How to Adjust Your Basal Rates for Endurance Exercise Training


Editor’s Note: Learn more about the 2021 team of Beyond Type 1 runners that participated in the TCS New York City Marathon. Beyond Type Run is sponsored by Dexcom and Tandem Diabetes Care.

Whether you’re running 3 miles or 13, learning how and when to adjust your insulin doses is a critical part of your training if you live with type 1 diabetes.

Training for any type of endurance sport—including running, cycling, swimming, or hiking—comes with a variety of factors that impact your diabetes management a bit differently than the average jog or power-walk.

Here, we’ll discuss how and when you may need to adjust your insulin doses throughout your endurance training.

When you might see changes in your sensitivity to insulin

“When you first begin any type of new exercise or training program, your sensitivity to insulin typically increases throughout the first two to four weeks,” explains Matt Schoenherr, CDCES and exercise physiologist who’s lived with type 1 diabetes (T1D) for nearly 30 years.

“But that increased sensitivity to insulin isn’t always sustained,” adds Schoenherr. “Don’t be surprised if your insulin requires drift back to pre-training levels. While preparing for something like a marathon, your insulin sensitivity may actually wax and wane throughout a training cycle.”

Schoenherr says you’re likely to be most sensitive to insulin (and thus, need a decrease in your doses) during:

  • The first month
  • The first rwo weeks of a new training phase (adding intervals, distance, etc.)
  • During the tapering phase
  • During a recovery week

And you may see your insulin needs increase during:

  • The “back half” of your training phase, when you’re most fatigued
  • Cumulative fatigue (exhaustion) = stress and stress = insulin resistance

Just how much will your insulin needs change?

Work closely with your healthcare team when making gradual changes to your insulin doses—everyone’s body is different!

While every person’s insulin needs are different, research on reducing basal/background insulin throughout endurance training offers some general expectations.

One factor to keep in mind is how active you were prior to training for a marathon or other endurance events. If you were an avid exerciser before, the change you’ll see in your insulin doses will likely be a bit less compared to someone who wasn’t regularly active until they began training.

Basal/background insulin: Throughout your marathon training and the day of a competitive endurance event, you can expect to see your basal/background insulin needs decrease by anywhere from 10 to 25 percent. (Read this research here and here.)

  • For example: If you used 25 total units of insulin per day for your basal/background needs before you began training for a marathon, it could decrease by 3.75 to 6.25 units over the course of your training.
  • In an insulin pump: When making changes to your basal rates, work closely with your healthcare team to determine if all of your basal rates should be adjusted or just certain parts of the day.
  • Keep in mind that you’ll likely need to set your reduced basal rate at least an hour before your run in order to reduce the amount of insulin on board when you start running.

Bolus/mealtime insulin: When it comes to your meal-time insulin doses, the biggest change you can expect to see is in how you dose for the meals you eat in the hours after training. Insulin sensitivity can be especially heightened in this window!

You may need anywhere from 25 to 50 percent less insulin for meals after a training session. This increased sensitivity to insulin after exercise can persist for hours. Taking notes to pinpoint how it affects your body is critical.

Correction doses: Correcting high blood sugars before, during and after training sessions should be done so with great caution. Remember that the exercise is going to hugely amplify the impact of your correction dose on your blood sugar. Most likely, your correction doses right before, during and immediately after training will need to be reduced by anywhere from 50 to 75 percent.

Work closely with your healthcare team when making gradual changes to your insulin doses.

Long-term changes in your insulin needs

If your training leads to notable changes in your physique—like increased muscle or loss of body fat—you may see more long-term changes in your insulin needs.

On the flip-side, endurance training also usually means you’re eating more calories, too, in order to fuel your training and recovery.

More calories can mean more insulin, both for meals and basal/background doses because your body will be working harder to keep your glycogen stores (stored sugar in your muscles and liver) full.

Needing more insulin is not necessarily a bad thing. Insulin is a powerful hormone that enables your body to use the sugar in your bloodstream for fuel. If you’re demanding more from your body through endurance sports, it makes sense that you may need more insulin, too.

Figuring out how much insulin your body needs to stay in your goal blood sugar range matters most.

Take time to fine-tune your insulin doses

“When you’re doing any type of sport or exercise with diabetes, part of the process is figuring out your body’s insulin needs,” encourages Schoenherr.

“Yes, there may be training sessions that are cut short because of low blood sugars—it’s going to happen. But your safety is key, so it’s important to go back and review what you did, to learn why and how to prevent it during your next training session.”

Schoenherr says taking good notes of what you ate, insulin doses, timing, etc. around every training session is especially important. The more details you track and reflect on, the more you can determine what helps you complete your training on any given day with the fewest fluctuations in your blood sugar.

“You should definitely practice fueling with food and drinks during your training, too, just like you would during the actual event,” reminds Schoenherr.

There is no “one-size-fits-all” in T1D

“Experimentation, record keeping and trial and error are key,” says Jacob Seltzer who worked with Integrated Diabetes Services to help him learn how to adjust his own insulin doses throughout his first marathon training experience.

Throughout the process, ask your endocrinologist and diabetes educators for help! Most likely, you aren’t the first endurance athlete with type 1 diabetes who they’ve encountered.

WRITTEN BY Ginger Vieira, POSTED 11/30/21, UPDATED 12/07/22

Ginger Vieira is an author and writer living with type 1 diabetes, celiac disease, fibromyalgia and hypothyroidism. She’s authored a variety of books, including “When I Go Low” (for kids), “Pregnancy with Type 1 Diabetes,” and “Dealing with Diabetes Burnout.” Before joining Beyond Type 1 as digital content manager, Ginger wrote for Diabetes Mine, Healthline, T1D Exchange, Diabetes Strong and more! In her free time, she is jumping rope, scootering with her daughters, or walking with her handsome fella and their dog.