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According to soyconnection.com, soybeans get about 35 to 38 % of their calories from protein compared to approximately 20 to 30 % in other legumes. That's pretty significant when you consider that legumes are known as a high source of protein anyway. The aforementioned article also states that under guidelines adopted by the FDA and the World Health Organization for evaluating protein quality for children and adults, soy protein isolate receives a rating of 1, which is the highest possible score. To put it in perspective, the quality of soy protein is equal to that of meat and milk proteins, which everyone recognizes as top sources of protein. But many people don't want or can't from a health standpoint get their protein that way.
Approximately 40 % of the calories in soy come from fat; most legumes (with the exception of peanuts) contain between two and 14 % fat. Now keep in mind that the fat in soybeans is primarily unsaturated. The polyunsaturated fat content of soybeans contains linolenic acid (7% of the total fat content), an omega-3 fatty acid. This is important, because omega-3 fatty acids may be essential nutrients for infants and they may also help to reduce risk of both heart disease and cancer.