Trypsin inhibitors are large, tightly folded proteins that are only deactivated after a considerable period of heat treatment. Even in low amounts, trypsin inhibitors prevented normal growth in rats. The main ingredient in soy infant formula is soy protein isolate, a powder extracted from soybeans through a process that involves not only high temperatures but also caustic chemicals. The alkaline soaking solution produces a carcinogen, lysinealine, and reduces the cystine content, which is already low in the soybean. Other carcinogens called nitrosamines are formed during high temperature spray drying.
Soybeans also contain high levels of phytic acid or phytates.
Soybeans have very high levels of a form of phytic acid that is particularly difficult to neutralize. As early as 1967, researchers testing soy formula found that it caused negative zinc balance in every infant to whom it was given. Scientists have found a strong correlation between phytate content in formula and poor growth, even when the diets were additionally supplemented with zinc. High amounts of phytic acid in soy foods and grains have caused retarded growth in children on macrobiotic diets. A reduced rate of growth is especially serious in the infant as it causes a delay in the accumulation of lipids in the myelin, and hence jeopardizes the development of the brain and nervous system.