Many people with diabetes seeking to improve their overall health wish to include daily exercise in their lifestyle. Unfortunately, the blood glucose (BG) response to exercise can often pose problems. BG can drop during or even hours after physical activity, or, to further complicate matters, it can rise. Why the inconsistency and varied response? Blame it on stress hormones and counter-regulatory hormones (CRH) activated during exercise. These hormones stimulate the liver to release glucose into the blood and this can send BG levels soaring.
Secretion of CRH is somewhat proportional to the intensity of the activity as well as the duration. For example, if you walk for 30 minutes, only a small amount of CRH are secreted and the liver releases only a small amount of glucose. If the rate that glucose enters the muscle during exercise is greater than the rate that the liver releases it, then BG levels drop slowly during the walk. Your BG may be 120 mg/dl at the onset of exercise and may fall to 100 mg/dl after 30 minutes.
A 30-minute jog burns considerably more glucose in the muscle than a 30-minute walk, and the result is that more BG enters the muscle from the blood. CRH secretion is higher with jogging, and consequently the liver is stimulated to release more glucose. If the energy cost of the jog is below 80% of your maximum capacity to use oxygen, then your BG may fall during the activity because more glucose enters the muscle than is released from the liver. Note the key role of exercise intensity: If the jog is above 80% of your maximum capacity to use oxygen, the CRH level greatly rises, stimulating an even higher rate of glucose secretion from the liver with BG rising during the exercise rather than falling.
Several recently published papers have observed this 80% intensity threshold. In fact, BG rises even in non-diabetics with exercise at this level of intensity! However, the non-diabetic pancreas quickly releases more insulin to match the rise in BG and soon BG returns to normal.
So, if you sometimes find that exercise causes your BG to rise rather than fall, it may be that the exercise is strenuous enough to hit or surpass the 80% mark. By reducing the intensity of the activity, you may find that your BG falls.
Many other factors, in addition to exercise intensity, determine the BG response to exercise. In a future column, some of these other factors will be discussed and a checklist of these factors will be presented. With this knowledge and the willingness to monitor and record BG levels before and after each exercise session, most people with type I and type 2 diabetes can successfully maintain good blood glucose control.