Fats have been nearly eliminated in most processed foods these days, and the fats that are included are unhealthy types of fats like trans-fats and most vegetable oils. Fats supply two types of required fatty acids that our bodies cannot manufacture. Even though they are complex carbohydrates, most grain products are digested very rapidly and enter the blood stream as sugars almost immediately after you eat them. This includes whole wheat bread. The body experiences a sharp spike in blood sugar and releases a corresponding amount of insulin in response.
High insulin is bad.
Insulin decreases the sugar in your blood and in your brain. Low sugar in the brain means poor concentration, fatigue, and other problems. At times my sugar would drop so low that my hands and legs became shaky, I could barely think, I was incredibly thirsty, flushed, and could think about nothing else but eating sugar. Of course this would only cause a repeat of the cycle. At the time, I was told to avoid sugars.