Glucose circulating in your bloodstream is the energy source on which every cell in your body depends. There's glucose stored in your muscles, and when you use that, such as by working those muscles in exercise, the sugar will be replaced from further stores in your liver and from the circulating glucose. Your blood-glucose level may fall measurably while you exercise or as much as a day later.
Implications
For most people, the decrease in blood glucose is just the matter of calories burned that was the point of exercising and the satisfying post-workout tiredness. If you're a diabetic, however, who doesn't make or use insulin well to get the glucose into your cells, blood sugar can fall precipitously with exercise to the level of clinical hypoglycemia. You may feel shaky or nervous, sweat more than is appropriate to the exercise, have a seizure or pass out.
Monitor for Control
The Mayo Clinic advises diabetics to check their serum glucose before and after exercise, and every 30 minutes during a workout of more than an hour. Particularly if you take insulin or other medication to help control glucose levels, you need to time your exercise to when your blood sugar is rising or stable; combining exercise with medication can contribute to too-low blood sugar. During and after exercise, test every 30 minutes or so until glucose levels stabilize at no lower than the target level you've set with your doctor. Exercising at glucose levels over 250 mg/dL is also dangerous, because it can lead to ketoacidosis; check your urine for ketones and stay out of the gym.
Countermeasures
If your blood sugar tests at 70 mg/dL or less, do not begin a walk, let alone a workout or swim, and stop if you've already started. The U.S. National Institute of Diabetes and Digestive and Kidney Diseases, NIDDK, advises against working out at a glucose reading below 100 mg/dL. Wait for your medication to stabilize serum glucose, or help the meds along by taking two to five glucose tablets or a dose of gel, drinking 1/2 cup of fruit juice or full-sugar soda pop or eating five or six small hard candies. You can use these same measures if your blood sugar drops too low at any time, including after exercising.
Hydration
Failing to keep yourself hydrated during exercise can actually raise your blood glucose, but not in a way that counters hypoglycemia. Dehydration means there's less liquid blood volume to carry around the same cells and dissolved substances, including glucose. Your blood can get too thick to circulate efficiently. Brian D. Johnston and Paul L. Liebert, in "The Merck Manual for Health Care Professionals," recommend drinking 1/2 to 1 cup of water every 15 to 20 minutes during exertion, more when you're working at peak load or in high temperatures. They recommend weighing yourself before and after your workout, and drinking an additional 2 cups of water for each pound of water lost. Don't overdo hydration, either, because water intoxication can lead to seizures from too little sodium, and perhaps too little glucose.
