This is an article written by RICKIE HATHAWAY – BuyBulkWhey.com In the Field Correspondent.

Soy protein is considered to be able to assist in muscle mass gaining,however there are many key fats that can disprove such considerations.Soy contains a hormone called Phytoestrogen which is indeed an estrogen.Estrogen in case you didn’t know is the female sex hormone which can cause men to lose muscle mass and grow breasts.In the muscle building world one of the last things you’d want is estrogen,you need testosterone to aid in muscle mass gaining. Soy foods can also require your body to use more vitamin B-12 and can cause deficiencies of calcium and vitamin D. In America 99% of soy is genetically modified and has one of the highest pesticide contaminations of all foods. Soy first was used for food purposes in 1134-236 B.C. after the Chinese learned to ferment soybeans. Ever since hard research has slowly diminished the “benefits” of soy proteins and isolates.

References:

http://www.becomehealthynow.com/article/soy/316/
http://www.soyonlineservice.co.nz/03soymyths.htm