Clinician Fact Sheet: Medicinal Foods
July 2000; Volume 3; s1-s2
Soy and Soybeans (Glycine max L., Family: Fabaceae)
Population-based studies have demonstrated a lower risk of and mortality from hormone-related cancers among Asian men and women who consume a traditional Asian diet compared to Western populations. Since traditional Asian diets are strikingly different from traditional Western diets, scientists have suggested that soy might play a role in the prevention of breast and prostate cancer.
Preparation
Soy is available in a number of foods and soy preparations, including soynut butter, soy flour, tempeh, tofu, soynut, whole cooked soybeans, roasted soynuts, soy milk, soy powder, textured soy protein isolate, and soy protein concentrate. Recently, purified isoflavonoid pills and powders have reached the market as well.
Dosage
• Soy is safe when consumed in typical food amounts.
• To reduce the risk of prostate cancer, a dose of 10 g/d of isoflavones is recommended.
• A dose of 25-50 g/d is recommended for cholesterol-lowering effects; 40 g/d for preventing osteoporosis; and 20-60 g/d for reducing the number and severity of hot flashes.
• A daily dose in excess of the equivalent of 100 mg of isoflavones is not recommended in women with a high risk of breast cancer.
Adverse Reactions
• Soy is used worldwide as a food, especially in Asian cuisine, without any known adverse effects.
• Allergy is rare.
• No documented cases were found regarding harmful effects associated with ingesting too much soy.
Interactions
• Theoretically, soy may competitively inhibit the effects of estrogen replacement therapy.
Contraindications
• Allergy to soy
• Excessive dose in women at high risk of breast cancer, as stated above
Recent Clinical Research
Data from population-based studies have suggested that a phytoestrogen-rich diet is protective against breast (evidence is strongest for premenopausal women), prostate, and bowel cancer, resulting in a significant increase in the consumption of phytoestrogen-rich foods and the promotion of dietary supplements containing isoflavone extracts.1
In a recent review of soy isoflavone studies, however, researchers at the University of North Carolina did not find a consistent, positive correlation between isoflavones and the prevention and treatment of breast, uterine, and colon cancers.2
In a 1997 randomized, controlled trial, urine samples from 144 women with newly diagnosed early breast cancer and 144 randomly selected controls were assayed for the isoflavonic phytoestrogens daidzein, genistein, and equol, and the lignans enerodiol, enterolactone, and matairesinol. A substantial reduction in breast cancer risk was found among women with a high intake of phytoestrogens, particularly the isoflavonic phytoestrogen equol and the lignan enterolactone.3
In another randomized, controlled trial, the effect of 60 g soy supplementation for 14 days on the normal breasts of 84 premenopausal women was examined.4 Serum concentrations were raised in patients following soy supplementation and nipple aspirate levels of genistein and daidzein were higher than paired serum levels both before and after soy supplementation. However, there was no significant increase in nipple aspirate isoflavone levels in response to soy and no effect of soy supplementation on breast epithelial cell proliferation, estrogen and progesterone receptor status, apoptosis, mitosis, or Bcl-2 expression. The researchers concluded that short-term dietary soy intake has a weak estrogenic effect on the breast.
In response to the association between greater breast cancer risk and higher levels of the carcinogenic metabolites of 17ß-estradiol, 4- and 16a-hydroxyestrogen, and lower amounts of anticarcinogenic metabolites, 2-hydroxyestrogens, a recent study tested the ability of genistein and daidzein to alter the metabolism of 17ß-estradiol to 2- and 16a-hydroxylated products.5 Eight premenopausal women were placed on a high soya diet (400 kcal from soymilk and 113-202 mg/d isoflavones) for a complete menstrual cycle. After a 4-month washout period, the women consumed the same diet, supplemented with isoflavone-free (< 4.5 mg/d) soymilk. Urine samples were collected daily for the entire cycle during each soya period and analyzed for daidzein, genistein, and 2- and 16a-hydroxyestrone. Subjects excreted measurable amounts of daidzein (11.6-39.2 mg/d) and genistein (2.9-18.2 mg/d) during the isoflavone-rich soya diet, but not during the isoflavone-free soya diet. The diet rich in isoflavones increased the cycle mean daily urinary excretion of 2-hydroxyestrone by 47%, but the mean daily excretion of 16a-hydroxyestrone did not change. The ratio of 2-hydroxyestrone to 16a-hydroxyestrone was 27% higher during the isoflavone-rich diet. The results of this study suggest that soya isoflavones increase the metabolism of endogenous estrogens to the protective 2-hydroxylate estrogens in women which may play a significant role in lowering 17ß-estradiol levels and the long-term risk for breast cancer.
Researchers at the M. D. Anderson Cancer Center in Houston recently examined phytoestrogen intake and prostate cancer risk in 83 Caucasians and 107 controls.6 To measure phytoestrogen intake, DietSys, the nutrient analysis program associated with the National Cancer Institute Health Habits and History Questionnaire (HHHQ) was modified to include components in foods on the basis of published values and the main foods contributing to phytoestrogen intake. Controls reported higher consumption of foods containing genistein, daidzein, and coumestrol and lower consumption of foods containing campesterol and stigmasterol. Multivariate analysis, after adjustment for age, family history of prostate cancer, alcohol consumption, and total calorie intake, showed an inverse association between coumestrol and daidzein and prostate cancer risk and a positive association between campesterol and stigmasterol and prostate cancer risk. Genistein showed a slight protective effect.
References
1. Davis SR, et al. Phytoestrogens in health and disease. Recent Prog Horm Res 1999;54:185-210.
2. Anderson JJ, et al. Health potential of soy isoflavones for menopausal women. Public Health Nutr 1999;2:489-504.
3. Ingram D, et al. Case-control study of phyto-oestrogens and breast cancer. Lancet 1997;350:990-994.
4. Hargreaves DF, et al. Two-week dietary soy supplementation has an estrogenic effect on normal premenopausal breast. J Clin Endocrinol Metab 1999;84:4017-4024.
5. Lu LJ, et al. Increased urinary excretion of 2-hydroxyestrone but not 16alpha-hydroxyestrone in premenopausal women during a soya diet containing isoflavones. Cancer Res 2000;60:1299-1305.
6. Strom SS, et al. Phytoestrogen intake and prostate cancer: A case-control study using a new database. Nutr Cancer 1999;33:20-25.
Additional Resources
Alternative Medicine Alert. Atlanta, GA: American Health Consultants; 1999;10:109-113, 1999;12:138-142.
Natural Medicines Comprehensive Database. Stockton, CA: Therapeutic Research Center, Inc.
The Review of Natural Products. St. Louis, MO: Facts and Comparisons.
July 2000; Volume 3; s1-s2
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