http://www.cfsan.fda.gov/~dms/fdsoypr.html
Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration
Soy protein products can be good substitutes for animal products because, unlike some other beans, soy offers a complete protein profile. Soybeans contain all the amino acids essential to human nutrition, which must be supplied in the diet because they cannot be synthesized by the human body. Soy protein products can replace animal-based foods--which also have complete proteins but tend to contain more fat, especially saturated fat--without requiring major adjustments elsewhere in the diet.