Diabetes and Exercise: How to Prevent Low Blood Sugar During Workouts

When you have diabetes, exercise shouldn’t feel like a gamble. Yet for many, the fear of a sudden blood sugar drop keeps them from moving - even when they know it’s good for them. The truth? You can work out safely. It just takes a little planning. Preventing lows during workouts isn’t about avoiding activity - it’s about understanding how your body responds and adjusting before you start.

Why Exercise Drops Your Blood Sugar

When you move, your muscles need fuel. They pull glucose from your bloodstream - even without insulin. That’s great for lowering high blood sugar, but if you’re on insulin or certain medications, it can push your levels too low. This isn’t just during the workout. It can keep happening for up to three days after. That’s why people with type 1 diabetes often crash hours later, even after a perfectly fine session.

The American Diabetes Association says about half of adults with type 1 diabetes avoid exercise because they’re scared of lows. But you don’t have to be one of them. The key is knowing your numbers and timing.

What Your Blood Sugar Should Be Before You Start

Don’t just guess. Check your glucose 15 to 30 minutes before you begin. Here’s what to do based on your reading:

• Below 90 mg/dL: Eat 0.5 to 1.0 gram of carbs per kilogram of body weight. For a 70 kg person, that’s 35 to 70 grams - think a banana and a handful of crackers.
• 90-150 mg/dL: Still eat carbs. Aim for the same amount. This isn’t optional. Even if you feel fine, your body doesn’t know that.
• Below 100 mg/dL: The ADA recommends 15-20 grams of fast-acting carbs. A juice box, 4 glucose tablets, or 6 hard candies will do. Wait 15 minutes, check again. If it’s still low, repeat.
• Over 150 mg/dL: You’re probably safe, but monitor closely. Some experts, like Dr. Anne Peters, suggest pushing to 150-180 mg/dL before intense workouts to give yourself a buffer.

Insulin on Board: The Hidden Risk

This is where most people get caught off guard. If you’ve taken insulin in the last few hours, some of it is still working - that’s called insulin on board (IOB). During exercise, that active insulin doesn’t just sit there. It can act like 2-3 times more insulin than normal. That’s why two identical workouts can lead to wildly different results.

UCLA Health says: “The more insulin you have on board, the higher the likelihood of a low glucose.” So before you head out, check your pump or insulin pen log. If your IOB is over 1.2 units, you’re at risk. Here’s how to adjust:

• Insulin pump users: Reduce your basal rate by 50-75% starting 60-90 minutes before exercise. Some systems, like the Tandem t:slim X2 pump (approved in March 2023), now do this automatically with their Exercise Impact feature.
• Multiple daily injection users: Cut your pre-workout bolus by 25-50%. If you’re doing a long session, consider reducing your long-acting dose the night before - especially if you exercise in the afternoon.

What Kind of Exercise Is Safest?

Not all movement is equal when it comes to blood sugar. Aerobic workouts - like running, cycling, or swimming - tend to drop glucose steadily. But resistance and high-intensity training? They can protect you.

Research shows:

• A 10-second all-out sprint before or after your main workout can blunt the drop. One user on Diabetes Daily said adding this cut their lows from four times a week to one every two weeks.
• Resistance training (weight lifting, bands, bodyweight) raises glucose during the session. Doing 45 minutes of strength work before 45 minutes of cardio cut glucose drops from 166 to 124 mg/dL in one study.
• HIIT (High-Intensity Interval Training) with short bursts (5-15 seconds) reduces lows both during and after. It’s not just about intensity - it’s about the pattern.
• Circuit training (minimal rest, high reps) can be risky. It mimics long aerobic sessions and often causes delayed lows.

What to Do During Your Workout

You checked your numbers. You adjusted your insulin. But things change. That’s why you need to check again every 30-60 minutes during longer sessions. If you’re using a CGM, great - but don’t rely on alerts alone. They can lag.

Carry fast-acting carbs with you. Always. Not just in your pocket - in your hand, on your wrist, in your sports bra. You won’t remember if you have to stop and dig through your bag. Glucose gels, juice boxes, or even candy work. Don’t wait for symptoms like shakiness or dizziness. By then, it’s too late.

Post-Workout: The Silent Danger

The biggest risk isn’t during the workout - it’s hours later. About 70% of people with type 1 diabetes get a low 6-12 hours after exercise. That’s why you can’t just go to bed after a run and assume you’re fine.

Here’s what works:

• Check your glucose before bed. If it’s below 120 mg/dL, eat a snack with 15 grams of carbs and some protein - like peanut butter on toast or a small cheese stick with an apple.
• Set an alarm to check again in 2-3 hours if you exercised hard or late in the day.
• Use your CGM’s overnight alerts. Most modern devices let you adjust low alerts for nighttime.

Technology Is Your Ally

CGMs have changed everything. In 2022, 68% of type 1 diabetes users with CGMs checked their glucose before exercise - compared to just 42% without. Dexcom’s G7, released in October 2022, even has an “exercise mode” that lowers alert thresholds by 20 mg/dL during activity so you’re warned earlier.

And it’s getting smarter. The Tandem t:slim X2 pump now predicts glucose drops based on your past workouts and adjusts insulin automatically. Clinical trials show this cuts lows by over 30%. The next step? Artificial pancreas systems that deliver glucagon during exercise. Early results from the NIH’s Exercise-AP trial show 52% fewer lows. These aren’t future tech - they’re here now.

Real-Life Strategies That Work

People with diabetes have figured this out. Here’s what’s working:

• “I do a 10-second bike sprint before my 30-minute walk. No more crashes.” - u/DiabeticAthlete, Diabetes Daily
• “I reduce my basal by 60% 90 minutes before gym day. I’ve gone from 3 lows a week to none.” - T1D Exchange member
• “I eat 20g of carbs 20 minutes before I run. I also drink a protein shake after. No overnight lows.” - Reddit user, r/Type1Diabetes

It’s not magic. It’s trial, error, and tracking. Most people take 3-6 months to find their rhythm. Keep a log. Note what you ate, when you worked out, your insulin dose, and your glucose before, during, and after. Patterns emerge.

What Not to Do

Avoid these common mistakes:

• Skipping pre-workout carbs because you “feel fine.”
• Exercising during peak insulin action (usually 1-3 hours after a bolus).
• Not checking glucose after exercise because “it’s over.”
• Using alcohol before or after - it blocks your liver from releasing glucose.
• Assuming your response will be the same every time. Stress, sleep, weather, and hormones all change things.

Final Thoughts

Exercise isn’t a threat to your diabetes - it’s one of the best tools you have. But you need to treat it like a variable, not a constant. The same workout on Monday and Friday can give you two different results. That’s normal.

Start small. Pick one thing: check your glucose before your next workout. Then add a pre-workout snack. Then try a short sprint. Slowly, you’ll build confidence - and your body will learn.

You don’t have to be perfect. You just have to be consistent. And you’re not alone. Millions are doing this every day - and so can you.