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Exercising with type 1 diabetes isn’t simple — but it is doable if you learn the essential science of juggling insulin, timing, nutrition, and aerobic vs. anaerobic exercise. 

Here’s a closer look at exercising safely with type 1 diabetes with excerpts from my book Exercise with Type 1 Diabetes.

Woman with diabetes exercising at the gym

How much exercise do you need?

Regular exercise can have a tremendous impact on your ability to manage your weight, your insulin sensitivity, and your blood sugars. The more consistent you are with daily exercise, the more you’ll reap those benefits. 

The Center for Disease Control & Prevention recommends 150 to 300 minutes per week of moderate exercise, 75 to 150 minutes per week of vigorous exercise, or a combination thereof.

This breaks down to about 20-25 minutes per day. A little goes a long way! (And don’t underestimate the value of a regular walk!)

If you are introducing regular exercise to your daily routine, be sure to plan with your healthcare team how to adjust your insulin doses — you might see an immediate and gradual increase in your sensitivity to insulin. This means you’ll need to reduce your insulin doses (basal and/or bolus) to prevent hypoglycemia.

Managing insulin-on-board

Exercising safely starts with managing your insulin-on-board (IOB) and the timing of your workout. IOB refers to how much insulin is currently active in your bloodstream. 

When you have too much IOB within the hours of your workout, your risk of low blood sugar is extremely likely.

There are three common approaches to managing IOB to prevent low blood sugar during exercise:

  • Exercising several hours after eating or taking insulin for a meal
  • Exercising right after eating but reducing your meal bolus and/or basal insulin 
  • Exercising when you feel like it and consuming a lot of candy or juice to get through it

If you’re getting too much or too little basal/background insulin via injected long-acting insulin or pumped rapid-acting insulin, fasted exercise simply ain’t gonna go as planned. Learning how to fine-tune and adjust your background dose(s) is a must.

Keep in mind that adjustments to your basal/background insulin doses should be done in very small tweaks — a change of no more than 1 or 2 units total at a time for insulin-sensitive people.

For those on larger doses, your doctor may recommend larger adjustments of 3+ units at a time. Watch the impact for a couple of days, then adjust again if needed.

Here are the basics to consider when fine-tuning background insulin doses for fasted exercise:

Low blood sugars during fasted exercise likely mean: You are getting too much basal/background insulin whether it’s aerobic or anaerobic.

  • If you’re on injected long-acting insulin: Sometimes a tiny adjustment can make a huge difference. A reduction of just 1 or 2 units can address those lows, but don’t be surprised if that means you need a bit more for meals. Many people are on significantly too much basal insulin which means the introduction of any physical activity sends their blood sugars repeatedly crashing. Get those doses down!
  • If you are on an insulin pump: Most people find they need to reduce basal rates by anywhere from 25 to 75 percent starting 1 hour before aerobic exercise. It certainly requires a great deal of trial and error. Most people also have already adjusted their basal rates to account for spikes in the early morning due to the dawn phenomenon, so that is less of a factor to consider if you’re pumping.

High blood sugars during fasted exercise likely mean: You’re not getting enough insulin either via basal/background OR you need a small bolus to address spikes caused by anaerobic exercise or dawn phenomenon.

Anaerobic exercise raises blood sugar: Some people find they need a small bolus before, during, or after intense strength training or other anaerobic exercise. This is normal! Experiment carefully and take good notes.

The tiniest adjustments can make a big difference! Just 1 or 2 units added or subtracted from your total basal/background insulin dose(s) can make it that much easier to enjoy exercise safely.

Why “fasted” exercise reduces your risk of lows

If you can time your workouts for when you have the least amount of rapid-acting insulin in your system (your IOB), you can hugely reduce your risk of low blood sugar. 

In the context of type 1 diabetes, the phrase “fasted” exercise refers to exercising at least several hours after the last time you ate and took mealtime insulin. By exercising before you eat a meal, you are exercising when you don’t have a large bolus of rapid-acting insulin in your bloodstream. Most rapid-acting insulin is fully active in your system for approximately 4 hours after dosing. 

Fasted exercise is not intended for endurance sports — hours of exercising — because your body is inevitably going to need re-fueling eventually for long athletic events. 

“Fasted” exercise still requires basal/background insulin — even if it is a significantly reduced dose via your insulin pump. If you’re on long-acting injected basal insulin, you may need to make gradual reductions to the daily dose when you add exercise to your routine. 

