Diabetes and exercise: When to monitor your blood sugar

Exercise is an important part of any diabetes treatment plan. To avoid potential problems, check your blood sugar before, during and after exercise.

By Mayo Clinic Staff

Diabetes and exercise go hand in hand, at least when it comes to managing your diabetes. Exercise can help you improve your blood sugar control, as well as boost your overall fitness and reduce your risk of heart disease and stroke.

But diabetes and exercise pose unique challenges, too. Remember to track your blood sugar before, during and after exercise. Your records will reveal how your body responds to exercise — and help you prevent potentially dangerous blood sugar fluctuations.

Before exercise: Check your blood sugar before your workout

Before jumping into a fitness program, get your doctor's OK to exercise — especially if you've been inactive. Discuss with your doctor which activities you're contemplating and the best time to exercise, as well as the potential impact of medications on your blood sugar as you become more active.

For the best health benefits, experts recommend 150 minutes a week of moderately intense physical activities such as:

  • Fast walking
  • Lap swimming
  • Bicycling

If you're taking insulin or medications that can cause low blood sugar (hypoglycemia), test your blood sugar 30 minutes before exercising and approximately every 30 minutes during exercise. This will help you determine if your blood sugar level is stable, rising or falling and if it's safe to keep exercising.

Consider these general guidelines relative to your blood sugar level — measured in milligrams per deciliter (mg/dL) or millimoles per liter (mmol/L).

  • Lower than 100 mg/dL (5.6 mmol/L). Your blood sugar may be too low to exercise safely. Eat a small carbohydrate-containing snack, such as fruit or crackers, before you begin your workout.
  • 100 to 250 mg/dL (5.6 to 13.9 mmol/L). You're good to go. For most people, this is a safe pre-exercise blood sugar range.
  • 250 mg/dL (13.9 mmol/L) or higher. This is a caution zone. Before exercising, test your urine for ketones — substances made when your body breaks down fat for energy. Excess ketones indicate that your body doesn't have enough insulin to control your blood sugar. If you exercise when you have a high level of ketones, you risk ketoacidosis — a serious complication of diabetes that needs immediate treatment. Instead, wait to exercise until your test kit indicates absence or a low level of ketones in your urine.
  • 300 mg/dL (16.7 mmol/L) or higher. Your blood sugar may be too high to exercise safely, as these high glucose levels may increase your risk of dehydration and ketoacidosis. Postpone your workout until your blood sugar drops to a safe pre-exercise range.
Feb. 22, 2014 See more In-depth