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One protein source that has been getting a lot of attention is soy protein (soybeans). We've been told that regularly eating soy-based foods lowers cholesterol, chills hot flashes, prevents breast and prostate cancer, aids weight loss, and wards off osteoporosis. Some of these benefits have been attributed a unique characteristic of soybeans-their high concentrations of isoflavones, a type of plant-made estrogen (phytoestrogen).

In 1995, the New England Journal of Medicine (Vol.333, No. 5) published a report from the University of Kentucky entitled, "Meta-Analysis of the Effects of Soy Protein Intake on Serum Lipids."

This meta-analysis concluded that soy protein is correlated with significant decreases in serum cholesterol, Low Density Lipoprotein LDL (bad) cholesterol and triglyceride concentrations. However, High Density Lipoprotein HDL (good) cholesterol did not increase.

On the basis of this research PTI,in 1998, filed a petition with FDA for a health claim that soy protein may reduce cholesterol and the risk of heart disease.

The FDA granted this health claim for soy: "25 grams of soy protein a day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease." One serving, (1 cup or 240 mL) of soy milk, for instance, contains 6 or 7 grams of soy protein.

In January 2006 an American Heart Association review (in the journal "Circulation") of a decade long study of soy protein benefits casts doubt on the FDA allowed "Heart Healthy" claim for soy protein. The panel also found that soy isoflavones do not reduce post menopause "hot flashes" in women nor do isoflavones help prevent cancers of the breast, uterus or prostate. Thus, soy isoflavones in the form of supplements is not recommended. Among the conclusions the authors state, "In contrast, soy products such as tofu, soy butter, soy nuts, or some soy burgers should be beneficial to cardiovascular and overall health because of their high content of polyunsaturated fats, fibre, vitamins, and minerals and low content of saturated fat. Using these and other soy foods to replace foods high in animal protein that contain saturated fat and cholesterol may confer benefits to cardiovascular health."

Soy is also rich in estrogenic compounds such as genistein and daidzein, however, research is conflicting as to whether or not it can cause increases in estrogen in men. Studies have found that in fact these compounds compete with estrogen for estrogen-receptors, leading to cognitive impairment in men.

A recent study found that 56g of soy protein powder per day caused serum testosterone to fall 19% in four weeks in a test group of twelve healthy males. According to the study, this data supports further studies of these hormonal effects as a mechanism in prostate cancer prevention. However, a study conducted by the Miami Research Associates refutes the finding of the Goodin study, finding that soy protein had no significant impact on testosterone levels in healthy males. In fact, only one participant in the Goodin study actually saw a drop in testosterone. The participant in question had testosterone levels 200% higher than all of the other subjects, and during the study, his levels dropped to bring him inline with the other participants. The Goodin study did not conclusively prove that the participant's erratic testosterone levels were related to the soy protein.

Media reports tout the joys of soy, and food makers are churning out new soy products that are moving into the mainstream. In Boston, for example, soymilk is now advertised on the radio during Boston Red Sox games, alongside donuts, oil additives, and beer.

Of any studied legume, whole soybeans have the highest levels of phytic acid, an organic acid and mineral chelator present in many plant tissues, especially bran and seeds, which binds to certain ingested minerals: calcium, magnesium, iron, and especially zinc — in the intestinal tract, and reduces the amount the body assimilates. For people with a particularly low intake of essential minerals, especially young children and those in developing countries, this effect can be undesirable. However, dietary mineral chelators help prevent over-mineralization of joints, blood vessels, and other parts of the body, which is most common in older persons.

The digestibility of some soyfoods are as follows: steamed soybeans 65.3%, tofu 92.7%, soy "milk" 92.6%, soy protein isolate 93–97%. Some studies on rats have indicated that the biological value of soy protein isolates is comparable to animal proteins such as casein if enriched with the sulfur-containing amino acid methionine.

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