Avoid Exercise Lows: How to Time Your Workouts Around Insulin Doses


Exercising as a person with type 1 or type 2 diabetes who takes insulin can be challenging. Nothing makes your efforts to exercise more frustrating than crashing with low blood sugars and having to eat mouthfuls of fast-acting carbs just to survive.

Looking for a way to avoid those mid-exercise low blood sugars? This method can help, whether you’re exercising first thing in the morning or later in the day!

What is “fasted” exercise?

Fasted exercise is a really simple way to encourage your body to burn fat for fuel instead of sugar from your bloodstream.

For instance, many bodybuilders do low-impact cardio first thing in the morning because exercising before eating helps them burn body fat instead of burning glucose or muscle for fuel.

This science is true in anyone, with or without diabetes—as long as your basal/background insulin dose(s) are accurate.

  • In a non-diabetic: As soon as you eat breakfast, you “break the fast” causing your body to switch gears and start burning glucose primarily for fuel.
  • In a person with diabetes: As soon as you take a bolus of rapid-acting insulin (via injection, pump, or inhaled insulin) for a meal, your body is going to burn glucose primarily for fuel while you exercise. The exercise will increase the rate of glucose you burn, increasing your risk of hypoglycemia.

If you exercise when you do NOT have a bolus of rapid-acting insulin on board, you hugely reduce your risk of low blood sugars.

If the idea of exercising first thing in the morning doesn’t sound ideal to you, keep reading!

Your background/basal insulin dose(s) needs to be accurate

A critical part of exercising in a “fasted” state is making sure your background/basal insulin dose via pump or long-acting insulin is accurate.

  • If you go low during fasted exercise: This means your background/basal dose is a bit too high. You can discuss reducing this dose slightly with your healthcare team!
  • If you start in your goal range but rise during fasted exercise: This could mean two things. Either you need a slight adjustment in your background/basal insulin, you’re doing a type of exercise that tends to raise blood sugar, or you’re spiking because of hormones related to dawn phenomenon or caffeine. You may need a very small bolus of rapid-acting insulin to keep you in your goal range while exercising—something to discuss with your doctor! (Keep reading to read examples of managing this rise in blood sugar.)

For example: My daily Lantus (long-acting) dose at the moment is 11 units taken at night. If I were taking 12 units, fasted exercise would definitely lead to low blood sugars. By adjusting my long-acting insulin dose to 11 units, fasted exercise does not cause lows.

That being said, I do need a very small bolus (about ¼ unit) of rapid-acting insulin to manage the rise I experience first thing in the morning.

Creating a “fasted” environment for exercise at ANY time of day

As a person with diabetes taking insulin, you can create that “fasted” environment:

  • If you take rapid-acting insulin (Novolog, Humalog, Fiasp, etc.) via injection or pump, that bolus of insulin is highly active in your bloodstream for approximately 3 to 4 hours. Exercising during that 4-hour window after taking injected rapid-acting insulin increases your risk of going low. Timing your workout for after that 3-4 hour window is ideal for exercising without low blood sugars!
  • If you use inhaled insulin (Afrezza), that bolus of insulin is highly active in your bloodstream for 60 to 90 minutes. Exercising during that 60 to 90-minute window after taking inhaled insulin increases your risk of going low. Timing your workout for after that 60 to 90-minute window is ideal for exercising without low blood sugars!

For example: If I want to exercise in the morning, and I wake-up in my goal blood sugar range, I usually see a small rise in my blood sugar due to the dawn phenomenon. I manage this by taking a very small ¼ unit once I see that rise beginning. Then I head out for a 45-minute dog walk, followed by 30 to 40 minutes of running or jumping rope — and 15 minutes of light weight lifting every other day.

What if your blood sugar is high before you want to exercise?

It’s important to correct high blood sugars — but you need to do this carefully if you’re about to exercise. You can take a significantly reduced correction dose of insulin to help bring your blood sugar down into your goal range without crashing low, still getting the overall benefits of “fasted” exercise because you haven’t eaten.

Remember: if your blood sugar is over 13.9 mmol/L250 mg/dL, you may be at risk of developing ketones. Exercising with extremely high blood sugars and ketones can be dangerous. It may be best to postpone your workout until you’re sure your ketones have cleared and your blood sugar is closer to your goal range.

For example: If I wake up with a blood sugar of 11.1 mmol/L200 mg/dL, I would take ¼ units (.25) of Novolog to ensure that my workout doesn’t cause it to rise further, and instead helps it come back into my goal range without crashing too low. This ¼ unit is a 75% reduction from the 1-unit I would normally take to correct a 11.1 mmol/L200 mg/dL blood sugar without the addition of exercise.

What if your blood sugar is low before you want to exercise?

It really depends on why you’re low.

If you’re low because you have a bolus of insulin still active in your bloodstream from a recent meal, it’s probably best to treat the low and wait until that insulin is out of your system.

If you’re just teetering on the edge of low when you wake up, you may be able to still do your fasted exercise after eating 5 to 10 grams of fast-acting carbohydrates. If you’re up to it and it feels safe! No, it’s not true “fasted” exercise because you’ve eaten. But if you keep the carb-consumption small so that it doesn’t require a bolus, you can still exercise with minimal risk of another low blood sugar.

