Topic: Eating Soyabean

I have read that soyabean is a good source of protein and soya milk, tofu and soya chunks can be used in diet.  I have perfect health and I exercise for two hours in the morning and I am fully vegetarian.  How much soyabean should I include in my diet? Does it have any side-effects?  If it is a good source of protein, why our traditional diet does not include soyabean?