Exercising with Diabetes: How to Prevent Lows with Fasted Workouts

Editor's Note: The information provided in this article on exercise and fasting is for general informational purposes only and should not be considered medical advice. Always consult with a healthcare professional before beginning any new diet, fasting regimen, or exercise program.

Want to avoid those frustrating lows during physical activity? This method can help—it’s my favorite method—and it doesn’t matter what time of day you prefer to exercise.

Get more support with exercise and insulin here:

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• Free 5-Day Email Course: Exercise with Type 1 Diabetes

What Is Fasted Exercise?

Fasted exercise is a simple way to train your body to burn fat for fuel instead of relying on glucose from your bloodstream.

For example, bodybuilders often do low-impact cardio before breakfast because working out before eating helps their bodies tap into fat stores rather than glucose or muscle tissue. This principle applies to everyone, with or without diabetes—as long as your basal/background insulin doses are properly adjusted.

(I didn’t believe this would work when it was first explained to me by my powerlifting coaching nearly 20 years ago!)

• For people without diabetes: Eating breakfast “breaks the fast,” shifting the body to primarily burn glucose for fuel.
• For people with diabetes: Once you take a bolus of rapid-acting insulin for a meal, your body will prioritize burning glucose, increasing the risk of hypoglycemia during exercise.

By exercising when there is no rapid-acting insulin active in your bloodstream, you can significantly reduce your risk of a low blood sugar episode.

If morning workouts aren’t your thing, don’t worry—there are ways to create a “fasted” exercise state at any time of day!

First, is Your Basal Insulin Dose Accurate?

For fasted exercise to be effective and safe, your basal (background) insulin dose must be properly adjusted.

• If you go low during fasted exercise: Your basal insulin might be too high. Talk to your healthcare provider about making small adjustments.
• If your blood sugar rises during fasted exercise: This could mean your basal insulin needs tweaking, your workout type is causing a spike, or hormones (like those from the dawn phenomenon or caffeine) are affecting your levels. A small bolus of rapid-acting insulin may help—discuss with your doctor for a personalized approach.

For instance, if I take 10 units of long-acting insulin at night, fasted exercise makes my blood sugar drop too much. By adjusting my dose to 9 units, I avoid those lows. However, I still take a tiny ¼ to ½ unit of rapid-acting insulin around 8:30 a.m. to counteract the spike of glucose from my liver because I’ve skipped breakfast. I’ve usually finished my morning workout by then but they might overlap sometimes. 

How to Exercise in a Fasted State Anytime

If you take rapid-acting insulin (Novolog, Humalog, Fiasp, etc.), it remains active in your bloodstream for about three to five hours. Exercising within that window increases your risk of hypoglycemia. Waiting until after this period reduces that risk significantly.

If you use inhaled insulin (Afrezza), which is active for about 60-90 minutes, the same rule applies—plan your exercise after that window to minimize lows.

For example, if I plan to eat my daily apple and peanut butter at 1 p.m., it would make more sense to time my dog walk before lunch, so I don’t have the big bolus of insulin on board to cover the food. I know any insulin I took in the morning has cleared my system by noon so the only insulin active in my system is the meal bolus.

What about insulin pumps?

Well, today’s pumps have definitely complicated some of this. 

If you’re using an insulin pump, you must make adjustments at least an hour before your workout, particularly for aerobic (cardio) exercise. Every system is a little different, and there are a variety of ways people have approached it, including:

• Set a temp basal rate (potentially 50 to 75 percent less) starting one hour before you exercise.
• Put the pump in exercise mode during your workout to prevent it from giving you additional insulin.
• Consider replacing lost basal insulin with a tiny bolus after you exercise if you experience post-workout highs. Remember that the basal insulin you get from 12 p.m. to 1 p.m. affects your blood sugars the most, starting at 1 p.m. 

