Question: Why does my blood glucose (BG) sometimes rise after exercise?

Answer: If an inadequate amount of insulin is present in the blood allowing the BG to rise to about 250 to 300 mg/dl, then exercise may cause a further rise in BG rather than the expected drop. Low insulin coupled with physical activity stimulates the secretion of several other hormones such as epinephrine, norepinephrine, cortisol, glucagon, and growth hormone. Collectively these hormones trigger the liver to release glucose into the blood, thereby increasing the BG rather than decreasing it. The hormones also increase the breakdown of fat but limit the uptake of fat by muscle cells. The liver converts some of the fats to strong acids called ketones. The ketones may build up in the blood producing a state called ketoacidosis. This event is far more likely to occur in people with type I diabetes.

To prevent the problem the BG should be checked before exercise and if the level exceeds 250, then exercise should be delayed until it decreases well below 250. The urine should be checked for ketones and if they are present, exercise at this time will exacerbate the problem.

A second cause of the rise in BG during and after physical activity is highly vigorous exercise. The more intense exercise is, the greater the secretion of glucose from the liver. During the strenuous session, stress hormones will be secreted in large quantities which will then stimulate the liver to release glucose. This is an interesting paradox: the more vigorous the exercise the more glucose released by the liver with a likely rise in BG rather than a fall. To make matters worse, the level of stress hormones in the blood may be elevated for several hours after intense exercise causing the liver to continue the outpouring of sugar. Thus, the rise in BG may last for a number of hours once the exercise is completed. Additional BG monitoring may be needed until the values have stabilized.

Athletes who workout strenuously on a regular basis may not need to eat as large a feeding as expected before strenuous sessions. Also, as the body adapts to intense training, the hormonal secretion decreases when the same intensity workouts are done. Consequently, an intense workout at the beginning of a season that caused a surge in BG may cause a lesser rise in BG at mid-season.

In summary, monitor BG before exercise, during exercise if the sessions last beyond an hour, and after exercise one or more times. For non-athletes, high-intensity exercise isn't needed to improve fitness, health in general, and improve BG control. It even tends to make BG management more difficult and increase the risk of injury, sore muscles, and heart attack, and may discourage you from sticking with your exercise regimen.