Diabetes Forecast

How should I eat to prepare for a workout so I won’t go low?

Michael Lynch, RDN, RCEP, CDE, CHWC, responds

Carb-containing snacks and blood glucose checks go a long way to ensuring your workout is both effective and safe.

What to Know

Exercise is a powerful tool in the management of diabetes and the prevention of diabetes-related complications. When you exercise, your muscles use glucose to fuel your workout. That, in turn, lowers blood glucose levels, an effect that lasts for hours after your workout. The goal for most sweat sessions: Get a good workout without going low.

Find Out More

A pre-workout snack is useful for people taking medications that raise the risk of lows, such as insulin, sulfonylureas, and meglitinides. A starting point for determining carbohydrate needs: Eat up to 10 grams of carb for every 30 minutes of low-intensity exercise (such as taking a stroll). Eat 5 to 10 grams of carb for every 15 minutes of moderate-intensity exercise (such as walking briskly). And eat up to 15 grams of carb for every 15 minutes of high-intensity exercise (such as running). These carb goals are only suggestions, though. Try them, check your blood glucose often, and adjust as needed.

From there, make blood glucose checks your best friend. Monitor your levels about 15 to 30 minutes before and after exercise. Then follow the recommendations below.

If your blood glucose level before exercise is:

  • Below 70 mg/dl: Eat 15 grams of carb, wait 15 minutes, then check your glucose again. Repeat until you’re no longer low, then appropriately fuel for exercise. Low blood glucose before exercise may heighten the risk of a mid-workout low; err on the side of caution by eating enough pre-workout carb.

  • From 100 to 250 mg/dl: The need for carbohydrate will depend on your medication regimen, blood glucose level, timing and type of exercise, time of day, and fitness level, among other things. Keep careful records of these factors, plus how many carbs you eat before a workout and your post-exercise glucose level. This can help you determine how a specific workout affects your blood glucose and whether you’ll need to eat a snack beforehand to avoid going low. 

  • Above 250 mg/dl: Consuming carbohydrate is generally not recommended when blood glucose is this high. Check for ketones and, if negative, drink 8 to 16 ounces of water and proceed with low- to moderate-intensity exercise. If you test positive for ketones, do not exercise.

If you aren’t on a medication that can cause low blood glucose, the need to eat to avoid going low during exercise is minimal—unless your workout lasts longer than an hour. Frequent blood glucose checks are probably not necessary, but it’s a good idea to check your level before exercise.

If you tend to go low during exercise, carry a glucose meter and fast-acting carbohydrate during a workout. Glucose tablets or gels are tops for their ability to quickly raise levels in measured amounts—and they do so without the excessive calories of some other high-carb snacks.


Learning how to eat to prepare for exercise is a trial-and-error process, but it’s worth the effort. Work with your diabetes care team to determine what works best for you.

Michael Lynch, RDN, RCEP, CDE, CHWC, is chair of the Registered Clinical Exercise Physiologist Committee for the American College of Sports Medicine and lead clinical exercise physiologist at the Diabetes Education and Nutrition Clinic at the University of Washington Medicine/Valley Medical Center. He has type 1 diabetes.



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