messina MJ, et al. Soy for breast cancer survivors: a critical review of the literature. J Nutr 2001 Nov;131(11 Suppl):3095S-108S.
3. Kapiotis S, et al. Genistein, the dietary-derived angiogenesis inhibitor, prevents LDL oxidation and protects endothelial cells from damage by atherogenic LDL. Arterioscler Thromb Vasc Biol 1997 Nov;17(11):2868-74.
4. Wiseman H, et al. Isoflavone phytoestrogens consumed in soy decrease F(2)-isoprostane concentrations and increase resistance of low-density lipoprotein to oxidation in humans. Am J Clin Nutr 2000 Aug;72(2):395-400.
5. Sanders TA, et al. Moderate intakes of intact soy protein rich in isoflavones compared with ethanol-extracted soy protein increase HDL but do not influence transforming growth factor beta(1) concentrations and hemostatic risk factors for coronary heart disease in healthy subjects. Am J Clin Nutr 2002 Aug;76(2):373-7.
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8. Crouse III JR, 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. Arch Intern Med 1999 Sep 27;159(17):2070-6.
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10. Wangen KE, et al. Soy isoflavones improve plasma lipids in normocholesterolemic and mildly hypercholesterolemic postmenopausal women. Am J Clin Nutr 2001 Feb;73(2):225-31.
11. van der Schouw YT, et al. Higher usual dietary intake of phytoestrogens is associated with lower aortic stiffness in postmenopausal women. Arterioscler Thromb Vasc Biol 2002 Aug 1;22(8):1316-22.
12. Teede HJ, et al. Dietary soy has both beneficial and potentially adverse cardiovascular effects: a placebo-controlled study in men and postmenopausal women. J Clin Endocrinol Metab 2001 Jul;86(7):3053-60.
13. Rivas M, et al. Soy milk lowers blood pressure in men and women with mild to moderate essential hypertension. J Nutr 2002 Jul;132(7):1900-2.
14. Zhang Y, et al. Daidzein and genistein glucuronides in vitro are weakly estrogenic and activate human natural killer cells at nutritionally relevant concentrations. J Nutr 1999 Feb;129(2):399-405.
15. Tosetti F, et al. Angioprevention: angiogenesis is a common and key target for cancer chemopreventive agents. FASEB J 2002 Jan;16(1):2-14.
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18. Ju YH, et al. Dietary genistein negates the inhibitory effect of tamoxifen on growth of estrogen-dependent human breast cancer (MCF-7) cells implanted in athymic mice. Cancer Res 2002 May 1;62(9):2474-7.
19. Gardner-Thorpe D, et al. Dietary supplements of soya flour lower serum testosterone concentrations and improve markers of oxidative stress in men. Eur J Clin Nutr 2003 Jan;57(1):100-6.
20. Mei J, et al. High dietary phytoestrogen intake is associated with higher bone mineral density in postmenopausal but not premenopausal women. J Clin Endocrinol Metab 2001 Nov;86(11):5217-21.
21. Scheiber MD, et al. Dietary inclusion of whole soy foods results in significant reductions in clinical risk factors for osteoporosis and cardiovascular disease in normal postmenopausal women. Menopause 2001 Sep-Oct;8(5):384-92.
22. Alekel DL, et al. Isoflavone-rich soy protein isolate attenuates bone loss in the lumbar spine of perimenopausal women. Am J Clin Nutr 2000 Sep;72(3):844-52.
23. Albertazzi P, et al. The effect of dietary soy supplementation on hot flushes. Obstet Gynecol 1998 Jan;91(1):6-11. Erratum in: Obstet Gynecol 2001 Oct;98(4):702.
24. Nikander E, et al. A randomized placebo-controlled crossover trial with phytoestrogens in treatment of menopause in breast cancer patients. Obstet Gynecol 2003 Jun;101(6):1213-20.
25. Han KK, et al. Benefits of soy isoflavone therapeutic regimen on menopausal symptoms. Obstet Gynecol 2002 Mar;99(3):389-94.
26. Ilich JZ, et al. Nutrition in bone health revisited: a story beyond calcium. J Am Coll Nutr 2000 Nov-Dec;19(6):715-37.
27. Henkel J. Soy: health claims for soy protein, questions about other components. FDA Consumer. May-June 2000.
28. [No authors listed]. The role of isoflavones in menopausal health: consensus opinion of The North American Menopause Society. Menopause 2000 Jul-Aug;7(4):215-29.
Natural Foods Merchandiser volume XXIV/number 11/p. 38-40