As people living with type 1 diabetes, we need some active insulin present at all times in order to stay alive. Without enough basal/background insulin at any given time, a person with type 1 diabetes can quickly go into diabetic ketoacidosis — which is extremely dangerous and can be fatal.

“Fasted” exercise is not equal at all times of the day. Pre-breakfast workouts tend to see less change in blood glucose because levels of hormones like cortisol are still high from overnight. Pre-lunch workouts when you had insulin at breakfast are a completely different animal.

Aerobic and anaerobic exercise

You must learn the difference between aerobic (cardio) exercise vs. anaerobic exercise to better predict the impact it will have on your blood sugar and insulin needs.

Aerobic exercise is an activity you can perform for an extended period of time without stopping. During aerobic exercise, your heart rate is still low enough that your body is able to cycle oxygen to your fat cells to burn for fuel. Or, if there’s plenty of IOB, your sources of fuel will likely be glucagon and the glucose in your bloodstream. 

Examples of aerobic exercise: walking, jogging, kayaking, rollerblading, power yoga, skating, vacuuming, gardening, cycling, dancing, hiking, chasing kids, etc.

Aerobic exercise increases how quickly insulin picks up glucose and carries it to cells — because your cells are demanding more glucose to keep your body moving and performing! In a person with T1D, this can cause dramatic low blood sugar.

You can significantly reduce how much glucose aerobic exercise uses for fuel by carefully timing your workouts to reduce IOB.

Anaerobic exercise is an activity that’s usually performed at an intensity you can only sustain for a few minutes at most. Then you take a break — for anywhere from 30 seconds to 2 minutes — then you perform another round, and so on.

Examples of anaerobic exercise: lifting weights, sprinting, CrossFit, HIIT workouts, spinning intervals, etc.

Your heart rate during anaerobic exercise is so high that your body must use glucose for fuel by converting stored glucose (glycogen) in your muscle tissue and lactic acid to glucose. That glucose is then cycled back to your muscles for fuel with a little help from insulin.

For some types of anaerobic exercise, the insulin from your basal rate/dose might be plenty. For more intense anaerobic exercise, you may need more insulin to manage that extra glucose. 

After anaerobic exercise, your liver might release stored glucose from your liver to help replenish the glucose stores in your muscles — with a little help from insulin. In a person with T1D, this can cause unexpected spikes in blood sugar.

Mixed workouts with aerobic & anaerobic exercise: When you combine both types of exercise into one workout, you’ll want to consider the impact of both. 

For example, if you lift weights for 30 minutes and then jog for 30 minutes, the rise in your blood sugar from lifting weights could be offset by the jogging that would lower your blood sugar. 

If you perform 30 minutes of cardio first, you could expect your blood sugar to trend downwards (even with little-to-no IOB), then expect the 30 minutes of weightlifting to bring it back up. 

Like any other type of workout, mixing aerobic and anaerobic simply requires some careful preparation, consistency as you study what happens, and good notes! 

When you shouldn’t exercise

There are a few situations in which you should not exercise as a person with T1D.

If your blood sugar is over 250 mg/dL: Exercising with blood sugars over 250 mg/dL can put you at risk of developing ketones — particularly if you do not have enough IOB. While exercise with a thoughtful dose of correction insulin could help bring your blood sugar down, it should be done with consideration of ketones. If your blood sugar is over 300 mg/dL before exercising, testing for ketones is a must! 

If you already have ketones: Exercising with ketones is very dangerous and can further the production of ketones, increasing your risk of diabetic ketoacidosis (DKA). Contact your healthcare team for support or visit the nearest emergency department if you exhibit signs of DKA.

Be prepared for lows and take good notes

No matter what type of exercise you’re doing, you should always carry fast-acting carbohydrates with you, like glucose tabs, juice, jelly beans, or gummies.

Lastly, take good notes!

If you’re experiencing low blood sugar after every Zumba class, that tells you loud and clear that you have too much insulin on board before and during class.

If your blood sugar is consistently spiking an hour after your weightlifting workout that tells you loud and clear that you probably need a post-workout bolus of rapid-acting insulin. 

Take good notes. While blood sugar fluctuations during exercise can be very frustrating, they’re not illogical. Take a deep breath and keep experimenting!

Learn more in Ginger’s easy-to-read book, Exercise with Type 1 Diabetes, on Amazon!