Actually it takes more energy to breakdown the wholegrain food than processed food like white rice (Imo wholegrain bread = white rice). Also wholegrain contains more fiber which is more filling and slow the absorbtion rate more. By this, this is not too much insulin spike will occur. Insulin spike result in reduce body fat burn and increase body fat storage. So they say. That's why it's not recommended to eat sugar or sweet things. It will cause insulin spike. So they say.

In the long run, i think will still have some difference. That's why alot of bodybuilder eat oat, brown rice, sweet potato, all the natural complex carb.