The answer to this question matters tremendously - and not just for tofu-eating vegetarians. Soy is a common meat substitute found in hamburgers, hot dogs, and sausages. It is used to fortify energy bars, sports drinks, cereals, granola bars, and imitation dairy products. Soy can be found in 60% of processed foods and now constitutes 25% of the US infant formula market .
What are soy’s benefits?
Soy is a vegetable protein source that is cholesterol-free, lactose-free, high in fiber, and rich in unsaturated fats. Soy can mildly reduce the risk of osteoporosis  and there is strong evidence that adding soy protein to the diet can moderately decrease bad cholesterol, but the effect is much smaller than previously thought .
So what’s the issue?
Phytoestrogens. They’re plant-derived compounds found in soy (and some other foods) that have a chemical compound similar to estrogen. They differ in that they sometimes mimic the effect of estrogen and sometimes interfere with it.
Wow, how does that effect babies who drink soy formula?
Animal studies reveal clear risks associated with feeding babies soy formula. Female rats show early signs of puberty and a decreased ability to maintain regular cycles . Female fertility, ovarian function and reproductive health are all disrupted later in life when these animals are fed enough soy formula to imitate human baby consumption . Male animal babies have a different problem - they develop more breast tissue and have lower testosterone counts .
A 35 year study tracking 19,000 women linked soy formula consumption to greater risk of developing benign tumors in the uterus called uterine fibroids .
What about adults who eat soy?
Pre-menopausal women showed similar negative effects - their cycle length increased and their LH and FSH levels (reproductive hormones) were suppressed. There is also evidence that diets rich in soy can suppress sexual motivation .
The question about soy’s influence on breast cancer has been intensely debated yet unanswered. To date, no clear consensus has been reached on whether or not phytoestrogens are helpful or harmful for breast cancer risk .
What’s the bottom line? What should I do?
It’s important to remember that many foods have pros and cons. Soy can be a healthy source of protein and might be mildly beneficial to older individuals looking to increase their bone and cardiovascular health.
However, consumers should be aware that soy can cause hormonal changes and make dietary choices accordingly. Women who are pregnant, nursing, or attempting to become pregnant should eat soy foods with caution and be aware that there are dangers associated with feeding soy formula to babies. Moderation is likely key and the incorporation of real foods, as opposed to supplements or processed foods to which soy protein is added, is probably essential for maximizing health benefits.
The National Toxicology Program rated soy infant formula as a 2 out of 5 on their risk scale, concluding there is “minimal concern for adverse effects on development” .
There are studies that suggest Asian women show a decreased rate in breast cancer after consuming soy  but their soy consumption is noticeably different than in the West. Asian populations consume their soy across their entire life except for a brief 6-8 month breastfeeding window. Most of their soy comes from unprocessed foods like tofu and tempeh. For those breastfed soy formula in the West, the pattern is just the opposite. Also, many processed foods in the West are enriched with soy protein isolate. There are also countless other dietary differences between the two cultures that makes a cross-cultural comparison very difficult.
Relieve menopausal symptoms? No proof - just a weak association.
“Given the evidence that adding soy foods to an already healthy diet may have modest but measurable benefits on bone and cardiovascular health, women without serious risk factors for breast cancer or a family history of breast cancer could likely incorporate soy into their diet without significant concern.” 
As with many other compounds, like alcohol or caffeine, there are many pros and cons associated with moderate soy intake. Older individuals, especially those with high cholesterol, may experience modest benefits including improved bone and cardiovascular health, and perhaps a decreased risk of carcinogenesis.