Blood Sugar Spikes After Exercise? Here’s What May Be Happening

Exercise is a cornerstone of diabetes and blood sugar management, so it can feel extremely frustrating when, after a great workout, your continuous glucose monitor (CGM) suddenly starts screaming at you with a high blood sugar alert. 

If you live with diabetes, you know that blood sugars can be anything but predictable, even during exercise. And sometimes, exercise can ironically be the cause of high blood sugars. 

Exercise-induced high blood sugars can be one of the most frustrating and unpredictable parts of living with diabetes. When you lace up your shoes and gear up for a healthy habit, suffering from a high blood sugar not only kills momentum, but can also be scary. 

So, why does this happen and what can you do about it? The good news is that these spikes are common and usually manageable.

What happens during exercise?

The first thing to know is that not all exercise affects blood sugar the same way. Slow, steady cardio, such as walking, hiking or running, tends to lower blood sugar because muscles use glucose for fuel. 

But intense exercise like strength-training, weightlifting and sprints can cause your blood sugar to do the opposite and go up.

That’s because these higher stress workouts lead your body to release hormones like adrenaline, cortisol, glucagon and other stress hormones designed to help you get through your workout. Then these hormones signal the liver to release glucagon, which raises blood sugar levels. 

If the effects of stress hormones are not balanced by the amount of glucose used for fuel during exercise, blood sugar levels can become too high. This also puts you at risk of dehydration as extra glucose spills into the urine. 

Other reasons blood sugar may rise during exercise

Other factors can lead to an unwanted rise in blood sugar. These include:

• Eating or drinking too many carbohydrates before or during a workout
• Overtreating a low blood sugar before or during a workout
• Disconnecting an insulin pump so no insulin is being delivered
• Reducing rapid-acting insulin too much at the meal or snack before a workout

What you can do about exercise-induced high blood sugars

If you do experience severe high blood sugar with intense exercise or if you have symptoms of a severe high, like fatigue or frequent urination, there are a few things you can do:

• Drink water to keep yourself hydrated and help dilute the extra glucose in your bloodstream
• Do a light aerobic warmup and cool down around anaerobic exercise, such as a 20-minute walk
• If you are experiencing intense anxiety or stress around a competition or exercise event, try visualization exercises to calm your nervous system and lower the release of stress hormones
• If your blood glucose levels remain elevated after these approaches, consider a conservative insulin correction dose. However, be careful when you do this, particularly later in the day, as it may increase the likelihood of a low blood sugar level overnight. 

When to check for ketones

If you experience unexplained high blood sugar during or after exercise, be sure to check for ketones. However, ketones can also develop without high blood sugar, so if you’re feeling sick during or after a workout, it’s a good idea to check for ketones regardless of your glucose level.

Ketones, especially when combined with dehydration and electrolyte imbalance, can increase the risk of diabetic ketoacidosis (DKA), a serious diabetes complication that requires prompt attention.

Be sure to check with your healthcare team about how to handle ketones before you start a new exercise routine.

Struggling with high blood sugar after exercise and want to meet others who may struggle with the same? Join in on the conversation on our community apps with people living with diabetes who get it.