Healthy people almost never think about exact blood-glucose levels. For most, eating a balanced diet and getting plenty of fuel before a workout is sufficient to keep blood sugar in acceptable ranges. Diabetics, on the other hand, must know throughout the day the exact measure of glucose in blood, particularly before and after mealtime and before, during and after exercise. To prevent hypoglycemia, or low blood sugar, diabetics should exercise when blood sugar is at optimal levels.
Physiology
The body uses glucose as its primary fuel during exercise, according to Susan Sample of the University of Wisconsin Department of Surgical Sciences. If you even think about a trip to the gym, your body sends a signal via hormone release to the liver and other associated tissues that they should prepare for activity by releasing glucose into the blood. Dangerous levels of blood glucose, measured as grams of sugar per deciliter of blood, on the high end are in the 270 gm/dL range, while a measure of 70 gm/dL indicates dangerously low blood sugar. Finding the proper range between those extremes is critical at all times, but especially before exercise.
Hypoglycemia
Hypoglycemia, in layman's parlance, is low blood sugar. If your blood sugar levels are down around 70 gm/dL, you are hypoglycemic and you should not engage in physical activity until you get your levels up to normal, or around at least 100 gm/dL and preferable up to 120 gm/dL. Diabetics, particularly insulin-dependent type 1 diabetics, must constantly balance spikes and troughs in blood sugar in order to stay within acceptable ranges.
Hyperglycemia
Hyperglycemia is the opposite of hypoglycemia, meaning the level of glucose in the blood is too high. Readings for hypoglycemia can go as high as 300 gm/dL or more, but exercising with a reading above 270 gm/dL is not recommended. According to the Johns Hopkins Medical, the ideal maximum of blood glucose on the high end is about 240 gm/dL before you begin exercising.
Glucose Testing
Test your blood-sugar levels 30 minutes before exercising to verify that you are in the safe range. Type 1 patients should wait until after their most recent insulin peak before beginning. Once you begin, retest your blood every 30 minutes that you remain active. Finally, test when your workout is complete. Blood-sugar levels continue to drop after exercising because the body rounds up all the remaining glucose and converts it to glycogen for storage, so test your blood immediately after exercising and again several hours later to be sure that you are still in the safe range.
