In high intensity sports the associated high adrenalin levels cause glucose to be released from the liver. In a diabetic athlete a tendency to hyperglycaemia is the result unless there is adequate insulin to counteract this problem.
In lower intensity but longer duration sports the ultimate problem is the opposite. There is a tendency to hypoglycaemia after about 30 minutes of exercise as the muscle glycogen stores become depleted. Normally blood glucose levels would be maintained by releasing glucose from the liver stores. In the diabetic athlete this mechanism may be impaired due in part to the action of inappropriately high insulin levels.
The way around this problem is to calculate a formula based upon the intensity and duration of the exercise whilst taking into account appropriate doses and timing of insulin injections together with a designed carbohydrate intake regime. It is for this reason that collaborating with a dietician specialised in sport can prove vital in optimising performance.