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

Working out with diabetes doesn’t have to mean avoiding exercise because you’re scared of crashing. But if you’ve ever felt shaky, sweaty, or dizzy mid-run - and your meter reads 65 mg/dL - you know the fear is real. About half of people with type 1 diabetes skip workouts just because they’re afraid of low blood sugar. The good news? You can train safely. It’s not about avoiding activity. It’s about planning it right.

Why Exercise Drops Your Blood Sugar

When you move, your muscles don’t wait for insulin to grab glucose. They pull it straight from your bloodstream to fuel movement. That’s a good thing - it helps lower blood sugar naturally. But for someone on insulin, this extra demand can tip the scales too far. Your body also becomes more sensitive to insulin for up to three days after a workout. So even if your glucose looks fine before you start, it might drop hours later - even while you’re sleeping.

Check Before You Start

Don’t just guess. Check your blood sugar 15 to 30 minutes before you begin. Here’s what to do based on the number:

• Below 90 mg/dL: Eat 0.5 to 1 gram of carbs per kilogram of body weight. For a 70 kg person, that’s 35 to 70 grams of carbs - think two bananas or a large granola bar.
• 90-150 mg/dL: Still eat 0.5-1 g/kg carbs before starting. Don’t skip this just because you think you’re "fine."
• Below 100 mg/dL: The American Diabetes Association recommends at least 15-20 grams of fast-acting carbs. Wait 15 minutes, check again. If it’s still low, eat another 15 grams.

Timing Matters More Than You Think

When you take insulin isn’t just about meals. It’s about movement. If you take a bolus at 8 a.m. and hit the gym at 10 a.m., you’re working out right when that insulin is peaking. That’s a recipe for a crash. Instead, aim to exercise when your insulin is stable - usually 2-4 hours after a meal. If you’re on a pump, look at your insulin-on-board (IOB). If you have more than 1.2 active units, your risk of low blood sugar jumps. That’s because during exercise, each unit acts like 2-3 units. One small bolus can turn into a big problem.

Not All Workouts Are Created Equal

A 30-minute jog? That’s a slow, steady glucose drop. But if you add a 10-second all-out sprint before or after? It can stop the drop cold. That’s because high-intensity bursts trigger adrenaline, which tells your liver to dump glucose into your blood. Same goes for lifting weights. A 45-minute strength session before cardio cuts glucose decline by almost half. One study showed people who did resistance first dropped from 166 to 124 mg/dL - compared to 166 to 105 mg/dL without it. That’s a 20-point buffer.

HIIT and Circuits - Use Them Wisely

High-intensity interval training (HIIT) - think 30 seconds sprint, 90 seconds walk - actually reduces hypoglycemia risk during and after. A 2019 study found glucose stayed stable for up to 45 minutes post-workout. But circuit training? That’s trickier. If you’re moving fast with little rest, your muscles keep burning glucose without the adrenaline spike to balance it. That can still cause lows. So if you’re doing circuits, check more often. And never skip the post-workout snack.

What to Eat and When

Carbs aren’t just for before. You need them during long sessions. For workouts longer than 60 minutes, eat 15-30 grams of carbs every hour. Think: half a banana, a handful of gummies, or a sports drink. And don’t wait until you feel low. Proactive fueling beats reactive fixing.

For overnight protection, especially after afternoon or evening workouts, eat a bedtime snack with 15 grams of carbs plus a little protein. A small apple with peanut butter or a cup of Greek yogurt with berries works. Why? Delayed hypoglycemia hits 70% of people with type 1 diabetes. You might feel fine at 10 p.m., but crash at 2 a.m. That’s not luck - it’s biology.

Technology Can Help - But It’s Not Magic

If you use a CGM, you’re already ahead. Dexcom’s G7 has an "exercise mode" that lowers alert thresholds by 20 mg/dL during activity. That means you get warned earlier. But even the best tech can’t predict everything. One user on Reddit said he did the exact same 5K run, at the same time, with the same insulin on board - and got 70 one day and 180 the next. That’s normal. Your body changes. Sleep, stress, weather, hormones - they all shift how you respond.

New pumps like the Tandem t:slim X2 now have an "Exercise Impact" feature. It uses machine learning to predict glucose drops based on your history and the workout you’re about to do. It can automatically reduce insulin delivery. That’s huge. But it’s still new. Most people still rely on manual adjustments.

Adjusting Insulin for Workouts

If you’re on an insulin pump, reduce your basal rate by 50-75% starting 60-90 minutes before moderate exercise. For example, if your normal basal is 1.0 unit/hour, drop it to 0.25-0.5 for the next few hours. If you’re on multiple daily injections, reduce your pre-workout bolus by 25-50%. Don’t just guess - track it. Write down what worked last time. Your body remembers patterns better than you think.

What People Are Actually Doing

On T1D Exchange, 78% of users use temporary basal reductions before exercise. Of those, 63% say it works. On Reddit, people swear by pre-workout sprints. One user went from four lows a week to one every two weeks just by adding a 10-second bike sprint before his run. Another said he stopped night lows after starting a bedtime snack with protein. These aren’t outliers. They’re people who learned by doing.

The Learning Curve Is Real

Don’t expect to master this in a week. It takes 3-6 months of consistent tracking to understand your body’s response to different workouts. Start small. Try one new activity at a time. Check your glucose every 15 minutes during the first few sessions. Log everything: what you ate, how much insulin you took, how hard you worked, and your glucose before, during, and after. Over time, you’ll see the patterns.

When to Stop and What to Carry

Always carry fast-acting carbs. Glucose tabs, juice boxes, gummies - something you can grab fast. Don’t rely on your phone or gym bag. Keep it in your pocket. If your glucose drops below 70 mg/dL during exercise, stop. Eat 15 grams. Wait 15 minutes. Check again. Don’t push through. A low can turn into a severe one fast.

What to Do After

Your work isn’t done when you finish. Check your glucose again 1-2 hours after. And if you exercised in the afternoon or evening, check before bed and possibly once during the night. Set an alarm if you need to. Delayed lows are the silent killer.

It’s Not About Being Perfect

Some days you’ll nail it. Other days, you’ll crash anyway. That’s okay. The goal isn’t perfection. It’s consistency. You don’t need to avoid exercise. You need to plan for it. Every low you prevent is a win. Every workout you complete without a crash is progress.

Final Thought

Exercise isn’t the enemy. Fear is. You can run, swim, lift, bike - and stay safe. It just takes a little prep, a little patience, and a lot of data. Start with one change: check your glucose before your next workout. Then add one more thing next time. Slowly, you’ll build confidence. And your body will thank you.