For example: I know that if I wake up just slightly low, I can allow my dawn phenomenon to correct the low and then perform my usual morning exercise routine. I often still eventually need that ¼ unit of Novolog to keep the dawn phenomenon hormones from spiking me above my goal range. If I wake up with a blood sugar below 3.1 mmol/L55 mg/dL, I would treat with a few jelly beans, waiting until I rise above 4.4 mmol/L80 mg/dL, and then do my workout.

What if you’re doing exercise that causes your blood sugar to rise?

Certain types of exercise — usually anaerobic exercise, like lifting weights, spinning, sprinting, or Crossfit — can cause your blood sugar to spike. This is normal, and it can happen during “fasted” exercise, too.

Reasons your blood sugar might rise during exercise include:

  • During anaerobic exercise, your body can convert lactic acid to glucose (also known as gluconeogenesis) giving your body fuel to perform. In a person with diabetes, this can cause a mini or a mega spike in your blood sugar.
  • The adrenaline you produce during competitive sports tells your liver to release stored glucose to give your body a competitive boost. Also known as “fight or flight,” a person with diabetes doesn’t produce the extra insulin to accompany that extra glucose, so it can cause a mega spike.
  • Lastly, simply skipping breakfast can cause your liver to release stored glucose in an effort to give your brain the fuel it needs to function. Totally normal, but those of us with diabetes need to manage that spike with a small bolus of insulin.

During anaerobic exercise —fasted or not — most people with diabetes who take insulin find that they need a small bolus of rapid-acting insulin when they start or halfway through that workout. This can be amplified during a “fasted” state because there isn’t already lingering rapid-acting insulin on board from your most recent meal.

For example: If I were to lift weights for a full hour first thing in the morning, I know my body needs 1 unit of Novolog halfway through that workout. (I don’t lift weights intensely anymore because of fibromyalgia.) Without it, I’ll end up around 13.9 mmol/L250 mg/dL by the time I’m done. When jogging, I see my blood sugar spike about 50 points as I up the intensity of my training, so I can take 1/4 of Novolog before my run to prevent the rise of a 2-unit Afrezza cartridge after to quickly correct a mild high.

Should you take a correction dose for highs and a dose for other factors that raise blood sugar during exercise?

No. If you wake up high, then take the reduced correction dose as described above. Do your workout. And then determine afterwards if you need another small dose to manage continued dawn phenomenon or other factors that caused your blood sugar to rise during your workout.

If you learn overtime that your body can handle both a small correction dose plus a dose to manage those other factors, then that is a safe plan. The first priority is to prevent lows, then study and tackle the highs.

For example: If I wake up at 9.7 mmol/L175 mg/dL, I will take a 2-unit cartridge of Afrezza to correct that high quickly during my 45-minute dog walk. But I know my blood sugar tends to rise during my 30-minute jog, so if I feel safe doing so, I will take a 1/4 unit of Novolog right before my jog or a 2-unit Afrezza cartridge to quickly correct a mild high. By splitting my correction dose and my exercise/dawn phenomenon dose, I prevent low blood sugars because both at the same time would cause me to crash.

Hey, what if you want to eat right before exercising?

Whether you want or need to eat before exercising, it usually means you’ll need to reduce the amount of insulin you took for that meal. For some people, it might be 25 percent less insulin. For others, it might be 50 or 75 percent less insulin.

It really depends on how long you’re going to exercise, whether you’re doing anaerobic (weight lifting, etc.) vs. aerobic (jogging, power-walking, etc.), and how finely-tuned your insulin doses are for your body’s current insulin needs.

In general, if your insulin doses aren’t accurate, then nothing is going to go as planned! Talk to your healthcare team to get help fine-tuning your doses. A slight increase or decrease in your doses can make a huge difference in your success and safety.

For example: If I’m just too darn hungry to do my afternoon dog walk before eating lunch, I cut my lunch bolus down by at least 50 percent, if it’s just a 15-20 minute walk. If it’s going to be another 45-minute walk, I’d cut my lunch bolus down by 75 percent.

What if you’re doing endurance exercise and need to eat?

Fasted exercise is definitely not a great fit for hours and hours of training for something like a marathon or triathlon. You can, however, use it to help you get started as a beginner athlete with goals of competing in endurance events.

Recommendations for nutrition during endurance training and endurance events is that you consume fuel after every 60-minutes of exercise.

Running a marathon as a person with diabetes taking insulin is no simple task.

This balancing act requires careful self-study to determine how much insulin you need with each feeding during your endurance training or event. It’s critical to work closely with your healthcare team and carry fast-acting carbohydrates and emergency glucagon with you at all times.

Trial and error—and always be prepared for lows!

Learning how to manage your blood sugar during exercise is a learning process. It requires patience and a curious mind, looking for solutions versus getting mad and giving up.

It can be done! The more consistency you can create in your routine around the time of day you prefer to exercise, the easier it will be to study what works and what doesn’t.

And above all else: always, always, always keep fast-acting carbohydrates (or emergency glucagon) with you in case you do go low.

WRITTEN BY Ginger Vieira, POSTED 09/17/21, 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.