The reason there isn’t always a positive impact on A1C has to do with the ever present risk of hypoglycemia and hyperglycemia whenever a person with type 1 diabetes exercises. If production of glucose by the liver cannot keep up with demand, hypoglycemia occurs.
Too much food and not enough insulin, you have hyperglycemia. Too much insulin and not enough food, and you have hypoglycemia. It also can generate glucose from non-carbohydrate sources or from glycogen in a variety of ways.
Simultaneously, levels of glucagon and epinephrine rise, which releases glucose from liver glycogen. If fasting continues beyond a few hours, glycogen stores become depleted and fatty acids are released from the fat cells to be oxidized into ketones.