What Would Cause Frequent Mood Swings?

Hi
My 8 year old daughter has had mood swings which are caused/controlled by diet. We started noticing this when she was only 2yrs. We have found over the years that if we feed her complex carbohydrates little and often then she is 'normal' for her age. When she goes without these foods for a couple of days we have a very tired, tearful and angry child. She is overweight despite having a healthy diet (dietician said diet was good) and regular exercise. GP did routine bloods, glucose, insulin levels and t4 last year, which they said were normal. She is overly sensitive to heat, wears very little clothing most of the time and has to have a fan on her at night, even when it is colder weather. The GP is now saying it could be her hormones. We adore her but she has almost destoyed family life and we are trying to reduce her calorie intake so that she will lose weight but this is causing the moods and hysteria to start. Any advice would be most grateful.