If you have type 1 diabetes and your blood glucose results are 250 mg/dl before exercise, stop and check for ketones. If you have moderate or large amounts of ketones, do not exercise. Ketones are a sign that your insulin level is too low. Exercise could cause the body to make more ketones.
Ketones add acid to the blood. When too many ketones are produced, they disrupt your body's chemical balance. This can be dangerous. Wait until your tests show negative or trace ketone levels before beginning to exercise.