GENERAL DESCRIPTION:
Soy-based foods such as soy milk, tempeh, and tofu, contain potent compounds called isoflavones that are chemically similar to the female hormone estrogen. Many scientists believe that the widespread use of soy in Eastern diets may help to explain why the incidence of hormone-related cancers is much lower among Asian women.
ROLE IN ANTIAGING:
Research suggests that two isoflavones present in soy – genistein and daidzein – appear to lower the risk of developing hormone- related diseases such as breast cancer, prostate cancer, and endometriosis. Results of one study revealed that women who ate the most soy and other phytoestrogen- rich foods were 54% less likely to develop endometrial cancer. Researchers believe that genestein, which has been shown to inhibit angiogenesis, may also block a protein called tyrosine kinase, which promotes the growth and proliferation of tumor cells.
In 1999 the Food and Drug Administration (FDA) in the USA decided that soy foods can be marketed as foodstuffs that can reduce the risk of heart disease by lowering “bad” LDL cholesterol levels. In one study, people who drank a milkshake containing 25 grams of soy protein for nine weeks experienced an average 5% reduction in LDL cholesterol levels, while those with the highest LDL levels experienced a drop of 11% – both of which could have a significant effect upon heart disease risk. As well as lowering LDL cholesterol levels, soy has been shown to inhibit the oxidation of LDL cholesterol and raise “good” HDL cholesterol levels. Soy isoflavones may also protect against osteoporosis. In a study of postmenopausal women by Potter et al, published in the American Journal of Clinical Nutrition in 1998, findings demonstrated that the consumption of 40 grams of soy protein each day led to a significant increase in bone mineral density of the spine. Although these results are encouraging, no long-term human study to date has examined the effects of soy or soy isoflavones supplements on bone density or fracture risk. Soy isoflavones are also recommended for menopausal women to help lessen the symptoms of menopause. Results of one study showed that women who added 45 grams of soy flour to their daily diet experienced a 40% reduction in hot flashes.
DEFICIENCY SYMPTOMS: Not applicable
THERAPEUTIC DAILY AMOUNT:
While it is now possible to buy soy isoflavone supplements, it is not yet proven whether such supplements have the same health benefits as soy isoflavones consumed in food. In Asian countries, where the incidence of hormone-related cancers such as breast cancer and prostate cancer is significantly lower than in the West, people obtain roughly 20 to 200 mg of soy isoflavones a day from their food.
MAXIMUM SAFE LEVEL:
Not established – however due to recent study findings some experts recommend that soy intake should be limited to no more than 100mg a day.
SIDE EFFECTS/CONTRAINDICATIONS:
In laboratory studies, soy has been shown to stimulate the growth of breast cells, whether or not this increases the risk of breast cancer remains unclear, and research is ongoing. However, women with a medical history or family history of breast cancer are advised to consult their doctor before taking soy isoflavone supplements. Results of a study in mice published in the Proceedings of the National Academy of Sciences in May 2002 suggests that the isoflavone genistein may hamper the immune system. Researchers found that when mice were injected with genistein levels of several immune system cells dropped, and the thymus, a gland where T-cells mature, shrank. However, more alarmingly, the thymus also shrank when mice were fed genistein in their diet. The authors of this study recommend that soy intake should be limited to no more than roughly 100mg a day. Several groups of people should avoid taking soy isoflavone supplements, these include: pregnant and nursing women, and people taking estrogens, ipratropium bromide, thyroid hormones, or warfarin.
