I heard that if you eat bread less than an hour before doing something mentally strenuous, something in the bread actually effects your ability to think, is this true? if so is it only bread but other foods aswell? and is there something that can be done to reverse the process? also, if this is true, does it only effect the brain or other organs, or functions aswell?

When you eat, your body diverts blood flow from other parts of the body to the digestive system. This can cause you to be drowsy and to lose concentration, as less blood is available and the conscious brain normally uses about 20% of the available blood flow.