Blood Sugar Management—During and After Exercise for People Who Use Insulin

Exercising with diabetes can be tricky. Because exercise can increase the body’s sensitivity to insulin, you never know exactly how your blood sugar will respond. Still, there are some general patterns that apply to most people living with diabetes. When you understand those patterns, it gets easier to spot trends and figure out your own “formula” for food and insulin. That can help make exercise feel a lot less stressful.

In this article, we’ll break down the two main types of exercise, how blood sugar typically responds to each one and strategies that may help you manage highs and lows during and after workouts.

Types of exercise

1. Aerobic, also known as steady-state cardio. This includes activities like fast-paced walking or bike riding.
2. Anaerobic, which includes activities like interval training, resistance exercise and weight lifting.  

Blood sugar management during and after steady-state cardio

Cardiovascular exercise includes any workout where your heart rate increases for the duration of the activity. This includes activities like walking, jogging, cycling, or swimming at a consistent pace.

The general rule with steady-state cardio is that it can cause blood sugar to drop if you have insulin on board (IOB) or bolus on board (BOB). IOB or BOB is how much insulin you still have in your body that can keep lowering your blood sugar. Having too much IOB or BOB increases your chances of having low blood sugar. Some people don’t start to see the blood sugar lowering effect of steady-state cardio exercise until several minutes into a workout session, and some will only see the effect during specific types of workouts.

What happens during steady-state cardio is that you increase the body’s use of blood glucose. So if you have high levels of IOB during your cardio session, your muscles will take up more blood glucose for energy, increasing the risk of low blood sugar. This risk is not only increased during the cardio session, but also up to 48 hours afterward.

Strategies for preventing low blood sugar during and after steady-state cardio

Before making any adjustments to your insulin, talk to your healthcare provider about:

• Reducing the amount of IOB (both basal and bolus) by adjusting your insulin
• Adjusting basal insulin for up to 48 hours after a cardio session

Or you can:

• Try doing cardio while fasting, or at least four hours after your last bolus
• Consider a snack with carbs and protein if exercising longer than 60 minutes

Ask your healthcare provider about glucagon to raise your blood sugar in emergencies.

Blood sugar management before, during and after aerobic exercise 

Interval Training 

Interval training is a form of cardio that alternates between short bursts of intense activity and periods of lower-intensity movement or rest. Examples include running intervals on the treadmill or playing group sports like basketball, soccer and so on.

Unlike steady-state cardio, interval training can sometimes cause blood sugar to rise instead of fall.

Your body is trying to fuel the workout so it’s dumping energy, in the form of glucose, into your system to support your activity level. Some people notice this effect during exercise, while others see it continue for up to two hours after the workout.

A post-workout blood sugar spike can definitely feel frustrating, but your body is actually not working against you. You just need to anticipate this possibility and learn to work with it.

Talk to your healthcare provider about any blood sugar trends you notice when exercising. You may want to discuss:

• Adjusting your insulin slightly up, before starting your workout (basal rate or a small bolus)
• Having a small meal (+bolus) before your workout
• Adjusting your basal rate or bolus during your workout
• Adjusting basal or bolus insulin for up to two hours after your workout

Be careful when you increase your insulin, particularly later in the day, as it may increase the likelihood of a low blood sugar level overnight. 

If you find yourself sweating from exercising or getting dehydrated, drink water to keep yourself hydrated and help dilute the extra glucose in your bloodstream (body). Talk to your healthcare provider or diabetes care team about the best ways to stay hydrated. 

Resistance training

Resistance training can be incredibly helpful for improving insulin sensitivity, in addition to building strength. That’s a powerful combo!

Resistance training can impact blood sugar in different ways, depending on the intensity and number of reps. If you’re lifting lighter weights with high repetitions and short breaks, you may see blood sugar decrease since this type of resistance training has a similar impact on the body as cardio.

On the other hand, heavier lifting with lower repetitions may cause blood sugar to rise during the workout. Afterward, many people experience improved insulin sensitivity for up to 72 hours.

The blood sugar patterns that come with heavy lifting are often similar to those seen with interval training, so some management strategies overlap.

Higher stress workouts like heavy weightlifting, sprinting and competitive sports can make your body release different hormones like adrenaline, cortisol and glucagon. These hormones can raise blood sugar levels by making your liver release sugar. 

Exercise can be fun and rewarding, as well as an amazing stress reliever. But it can also feel frustrating when blood sugars seem unpredictable. Use these guidelines as a starting point to learn how your body responds to different types of movement and what strategies work best for you.

Key takeaways

In addition to these guidelines, if you don’t have access to diabetes technology, consider adding or using it to help you manage your IOB and blood sugar safely. Monitoring your blood sugar and paying close attention to your nutrition like the foods and beverages you eat and drink are key to supporting your exercise goals and diabetes management.

And remember: even when you know the general rules and have your own routine, exercise-related highs and lows can still happen. Always be prepared and have your diabetes gear (insulin, blood glucose monitoring kit or CGM) and fast-acting carbs within reach.

While experts recommend exercise for people living with diabetes, it’s important to do so safely. If you’re new or restarting an exercise routine, start slow, take breaks when needed and increase all with the guidance of your healthcare team. You’ll especially want to check with your healthcare professional before starting any high-impact or strenuous exercise routine like heavy weight-lifting to avoid injury and complications. 

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