Worldwide statistics indicate that Japan enjoys a 75 percent decreased incidence of breast cancer and an 80 percent decreased incidence of prostate cancer compared to the United States, Canada and many other developed countries.
The question many patients ask regarding soy intake is, "How much soy, soy protein and/or soy isoflavones should I consume to derive the potential health benefits?" To help answer this question, a recent research paper published by M. Messina, et al., looked at soy intake surveys from Japan, China, Hong Kong and Singapore. Surveys of individuals living in Japan suggest that in 1961, soy protein accounted for 13.4 percent of the total daily protein intake, on average. As of 2002, this figure dropped to 9.5 percent, due to the steady increase over the past four decades of more animal protein foods in the Japanese diet. However, total soy intake has remained consistent at about 65 gm per day since the early 1960s.
Older Japanese, 60-69 years of age, consume 91.7 gm of soy each day on average, which is significantly higher than the population average. As such, one may conclude that the traditional Japanese diet contains approximately 90 gm of soy per day, which yields 6-11 gm of soy protein and 25-50 mg of soy isoflavones.3 A number of investigators have suggested this is the level of intake that may be prudent to reduce risk of certain degenerative diseases. The daily average of soy isoflavone intake across the entire Japanese population is reported to be approximately 35 mg per day.3
How Much Isoflavone Should One Consume?
Based on reports such as the one by Messina, et al., I routinely recommend to both my male and female adult patients who are free from reproductive disease and soy allergies or sensitivities, that they ingest at least 35-50 mg of soy isoflavones per day. This can be achieved by consuming any combination of soy foods, functional foods (e.g., soy protein shakes) and/or supplements containing soy extract, as a means to support health and help prevent certain degenerative diseases. Many health experts suggest an intake of 50-75 mg per day of soy isoflavones for health optimization.
Patients with a previous history of reproductive-organ cancers deserve special consideration when making recommendations about soy and soy isoflavone intake. Some studies indicate that women with breast cancer may slow or reverse tumor growth by ingesting 200 mg per day of soy isoflavones via supplementation. Men with prostate cancer have been shown to benefit from soy isoflavone supplementation of 100 mg per day, which resulted in a slower rise in serum prostate-specific antigen (PSA) levels. These findings require confirmation from further studies; however, there is the suggestion that certain patients may benefit from higher intakes of soy isoflavones as a therapeutic measure.1,4
To help control hot flashes and other symptoms of menopause, and to slow bone demineralization associated with menopause, some studies indicate that 50-100 mg of isoflavones are beneficial as a therapeutic intervention.2
Anti-Cancer Properties of Soy
In regard to the prevention and management of reproductive-organ cancers, soy isoflavones were first shown to have anti-estrogenic effects in rodents in 1966. Most recently, isoflavones have been shown to bind to beta-estrogen receptors. Some data indicate that when these receptors are activated by isoflavones, it inhibits the proliferation of prostate cancer cells and estrogen-stimulated growth of breast cancer cells.3 Many other anticancer mechanisms for isoflavones have also been reported in the scientific literature. For example, soy isoflavones have been shown to:
- lower endogenous estrogen levels;
- stimulate the production of sex hormone-binding globulin by the liver, leading to less free estradiol, reducing the amount of estrogens available for binding with estrogen receptors;
- inhibit the enzymes that promote cell proliferation (protein tyrosine kinase, DNA topoisomerase and ornithine decarboxylase);
- inhibit angiogenesis, which prevents the building of life-supporting blood vessels in and around malignant tumors; and
- provide antioxidant defence and induce cell differentiation.
| Soy Food | Protein Content Per 3.3 Ounces (100 gm) | Isoflavones Content Per 3.3 Ounces (100 gm) |
| Tofu | 7-8 gm | 28 mg |
| Soy milk | 4-5 gm | 10 mg |
| Soy nuts, dry roasted | 39.6 gm | 9 mg |
| Soy beans (cooked) | 16.6 gm | 54 mg |
Furthermore, the weak estrogenic potential (more than 1,000 times weaker than estradiol) of soy isoflavones do not elicit a strong estrogenic response, and thus have an anti-estrogenic effect that tends to inhibit the growth and proliferation of estrogen-dependent cancer cells, as demonstrated by the research of A. Molteni, et al.5,6,7
Soy Protein and Soy Isoflavone in Common Soy Foods
In regard to soy protein and soy isoflavone content of common soy foods, the table above provides a quick reference guide for practitioners.3 Many functional foods made of soy, including protein shakes, bars (e.g., chocolate bars with soy), sausage, hotdogs and cheese, list the protein and isoflavone content on the label. It is not uncommon for a 3.3-oz soy hotdog to contain 15 mg of isoflavones, or for a 3.3-oz soy burger to contain 15-25 gm of isoflavones. Some soy protein bars contain up to 60 mg of isoflavones in 2 oz. Soy cheeses often contain 8 mg of isoflavones in 3.3 oz.
Summary
Soy foods are unique in that they are the only nutritionally relevant, naturally occurring food source of isoflavones. People around the world who rely on soy as a dietary staple have significantly lower incidence of reproductive organ cancers and other health problems. Investigative research has demonstrated that soy isoflavones and other constituents of soy foods exhibit numerous anticancer properties, and several clinical trials have demonstrated the ability of soy isoflavone supplementation to slow the growth of breast and prostate cancer.
For practitioners who are convinced that soy and isoflavones are an important aspect of health optimization, it appears that encouraging patients to consume at least 35-50 mg of soy isoflavones per day is a worthy target. The table and other data included in this discussion are intended to provide practitioners with a reference from which to help patients achieve this goal.
References
- Sartippour MR, Rao JY, Apple S, et al. A pilot clinical study of short-term isoflavone supplements in breast cancer patients. Nutr Cancer, 2004;49(1):59-65.
- Messina M. Legumes and soybeans: overview of their nutritional profiles and health effects. Am J Clin Nutr, 1999;70(suppl):439-50.
- Messina M, Nagata C, Wu AH. Estimated Asian adult soy protein and isoflavone intakes. Nutr Cancer, 2006;55(1):1-12.
- Hussain M, Banerjee M, Sarkar FH, et al. Soy isoflavones in the treatment of prostate cancer. Nutr Cancer, 2003;42(2):111-7.
- Molteni A, Brizio-Molteni L, Persky V. In vitro hormonal effects of soybean isoflavones. J Nutr, 1995;125(suppl):751S-756S.
- Chen X, Anderson JJB. Isoflavones inhibit proliferation of ovarian cancer cells in vitro via an estrogen receptor-dependent pathway. Nutr Cancer, 2001;41(1-2):156-71.
- Constantinou AI, Lantvit D, Hawthorne M, et al. Chemopreventive effects of soy protein and purified isoflavones on DMBA-induced mammary tumors in female Sprague-Dawley rats. Nutr Cancer, 2001;41(1&2):75-81.
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