The goal is to reduce/limit/control how much that automated system gives you. It needs to know you’re exercising. Some people turn their automated system off during exercise and go manual—but this comes with the risk of it not shutting insulin delivery off during lows.

Again, each system is a little different, so it’s hard to give one specific method that works across all pumps. 

High Blood Sugars Before Exercise…

If your blood sugar is high before working out, correcting it requires caution. A reduced correction dose can help bring it into range without causing a sudden drop. This allows you to exercise in a fasted state without consuming food.

However, if your blood sugar is over 250 mg/dL, you might have ketones. Exercising with ketones can be dangerous, so check your levels and wait until your blood sugar is back in range.

For example, if my blood sugar is 180 mg/dL, I take just ¼ unit of Novolog, which is 75% less than my usual correction dose, to prevent a drop while still allowing my workout to help bring it down.

Correct highs with caution!

Low Blood Sugars Before Exercise…

Yes, you can potentially exercise after a low, but it depends on why you’re low:

• If it’s due to active insulin from a recent meal, treat the low and wait until the insulin is out of your system OR treat the low with more than 15 grams of carbs and proceed with your exercise. Beware that you run the risk of going low again.
• If you’re slightly low upon waking, you may be able to exercise after eating 5-10 grams of fast-acting carbs, as long as you feel safe. Though it’s not technically a fasted workout, keeping the carb intake minimal avoids the need for a bolus dose. For example, if I wake up at 50 mg/dL, I could eat a packet of glucose powder and wait until I reach at least 80 mg/dL (4.4 mmol/L) before starting my workout. Since I know the low isn’t from a meal bolus (because I haven’t eaten yet), the low is easier to treat and then exercise. There isn’t excess insulin on board.

High Blood Sugars During Exercise…

Some exercises, especially anaerobic activities like weightlifting, sprinting, or high-intensity interval training (HIIT), can cause blood sugar spikes.

Possible reasons include:

• Dawn phenomenon: If your workout is first thing in the morning, you’re overlapping with dawn phenomenon hormones that need attention regardless of exercise. 
• Gluconeogenesis: The body converts lactic acid into glucose for fuel.
• Adrenaline spikes: Competitive or intense workouts signal the liver to release glucose.
• Skipping breakfast: The liver may release stored glucose to fuel your brain.

Most people with diabetes find that taking a small bolus of rapid-acting insulin before or during anaerobic workouts helps prevent excessive spikes.

For example, if I lift weights for an hour in the morning, I take one unit of Novolog halfway through to prevent my blood sugar from skyrocketing. When I jog, I take ¼ unit before starting or a small dose of Afrezza afterward to correct mild highs.

If You Need or Want to Eat Before Exercising

If you eat before working out, you’ll likely need to reduce your mealtime insulin dose. How much depends on:

• The type of exercise (aerobic vs. anaerobic)
• The duration of your workout
• How well your insulin doses match your body’s needs

For example, if I eat lunch before a short 15-20 minute walk, I reduce my mealtime bolus by 50%. If it’s a 45-minute walk, I cut my bolus by 75%.

When Fasted Exercise Doesn’t Make Sense

Fasted workouts don’t make sense for endurance training like marathons or triathlons. Once you exceed 60 minutes of exercise, fueling with carbohydrates becomes pretty critical!

This is a whole other ball of wax and requires much more customized training and support. I highly recommend working with coaches like Glucose Advisors—experts in helping people with T1D learn the deeper science of endurance training and insulin.

Hey, Be Prepared No Matter What!

We all have to experiment, take notes, and be persistent when it comes to exercise and insulin. Every little factor makes a difference. If you normally run four miles at 6 a.m. and suddenly you do the same run at 4 p.m., you’re gonna have to make some adjustments to your plan. 

No matter what, prepare for lows. 

Keep fast-acting carbs with you at all times and focus on consistency—exercising at the same time each day can make it easier to track patterns and make necessary adjustments–at least while you’re learning. Once you better understand what’s going on, you can start mixing things up more confidently!