Its kinda like when you get a new car and all of a sudden you start see that same car everywhere you look. Ya know what I mean?
I found this article to present some great information on both sides of the debate.
Some stand out pieces are:
There is a debate about whether soy prevents or causes breast cancer. Some women have estrogen-positive breast cancer, meaning their tumors have estrogen receptors and are thought to grow from contact with estrogen. Soy contains isoflavones which are weak estrogens. It is not known if this is good or bad. The isoflavones could stimulate estrogen positive breast tumors, or they could dull the effect of real estrogen on the tumor. There has been little research performed on humans.
In April of 2009, a paper was published in the American Journal of Clinical Nutrition, written by Mark Messina and Anna Wu.6 Here are highlights from this paper:
-Daily per capita isoflavone intake in the United States and in Europe is about 3 mg per day.
-Daily per capita isoflavone intake among older adults in Japan and in Chinese cities such as Shanghai is about 25-50 mg per day.
They noted that the study by Trock et al (mentioned above) found no statistially significant protective effect of higher intake on breast cancer rates in Asian countries.
Reduced risks were found for both pre- and postmenopausal breast cancer.
When you pick apart these various studies, there appears to be evidence that eating soy early in life is important for gaining a protection against breast cancer.
Four studies have taken breast tissue biopsies before and after exposure to isoflavones and none found any increased breast cell proliferation.
Two epidemiologic studies looking at survival after breast cancer diagnosis found no reduced survival among women eating more soy.
The authors conclude, "The concern that products containing isoflavone might be contraindicated for patients with breast cancer and women at increased risk of breast cancer is based almost exclusively on results from rodent studies. In contrast, however, the clinical and epidemiologic data suggest that isoflavones pose no risk to such women. This suggestion is consistent with the relatively unimpressive data showing that postmenopausal therapy with oral estrogen increases breast cancer risk.
Nevertheless, it must be acknowledged that, because there is an imprecise understanding of the effect of soy and isoflavones on breast tissue, women at high risk of developing breast cancer and patients with breast cancer should first discuss any dietary changes with their primary health care provider."
Also in April of 2009, a prospective study on soy intake and breast cancer risk in the Shanghai Women's Health Study was published. They found that among premenopausal women, higher soy intake was protective, but not among postmenopausal women.
A 2008 study from Indonesia found that among people aged 52 to 98, increased tofu consumption was linked to slightly worse memory scores (-0.18, p = .05).9 Tempeh, on the other hand, was linked to slightly better memory scores.
The authors state that:
"According to the Departments of Public Health at the Universities of Jakarta and Yogyakarta, formaldehyde is often added to tofu (but not tempeh) to preserve its freshness. Formaldehyde can induce oxidative damage to the frontal cortex and hippocampal tissue...."
As of September 2008, the government was trying to end the practice (Formaldehyde-laced tofu still selling despite raids, The Jakarta Post, 09/11/2008). In light of the formaldehyde problem and how slight the differences were in memory scores, there is probably little to worry about.
The authors surmised that tempeh might be good for memory because the bacteria used in the tempeh starter, Rhizopus oligosporus produce folate, which is thought to protect memory.
There is evidence that soy intake may be protective against heart disease, prostate cancer, osteoporosis4, 10, and menopausal symptoms. Tempeh is also a good source of absorbable zinc.
Of course, some people are allergic to soy and should avoid it. Other people say they feel better when not eating soy. And other people feel better when they do eat soy. But the research indicates it’s “not unsafe” for most people at 2 to 3 servings a day
This is another good read if you are interested:
Is It Safe to Eat Soy? by Virginia Messina, MPH, RD & Mark Messina, PhD
Oh and before I forget:
Soy milk and infants:
A study published last year in the Proceedings of the National Academy of Sciences found that the highly concentrated phytoestrogens in soy formula might weaken the immune systems of babies. The formulas have more of these compounds than soy foods do. But researchers emphasized that this risk is largely theoretical. There's no evidence that soy formula is unsafe, or that infants drinking it have been harmed. Breast milk is still the first choice, however, followed by milk-based formulas. Only infants allergic to milk should drink soy formula.
During pregnancy, isoflavones from the mother's diet appear to be passed on to the fetus. High levels of isoflavones were found in healthy Japanese infants whose mothers also had high blood levels of isoflavones, probably due to high intakes of soy.18 These levels of isoflavones have not been associated with any health problems in infants. One report has found that a birth defect of the penis called hypospadias occurred more frequently in infants whose mothers followed a vegetarian diet during pregnancy.19 Although some have attributed these results to use of soy, there was no significant association between use of soymilk and other soy products and development of hypospadias.19 Isoflavones from the mother's diet also appear in breast milk, although the daily isoflavone intake of breastfed infants remains negligible,20 even when breast milk levels are increased as much as tenfold by the mother's use of soy foods.21 At this point there is no scientific evidence of a need to avoid soy foods in pregnancy or during breastfeeding; 2-3 servings a day is a reasonable amount.
Ideally, all infants would be breast-fed. There are circumstances, however, where soy formula is the next-best alternative. Researchers have concluded that use of soy formula appears to have no effect on fertility, miscarriage rate, birth defects in offspring, or maturation.22 Based on the results of this study, and calculating isoflavone intake on a body weight basis, and assuming that older children absorb and metabolize soy isoflavones similarly to infants, a daily soy intake of 2-3 servings per day appears reasonable for children.
Children can use soy products as a part of a healthy diet. Sometimes, perhaps because of convenience, or perhaps because they look like what other children are eating, vegetarian children become over-dependent on soy products like veggie burgers, veggie dogs, and veggie deli slices. Using seitan or bean burgers, peanut butter sandwiches, or other foods can encourage some dietary variety rather than focusing strictly on soy.