for full article and references click The Magic (Or Lack Thereof) of Soy
Soybeans are used for damned near everything: as industrial lubricants, cleaners, diesel fuel additives, paint removers, crayons, meatless burgers, non-dairy ice cream, infant formulas, and high-concentration protein powders. Read the ingredient labels of various common food products, and you’ll see that many of them are fortified with some type of soybean product. For the scope of this piece, the majority of our attention will focus on the generalities of soy products, because all soy products carry hormonally active substances called isoflavones.
Soy protein is highly digestible for a plant protein, but it’s nothing special in terms of bioavailability (BA). Its level in this regard is only about 92 percent of that of regular milk protein. In fairness, however, when compared to milk, a much higher rate of essential amino acids is lost, and therefore unusable, during soy protein digestion. Studies on the BA of soy protein place it just below egg and milk, but far below that of whey proteins[2-6, 202]. This makes soy equivalent to most animal meat sources in this regard. Long story short, soy protein is an extremely cheap and moderately potent protein.
Be careful when considering the implications of the paragraph above. Soy protein only possesses a relatively high BA when it’s isolated. This high BA doesn’t apply to soy products in general, because they’re low in the essential amino acid methionine[4,7]. More importantly, they contain chemicals that decrease the digestibility of protein[8, 9, 198, 199]. Consuming soy products can decrease the protein quality of an entire meal. For example, beef normally has a BA of 90 percent. When consumed with soy, this reduces to 26 percent. Adding soy products to a diet—other than soy protein extracts—severely quiets the body’s ability to digest protein.
Soy has been shown to lower cholesterol levels[7, 12-55], and has been implicated in attenuating breast cancer risk[56-70], softening the severity of hot flashes during menopause[71-75], and decreasing prostate cancer risk[35, 76-96]. It should be noted that the results in humans are somewhat less than promising for prostate cancer[42, 97-100], and the significance of soy’s impact on blood lipids has been called into question[26, 27, 101-105].
Also, if you think drinking soy milk is a convenient way to lower your cholesterol levels, think again. Soy milk is far less effective at this than other soy products[106-110]. Regardless, this is still an interesting list of characteristics, some of which are beneficial.
Most of these effects aren’t caused solely by the soy protein itself. Soybeans contain high levels of potent substances called isoflavones. Isoflavones, chemically active in humans and other animals, appear to be responsible for several of the major benefits of eating soy protein. Understand, however, that several other plants also possess isoflavones which may have actions in humans[50, 111-117]. Soy just happens to have very high levels of many different isoflavones.