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Why Is My Blood Glucose Sometimes Low after Physical Activity?

Low blood glucose is defined as a blood glucose level below 70 mg/dl if your meter measures whole blood, or 80 mg/dl or below if it measures plasma glucose (a plasma blood glucose of 90 mg/dl or below with symptoms is also a sign of hypoglycemia).

One of the most common causes of low blood glucose is too much physical activity. In fact, moderate to intense exercise may cause your blood glucose to drop for the next 24 hours following exercise. This post-exercise hypoglycemia is often referred to as the "lag effect" of exercise.

Basically, when you exercise, the body uses two sources of fuel, sugar and free fatty acids (that is, fat) to generate energy. The sugar comes from the blood, the liver and the muscles. The sugar is stored in the liver and muscle in a form called glycogen. During the first 15 minutes of exercise, most of the sugar for fuel comes from either the blood stream or the muscle glycogen, which is converted back to sugar. After 15 minutes of exercise, however, the fuel starts to come more from the glycogen stored in the liver. After 30 minutes of exercise, the body begins to get more of its energy from the free fatty acids. As a result, exercise can deplete sugar levels and glycogen stores.

The body will replace these glycogen stores but this process may take 4 to 6 hours, even 12 to 24 hours with more intense activity. During this rebuilding of glycogen stores, a person with diabetes can be at higher risk for hypoglycemia. Here are tips for safe exercising.

Guidelines for preventing exercise related hypoglycemia

  • Check your blood glucose before exercising to make sure your blood glucose is sufficient and/or consume an appropriate snack.
  • Avoid exercise at the peak of your insulin action.
  • Avoid late evening exercise. Exercise should be completed 2 hours before bedtime.
  • Avoid alcohol consumption prior to or immediately after exercise.
  • Avoid hot tubs, saunas and steam rooms directly after exercise. These continue to maintain an increased heart rate and may continue to lower your blood glucose.
  • Limit your exercise sessions to 1 or 2 per day. Additional sessions increase the likelihood of hypoglycemia.
  • In the past, it was believed that injecting insulin into exercising muscle increased absorption of the insulin resulting in hypoglycemia; now it is believed that the timing and action of the insulin are more likely to be the key factors.
  • Check your blood glucose immediately after exercise to prevent low blood glucose from occurring hours after exercise. It may also be necessary to check your blood glucose more often for 2 to 4 hours after exercise. Moderate intense exercise may cause your blood glucose to drop for the next 24 hours following exercise

If blood glucose is less than 100 mg/dl immediately after exercise

  • Follow post-exercise snack guidelines. If you are not scheduled for a snack or a meal for 30-60 minutes after exercise, 15 grams of carbohydrate should be sufficient to prevent a low blood glucose. If no meal or snack is scheduled for more than one hour, take 15 grams of carbohydrate and 7-8 grams of protein.
  • Increase carbohydrates before exercise.
  • Decrease the dose of active insulin for the next exercise session.
  • Consider decreasing the insulin dosage following exercise.
  • If your blood glucose at bedtime is still less than 100 mg/dl, double your bedtime snack, or if possible, decrease your insulin dose acting during bedtime.

Guidelines for unplanned exercise

Sometimes exercise or physical exertion occurs spontaneously or is unplanned. You may need extra food at these times in order to maintain the balance between your insulin or oral medication and the energy needed to exercise. A number of factors will determine if you need food and how much food is necessary.

  • Snacks may be necessary for exercise beginning 2 hours or more after your last meal.
  • Snacks may be needed for exercise lasting 1 hour or more.
  • Long duration or all-day activities may require both a snack adjustment and an insulin adjustment.
  • If you are attempting to lose weight, adjust your insulin or medication rather than eating extra food.

Be sure to talk to your diabetes educator for more information about how exercise will work best with your diabetes.

Find more information about physical activity and diabetes in Staying Healthy with Diabetes – Physical Activity & Fitness available from the Joslin Online Store.

      Attention visitors from outside the US: You can easily convert US to non-US blood glucose levels by clicking here.

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      Page last updated: September 18, 2019