Working out with diabetes doesn’t have to mean avoiding movement because you’re scared of crashing. But if you’ve ever felt shaky, sweaty, or dizzy halfway through a run - or woke up in the middle of the night with a pounding heart after a morning gym session - you know the fear is real. Preventing hypoglycemia during exercise isn’t about giving up activity. It’s about working smarter with your body’s natural responses.
Why Exercise Drops Your Blood Sugar
When you move, your muscles need fuel. They don’t wait for insulin to bring glucose in - they pull it straight from your bloodstream. That’s great for burning fat and improving insulin sensitivity, but it’s also why your blood sugar can plummet during or after a workout. This isn’t just a minor inconvenience. For people using insulin, especially those with type 1 diabetes, exercise can trigger lows that last for hours - even overnight. Research shows that up to half of adults with type 1 diabetes avoid physical activity because of this fear. But you don’t have to be one of them. The key isn’t to stop exercising. It’s to understand how your body reacts and adjust before you start.Check Your Numbers Before You Start
Never jump into a workout without checking your blood sugar. If it’s below 90 mg/dL, you’re in the danger zone. At this level, you need to eat before you move. The rule of thumb? Eat 0.5 to 1 gram of carbs per kilogram of your body weight. For a 70 kg person, that’s 35 to 70 grams of carbs - think a banana and a handful of pretzels, or a granola bar and a small juice box. If your glucose is between 90 and 150 mg/dL, you’re in the safe zone - but still not out of the woods. Eat 15 to 20 grams of fast-acting carbs (like glucose tablets, fruit juice, or honey) 15 to 30 minutes before starting. Recheck after 15 minutes. If it hasn’t risen, eat another 15 grams. Don’t guess. Test.Timing Matters More Than You Think
When you take insulin matters as much as when you exercise. If you take a bolus for lunch and hit the gym at 3 p.m., you’re asking for trouble. That insulin is still active - and your muscles are hungry. The result? A steep drop. The best strategy? Exercise at the same time every day. Consistency helps your body predict what’s coming. If you always work out at 7 a.m., your body learns to expect it. That makes your glucose responses more stable. Also, check your insulin-on-board (IOB). If you’ve got 1.2 units of insulin still working in your system, your body might treat it like 2 or even 3 units during exercise. That’s a recipe for a low. Most insulin pumps let you see your IOB. Use it. If it’s over 1 unit, consider reducing your pre-workout bolus by 25-50% or lowering your basal rate by 50-75% for 60-90 minutes before you start.Not All Workouts Are Created Equal
A 30-minute jog? High risk for a low. A 10-second sprint? That might save you. Here’s the surprising truth: anaerobic exercise - short bursts of high intensity - actually helps prevent lows. A 10-second all-out sprint before your workout can block glucose from dropping too far. One study showed people with type 1 diabetes who added a quick sprint before cycling cut their hypoglycemia episodes from four times a week to just once every two weeks. Resistance training is another game-changer. Doing 45 minutes of weight lifting before your cardio session can reduce glucose drops by nearly 40%. That’s because lifting pulls glucose into muscles without relying on insulin. It’s like hitting a pause button on the crash. High-intensity interval training (HIIT) - think 30 seconds of sprinting, 90 seconds of walking - also helps. The bursts of intensity raise your blood sugar temporarily, protecting you during the recovery phases. On the flip side, long, steady-state aerobic workouts - like running, swimming, or cycling for over 45 minutes - are the biggest risk. They keep pulling glucose out without giving your body a chance to rebound.
What to Eat During Long Workouts
If you’re doing something longer than 60 minutes - say, a hike, a long bike ride, or a 5K race - you’ll need to refuel. Don’t wait until you feel low. Start early. Aim for 15-30 grams of carbs every 30-60 minutes. Easy options: energy gels, sports drinks, dried fruit, or even a few pieces of candy. Keep them in your pocket, your waistband, or your bike bag. You won’t have time to dig through your bag when your hands are shaking. And don’t forget to recheck your glucose every 30 minutes. Even if you feel fine, your numbers can drop fast. A CGM with alarms set to 80 mg/dL is your best friend here.The Hidden Danger: Delayed Lows
The scariest lows don’t happen during your workout. They happen hours later - often while you’re asleep. Up to 70% of people with type 1 diabetes experience delayed hypoglycemia 6 to 12 hours after exercise. That’s why you can feel great after your run, eat dinner, go to bed - and wake up at 3 a.m. with a heart racing and sweat soaking your sheets. To prevent this, eat a small bedtime snack if you exercised in the afternoon or evening. It doesn’t need to be huge - just 15 grams of carbs with a little protein. Think: a slice of whole grain toast with peanut butter, or a small cup of Greek yogurt with a few berries. If you use a pump, consider setting a reduced basal rate overnight. Some pumps now have automatic features that do this for you. If you’re on injections, talk to your doctor about adjusting your evening dose after active days.Technology Is Your Ally
Continuous glucose monitors (CGMs) have changed everything. They don’t just show your number - they show the trend. A falling arrow means you need to act before you crash. Newer models like the Dexcom G7 even have an “exercise mode” that lowers alert thresholds during activity. That means you’ll get an alert at 80 mg/dL instead of 70 - giving you time to eat before you feel awful. Insulin pumps with smart features, like the Tandem t:slim X2, now use machine learning to predict glucose drops during exercise. They can automatically reduce insulin delivery before and during your workout. If you have access to one, turn it on. Even if you don’t use advanced tech, basic tools help. Keep a log: write down what you ate, what you did, your starting glucose, and how you felt. After a few weeks, patterns emerge. You’ll start to see: “Cycling after lunch = low. Weightlifting before cardio = safe.”
