Soy refers to soybean, which is a species of legume native to East Asia, widely grown for its edible bean, which has no healthy uses whatsoever.
The Word of Wisdom (the health code for The Church of Jesus Christ of Latter-day Saints) does not explicitly prohibit soybean, but according to Carson Owen Kimball in his magnum opus "Window To His Love", it should be. This is primarily because the chemical compounds therein closely mimic human estrogen, thereby causing any male who consumes it to become more feminine, potentially leading to "furry" behavior and homosexuality.
Soybean and all products derived from are extremely unhealthy for the human body and should be avoided at all costs. It is believed that the soy-based meat in the pizza that Brett O. Call consumed shortly before The April Incident was ultimately the cause of the unusual and offensive behavior exhibited by Brett on that day.