Soy protein is the most commonly consumed source of isoflavones, which are plant pigments found in soybeans. Examples of soy products include edamame (young soy beans), soybeans, soy milk, soybean sprouts, soy cheese, soy flour, soy yogurt, soy protein powder, miso soup, tempeh, tofu, soy burgers and soy cereal. Soy protein can supply all of the essential amino acids needed by the human body. The two main isoflavones in soy are genistein and daidzein, both of which have been reported in laboratory studies to exhibit anti-cancer effects at specific dosages.

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Discussion of Soy Protein from the 2012 AACR conference

Nutrient Content of Soy Foods

Overall, isoflavones may have a role in the management of prostate cancer. The beneficial effects of isoflavones include:

A decrease in blood androgen (testosterone) levels by increasing the level of SHBG (sex-hormone binding globulin). SHBG binds to testosterone. Therefore, less testosterone is available to help the cancer grow.
Binding to androgen receptors. As a result, more potent sex hormones (testosterone, dihydrotestosterone) are blocked from binding to the receptors and stimulating cancer growth.
Inhibition of alpha-5 reductase, an enzyme that converts testosterone to its most potent form (dihydrotestosterone).
Restriction of other enzymes associated with cancer cell growth.
Inhibition of tumor blood vessel formation. Blood vessel growth within the tumor allows the cancer to grow and spread.
Decrease in insulin growth factor-1 (IGF-1), which may be a marker for increased prostate cancer risk.
Medical Research/Studies
Prostate cancer incidence and mortality rates in Asian countries are much lower than in the United States. Research suggests that one of the reasons for this difference in incidence rates may be the high soy protein content in the Asian diet. In countries such as Japan, Korea, China and Taiwan, the estimated isoflavone mean daily intake is between 10-50 mg per day, as compared to 1-3 mg per day for Americans.

Daily Intake of Soy Protein in American and Asian Diets.
Studies point to diet as a major factor in the incidence of prostate cancer. In fact, migration studies have shown men from low-risk countries who move to the United States ultimately have the same risk for prostate cancer as the rest of the U.S. population. Researchers believe that this risk increase may be in part due to the change from a diet high in isoflavones to a more Westernized diet, which is low in isoflavones, lower in fruits and vegetables and higher in total fat and saturated fat.

Cell culture and animal studies have shown that genistein inhibits tumor growth. In one study, a group of human prostate cancer cells was treated with genistein and another group was left untreated. Prostate cancer cell growth was inhibited only in the cells treated with genistein. In another study, prostate cancer cells were transplanted into animal models. These animals ate either a soy-free diet or a soy-based diet. The progression of prostate cancer was reduced by 25% in the animals on the soy-based diet versus the animals on the soy-free diet.

Soy protein intake should be 35 to 40 grams per day.
A good way to add soy protein to your diet is by having a soy-protein smoothie with breakfast. Some soy protein isolate powders have up to 20 grams of soy protein and 20 mg of isoflavones per serving. (This is half of your recommended intake!)
Avoid soybean oils. Soybean oil does not contain beneficial isoflavones.
It is best to get your isoflavones as they occur in soy products, such as soy protein isolate powder, tofu and soy meat substitutes. Avoid isoflavone supplements, which may not provide the proper balance of genistein to daidzein.
Start increasing your soy intake gradually. Large amounts of soy contain high amounts of soluble fiber, which can cause gastrointestinal discomfort.