Soy has been promoted so much in recent years as a wonder food, wonder supplement and all around health good guy. I say...hold your horses! I think part of it was a marketing scheme to help the agricultural community out -- as the studies come in, soy is turning out to be a bit on the dangerous side.
First some personal experience. I had some blood work done...thyroid stimulating hormone My test came back a nasty 3.65 (It should be .3 to 2.0, preferrably for me under 1.0). For some reason I was drinking a bunch of soy protein that I'd gotten on sale and tried some soy yogurt. I had another blood test 3 weeks into my soy binge. My TSH had risen to 4.85. I pulled out all the soy and started on thyroid hormone...and it went down accordingly. Soy is a goitregen. It blocks your thyroid from using iodine to create necessary thyroid hormones. Anyone with a thyroid problem should stay far, far away from soy. It can even be a problem with people with healthy thyroids, so if you're having a hard time losing weight in spite of watching your diet and exercising, ditch the soy and see if that helps.
Supposedly soy is good for the heart, there is some report that consuming soy instead of other proteins adds in lowering consumption of heart-damaging saturated and trans fats, just by replacement. HOWEVER< there is absolutely NOT a shred of scientific basis to believe that the phytoestrogens in soy are heart protective.
In some studies, dietary soy has been shown to INCREASE the risk of breast cancer. The phytoestrogens in soy appear to promote cell growth and division, which is not so good when you're talking about cancerous cells.
It takes LARGE amounts of soy to lower LDL cholesterol. It also takes large amounts to have an effect on slowing bone loss. The amount of soy necessary means a great bombardment of the body with phytoestrogens and the goitregenic effects. Not a good trade off. Soy beans are high in phytic acid, which in large amounts can blockt he uptake of essential minerals in the intestinal track.
According to Dr. James Mercola: "One study of close to 4,000 elderly Japanese-American men found that those who ate the most tofu during midlife had more than double the risk of later developing Alzheimer's disease."
Some more insight from Dr. Mercola's site, from an article written by Dr. Mary G Enig and Sally Fallon:
"In 1997, researchers from the FDA's National Center for Toxicological Research made the embarrassing discovery that the goitrogenic components of soy were the very same isoflavones.50
Twenty-five grams of soy protein isolate, the minimum amount PTI claimed to have cholesterol-lowering effects, contains from 50 to 70 mg of isoflavones. It took only 45 mg of isoflavones in premenopausal women to exert significant biological effects, including a reduction in hormones needed for adequate thyroid function. These effects lingered for three months after soy consumption was discontinued.51
One hundred grams of soy protein - the maximum suggested cholesterol-lowering dose, and the amount recommended by Protein Technologies International - can contain almost 600 mg of isoflavones,52 an amount that is undeniably toxic. In 1992, the Swiss health service estimated that 100 grams of soy protein provided the estrogenic equivalent of the Pill.53
In vitro studies suggest that isoflavones inhibit synthesis of estradiol and other steroid hormones.54 Reproductive problems, infertility, thyroid disease and liver disease due to dietary intake of isoflavones have been observed for several species of animals including mice, cheetah, quail, pigs, rats, sturgeon and sheep.55
It is the isoflavones in soy that are said to have a favorable effect on postmenopausal symptoms, including hot flushes, and protection from osteoporosis. Quantification of discomfort from hot flushes is extremely subjective, and most studies show that control subjects report reduction in discomfort in amounts equal to subjects given soy.56 The claim that soy prevents osteoporosis is extraordinary, given that soy foods block calcium and cause vitamin D deficiencies. 50. Divi, R.L. et al., "Anti-thyroid isoflavones from the soybean", Biochemical Pharmacology (1997) 54:1087-1096.
51. Cassidy, A. et al., "Biological Effects of a Diet of Soy Protein Rich in Isoflavones on the Menstrual Cycle of Premenopausal Women", American Journal of Clinical Nutrition (1994) 60:333-340.
52. Murphy, P.A., "Phytoestrogen Content of Processed Soybean Foods", Food Technology, January 1982, pp. 60-64.
53. Bulletin de L'Office Fédéral de la Santé Publique, no. 28, July 20, 1992.
54. Keung, W.M., "Dietary oestrogenic isoflavones are potent inhibitors of B-hydroxysteroid dehydrogenase of P. testosteronii", Biochemical and Biophysical Research Committee (1995) 215:1137-1144; Makela, S.I. et al., "Estrogen-specific 12 B-hydroxysteroid oxidoreductase type 1 (E.C. 184.108.40.206) as a possible target for the action of phytoestrogens", PSEBM (1995) 208:51-59.
55. Setchell, K.D.R. et al., "Dietary oestrogens - a probable cause of infertility and liver disease in captive cheetahs", Gastroenterology (1987) 93:225-233; Leopald, A.S., "Phytoestrogens: Adverse effects on reproduction in California Quail," Science (1976) 191:98-100; Drane, H.M. et al., "Oestrogenic activity of soya-bean products", Food, Cosmetics and Technology (1980) 18:425-427; Kimura, S. et al., "Development of malignant goiter by defatted soybean with iodine-free diet in rats", Gann. (1976) 67:763-765; Pelissero, C. et al., "Oestrogenic effect of dietary soybean meal on vitellogenesis in cultured Siberian Sturgeon Acipenser baeri", Gen. Comp. End. (1991) 83:447-457; Braden et al., "The oestrogenic activity and metabolism of certain isoflavones in sheep", Australian J. Agricultural Research (1967) 18:335-348.
56. Ginsburg, Jean and Giordana M. Prelevic, "Is there a proven place for phytoestrogens in the menopause?", Climacteric (1999) 2:75-78."
A good book to read is The Whole Soy Story: The Dark Side of America's Favorite Health Food by Dr. Kaayla Daniel. Some insight the books shares:
Soy impedes the sexual maturation of boys (p. 335)
Soy accelerates the sexual maturation of girls (p. 339)
In newborns, the hormonal effects of soy may be irreversible (p. 333)
The average daily dose of soy estrogens in soy formula (38mg) is higher than the amounts that cause thyroid problems and endocrine disruption in adults (p. 334)