Insulin Dosing and Timing Before and After Intense Cardio Exercise

Insulin dosing around exercise can be very frustrating and overwhelming for many people living with type 1 diabetes. 

‘Steady-State’ vs ‘Short-Burst’ Cardio

When you’re walking or jogging — often referred to as “steady-state” cardio — you’re at risk of experiencing low blood sugar because your heart is pumping steadily. 

Most people focus on reducing the risk of low blood sugar during this kind of ‘steady-state’ exercise. If you don’t adjust your insulin dose or consume carbohydrates, you’ll inevitably be burning glucose for fuel and increasing your risk of low blood sugar. 

However, you might have noticed that implementing those steady-state cardio strategies will make your blood sugars skyrocket if you do ‘short-burst’ intense cardio exercise — like sprints, high-intensity intervals, or Bootcamp-like classes. 

And finishing an intense cardio exercise session only to see your blood sugars go through the roof can feel even more frustrating than low blood sugars.

Different Strategies Needed

You need a different insulin dosing strategy for short-burst intense cardio exercise than you do for steady-state cardio.  This article covers which strategies work, and how you can customize them to your body.

Intense cardio impact on BG during exercise

Our bodies are clever machines. During exercise, a variety of glucose-raising hormones are released into our system to make sure our muscles can perform and we have enough energy to complete the workout. 

In people living with type 1 diabetes, intense cardio exercise or any anaerobic exercise can cause a sudden spike in blood sugars due to the body’s release of glucose-raising hormones such as glucagon and adrenaline. 

This release of hormones can also leave you more insulin resistant immediately after your workout.  Studies have shown that blood sugars can be elevated for up to 2 hours after intense exercise.

Intense cardio impact on BG after exercise 

While you might see that your blood sugar rises during intense cardio exercise and the hours after, this type of exercise also has the potential of improving your insulin sensitivity significantly for 24-48 hours after you are done exercising. 

This increase in insulin sensitivity can also significantly increase the risk of ‘late-onset hypoglycemia’ (also known simply as low blood sugar) because your body is still recovering and replenishing the glucose it stores in muscle tissue after your workout. 

Late Afternoon or Evening Exercise has higher risk

Gary Scheiner, MS, CDE points out in his book “Think Like A Pancreas” that late-onset hypoglycemia typically occurs 6-12 hours after intense exercise. 

If you exercise in the afternoon or evening, it means you have a higher risk of low blood sugar while you’re sleeping. Figuring out how to adjust your insulin doses or carb-intake before bed when you exercise late in the day is critical to ensure you’re safe while you sleep.

Managing your blood sugar before and after intense cardio exercise

Establishing a good understanding (by reading articles like this and through careful experiments and observation) of how intense cardio can impact blood sugars during and after exercise makes it easier and more fun to exercise safely! 

Learning to be proactive and plan wisely before, during, and after exercise is a definite must for all of us living with type 1 diabetes.  

When it comes to managing blood sugars during and after intense cardio, you have to learn to manage 4 variables that all impact your blood sugar.

1. Insulin dosing
2. Carbohydrates
3. Timing
4. type, duration and intensity of your cardio exercise

Guidelines for insulin dosing and timing before intense cardio exercise

It is generally recommended that people living with type 1 diabetes aim for a blood sugar of 7–10 mmol/L (~126–180 mg/dL) before exercise. 

However, since intense cardio exercise can make blood sugars rise, a lower starting blood sugar of 5–7 mmol/L ( ~90–126 mg/dL) might be more appropriate to not end up with blood sugars out of range. 

It is critical to remember that if you have a large amount of insulin on board from a meal or a correction dose, any type of exercise can still lower your blood sugar significantly.

Finding your Dosing Strategy

Establish that a certain type of exercise you’d like to keep doing (like a CrossFit class or jumping rope intervals or sprints) does indeed raise your blood sugar. Keep in mind other critical variables, especially how much insulin you already have onboard from a recent meal or correction for a high blood sugar.

To prevent a blood sugar spike during intense cardio, Gary Scheiner recommends taking a small dose of insulin before exercise. Sheri R. Colberg, Ph.D. seconds that strategy in her book, “The Athlete’s Guide to Diabetes”.

• Both agree that precaution should be taken when injecting insulin to prevent high blood sugars during or after intense cardio exercise since exercise increases insulin sensitivity. 
• Be careful when you begin experimenting with this approach. Just a small amount of additional insulin than you needed can easily lead to dangerous and potentially fatal low blood sugars while performing intense cardio. 

If you choose to correct your blood sugar with more insulin, both Sheri Colberg and Gary Scheiner recommend taking a significantly reduced dose. (For example, up to 50 percent of what you would normally take to correct that high blood sugar after the fact.) 

To reduce the risk of dangerously low blood sugars some guidelines even recommend not taking any correction doses until the exercise is completed.

An alternative strategy to additional insulin dosing is to utilize blood sugar lowering exercises such as steady-state cardio to reduce high blood sugars after intense cardio exercise. You could add a 10-15 minutes steady-state cardio cooldown, such as a light jog or walk, after intense cardio exercise to help drive your blood sugars down rather than inject any additional insulin.

Guidelines for insulin dosing and timing after intense cardio exercise

To prevent low blood sugars after exercise, Gary Scheiner recommends that you keep a detailed exercise and blood sugar log to learn when, if at all, your blood sugars drop after different types of intense cardio exercise.

If you do see a drop, he suggests one or more of the following strategies:

1. Reduce your basal insulin, either your pump’s basal rate or long-acting insulin, before the expected low. Some guidelines suggest up to 20% reduction 
2. If the low blood sugar is after a meal, reduce the mealtime bolus. Some guidelines suggest up to 50% reduction
3. Eat a slow-digesting snack before the expected drop, such as an apple with peanut butter or beans and veggies 

Adapting these insulin adjustment guidelines to you

The guidelines above are great starting points for finding the right insulin adjustment to keep blood sugars in range during and after intense cardio exercise. 

Since everyone is different, you’ll need to do some work to learn exactly how to adjust your insulin doses to fit your body’s needs and the types of exercise you enjoy the most.

3 step guide to your insulin adjustment needs

1. Read and understand the insulin adjustment guidelines for intense cardio exercise.
2. Learn how your blood sugars react by keeping a detailed exercise and blood sugar log.
3. Apply an analytic mindset and make incremental changes to your insulin dosages utilizing the information in your log (if needed collaborate with your doctor on this, the log will help her make the adjustments).