What Doesn’t Work
A lot of people think the solution is to eat more carbs. But that’s not always right. Eating too much before a workout can cause a spike, then a crash. Or you might end up eating more than you burn - gaining weight, not gaining control. Others skip insulin entirely before exercise. That’s dangerous. It can lead to high blood sugar, ketones, and diabetic ketoacidosis - a medical emergency. And never rely on how you feel. You can feel fine with a glucose level of 60 mg/dL. Or you can feel awful at 120. Your body doesn’t always tell you the truth. Only your meter or CGM does.Start Small. Build Confidence.
If you’re new to exercising with diabetes, start slow. Try 15 minutes of walking after dinner. Check your glucose before, during, and after. See how your body responds. Then try a light resistance band workout. Then add a short sprint. It takes 3 to 6 months to learn your patterns. Don’t get discouraged if you have a low. That’s part of the process. Every low teaches you something. The goal isn’t perfection. It’s consistency. You don’t need to run a marathon. You just need to move - safely - every day.Final Rule: Always Have a Backup
No matter how confident you are, always carry fast-acting carbs. Glucose tabs. Juice boxes. Hard candy. Don’t rely on your gym having them. Don’t assume someone else will help you. Wear a medical ID. Let your workout buddy know you have diabetes. Tell them what to do if you get low - give you carbs, call 911 if you’re confused or unconscious. Exercise isn’t the enemy. Fear is. And fear fades when you have a plan.Can I exercise if my blood sugar is below 70 mg/dL?
No. If your blood sugar is below 70 mg/dL, treat it first. Eat 15 grams of fast-acting carbs (like glucose tablets or juice), wait 15 minutes, and recheck. Only start exercising once your glucose is above 90 mg/dL. Exercising while low can make things worse and lead to severe hypoglycemia.
Do I need to eat before every workout?
Not always. If your blood sugar is above 150 mg/dL and you’re doing light activity like a short walk, you may not need extra carbs. But for moderate to intense workouts - especially if you’re on insulin - it’s safer to eat 15-20 grams of carbs before starting. Better safe than sorry.
Why do I get low after exercise, even if I felt fine during?
After you stop moving, your muscles keep pulling glucose from your blood to refill their stores. Plus, your body becomes more sensitive to insulin for up to 72 hours. This means your insulin - even if it was working fine during your workout - can keep lowering your glucose long after you’ve cooled down. That’s why nighttime lows are so common after afternoon workouts.
Should I lower my insulin before exercise?
Yes - if you use insulin. For pump users, reduce your basal rate by 50-75% starting 60-90 minutes before exercise. For those on multiple daily injections, reduce your pre-workout bolus by 25-50%. Never skip insulin entirely. Talk to your healthcare provider to find your personal adjustment range.
Is it safe to do HIIT with diabetes?
Yes - and it might be safer than steady cardio. Short bursts of high-intensity exercise raise blood sugar temporarily, which can protect against lows during and after. Studies show HIIT reduces hypoglycemia risk compared to long, slow workouts. Still, check your glucose before, during, and after. Start with 2-3 intervals and build up.
What’s the best time of day to exercise with diabetes?
There’s no single best time, but consistency matters most. Exercising at the same time each day helps your body predict glucose changes. Avoid working out during peak insulin action - like right after a meal bolus. Morning workouts before breakfast can be safer for some, but only if you’re not prone to overnight lows. Track your patterns for a few weeks to find what works for you.
Can I use a CGM to prevent lows during workouts?
Absolutely. CGMs are one of the most effective tools for preventing exercise-related lows. Set alerts for 80-90 mg/dL to give yourself time to act. Use trend arrows - a falling arrow means you need carbs now. Some newer models even have exercise modes that adjust alerts automatically. If you don’t have one, consider talking to your doctor about getting one.
What should I do if I feel low during a workout?
Stop immediately. Check your blood sugar. If it’s below 70 mg/dL, consume 15 grams of fast-acting carbs. Wait 15 minutes and recheck. Don’t try to power through. Pushing through a low can lead to passing out, injury, or worse. Once your glucose is back above 90 mg/dL, you can resume - but keep checking every 15-20 minutes.