Preventing Low Glucose During and After Exercise

Prevent low blood sugar during exercise by starting at a safe glucose level, reducing “insulin on board,” and fueling with the right carbs at the right time.

Exercise is one of the best tools for health—but for people using insulin (and some glucose-lowering meds), it can also cause sudden lows during activity and “delayed lows” hours later (often overnight). This 2026 guide gives you a simple plan you can repeat for walks, workouts, sports, and long endurance days.

Searching for a specific answer? Jump to our Frequently Asked Questions for quick tips on exercise safety.

What’s the fastest way to stop sugar crashes during workouts?

Use this 3-step plan:

Start safe: aim to begin around 90–180 mg/dL with a steady or slightly rising trend.
Lower insulin risk: reduce meal bolus or use Exercise/Activity Mode (start it early) so you have less active insulin during the workout.
Fuel on purpose: carry fast carbs and “dose” carbs during longer activity—especially if your CGM trend is dropping.

When	Do this	Why it works
60–120 min before	Turn on Exercise/Activity Mode or set a temporary higher target (pump/AID). If eating, consider lowering the meal bolus.	Gives insulin time to “ease off” so your glucose is less likely to drop mid-workout.
Right before	Check CGM trend + IOB. If <90 mg/dL or dropping, take 10–20g fast carbs.	Most exercise lows happen when you start low or with too much active insulin.
During	Recheck every 30–60 minutes (more often for long/intense sessions). Use small carb “boosts” as needed.	Prevents small drops from turning into a full low.
0–2 hours after	Refuel (carb + protein). Consider a smaller bolus than usual if you’re more sensitive after exercise.	Rebuilds glycogen and reduces delayed lows.
6–36 hours after	Watch for delayed/nighttime lows: consider a temporary basal reduction, higher CGM alerts, and/or a bedtime snack.	Muscles keep pulling glucose to restore glycogen and insulin sensitivity stays higher.

Safety note: If you feel low symptoms (shaky, sweaty, confused), treat immediately—even if CGM looks “okay.” Confirm with a fingerstick if symptoms don’t match the CGM.

When are you most at risk for exercise lows?

Starting low or trending down on your CGM
High insulin on board (IOB) from a recent bolus (often within the past 2–4 hours)
A moderate-to-severe low in the past 24–36 hours (your “low protection” hormones may be blunted)
Long, spontaneous, or unfamiliar activity (untrained muscles can soak up more glucose)
New intensity, new sport, hills, wind, heat/cold (all can change glucose needs fast)
No insulin reductions (basal and/or bolus left unchanged)

For a deeper dive on exercise planning, see Exercising with Diabetes.

What blood sugar should I start exercise with?

A practical starting range

Ideal start (most people): 90–180 mg/dL with a steady or slightly rising trend.
If <90 mg/dL or dropping: take 10–20g fast carbs (more if you have high IOB), wait, and recheck.
If you’re often dropping early: you likely need less active insulin going into exercise (see the insulin sections below).

If glucose is high

If you’re above ~250 mg/dL, check for ketones (especially for type 1). Avoid vigorous activity if ketones are present.
If you have marked high glucose (example: ~350 mg/dL) plus moderate/large ketones, postpone intense exercise and treat the insulin deficiency first.

How do I reduce insulin safely before exercise?

The #1 reason for exercise lows is too much insulin in the system. Here are common, safer ways to reduce that risk.

If you ate within the last 2 hours

Reduce the meal bolus (commonly ~25–50% depending on intensity and your history).
If you use a pump/AID, consider pairing the bolus reduction with Exercise/Activity Mode.

If you use a pump or AID (Exercise/Activity Mode)

Start Exercise/Activity Mode early: typically 60–120 minutes before aerobic activity, and keep it on through the workout.
If activity is unplanned, turn it on immediately and be ready to use carbs early.

If you’re frequently dealing with “extra insulin,” review insulin timing and stacking with Avoid Insulin Stacking.

What should I do during exercise to prevent a low?

Check trend often: every 30–60 minutes (or more for long/intense sessions).
Carry fast carbs every time: glucose tabs, juice, regular sports drink, gummies, or candy.
Treat early: if your CGM arrow is down and you’re near 100 mg/dL, a small carb dose now can prevent a big low later.

Need a simple “what to take and how fast it works” guide? See How to Raise a Low Blood Sugar Quickly and Safely.

Use ExCarbs to plan fuel (especially for longer workouts)

The ExCarb system estimates how many grams of carbohydrate your body may burn during specific activities—so you can replace carbs (or reduce insulin) more precisely.

Learn the method here: ExCarbs for Exercise With Diabetes
For many people, adding small carbs every 30–45 minutes during prolonged activity prevents drops.

Why do I go low hours later or overnight after exercise?

Delayed lows (often 6–36 hours later) happen because your body is:

Refilling glycogen in muscles and liver (pulls glucose from blood)
More insulin-sensitive after activity
Using glucose in the background even after you stop moving

Fast glycogen rebuilding reduces delayed hypoglycemia

After prolonged or strenuous exercise, there’s often a short window where muscles are ready to refill glycogen quickly. A carb + protein recovery snack can help. Chocolate milk is one convenient example because it provides both carbohydrate and protein.

How can I prevent overnight lows after exercise?

Consider extra slow-acting carbs at dinner or bedtime (especially after long/strenuous activity).
Consider reducing basal insulin (commonly ~10–40% for ~6–12 hours) if you use a pump/AID, based on your history and clinician guidance.
Use CGM alerts strategically: set the low alert a bit higher overnight (example: 90 mg/dL) on hard training days.
Check patterns the next day—some people stay more sensitive for 24–36 hours.

If you have reduced awareness of lows, review prevention steps here: Hypoglycemia Unawareness.

What are “surprise activities” and why do they cause lows?

When you use untrained muscles (or do an unusually hard day), your body may “load” those muscles with glycogen afterward—pulling extra glucose from the blood for hours. Common surprise activities include:

Spring cleaning, home remodeling, heavy yard work
Hiking/backpacking, skiing, snow shoveling
Spontaneous sports (soccer, basketball, football)
Walking in sand, snow, or strong wind (often needs more fuel than expected)

How much should I reduce insulin or add carbs for different workouts?

In general: the longer and more aerobic the activity, the more you’ll need less insulin and/or more carbs.

Duration	Intensity	Approx. carb need (100 lb person)	Typical bolus reduction	Typical basal reduction
30 min	Moderate	~20g	-10%	None
60 min	Intense	~60g	-40%	-10%
90 min	Intense	~90g	-50%	-20%

Adjust for body weight, fitness level, insulin sensitivity, and whether your workout is mostly aerobic (often lowers glucose) or anaerobic/high-intensity intervals (can raise glucose first and drop later).

How can I learn my personal exercise “playbook” more quickly?

What works for one person may not work for another—so keep quick notes for 2–3 weeks and you’ll usually see patterns.

Starting glucose + CGM trend
Insulin on board (IOB) and recent boluses
Type, duration, and intensity of activity
Carbs taken (amount + timing)
What happened 0–2 hours after and overnight

For a structured approach to finding patterns, see Use Pattern Management to Solve Specific Issues.

Quick summary: preventing low glucose during and after exercise

Start in a safe range (often 90–180 mg/dL) and don’t ignore a downward trend.
Reduce risk by lowering insulin on board (bolus reduction and/or Exercise/Activity Mode started early).
Monitor during activity and carry fast carbs every time.
Refuel after long/hard exercise and plan for delayed/nighttime lows.
Log a few key details to build your personal plan.

Helpful Resources & Research

Frequently Asked Questions

How can I prevent low blood sugar during exercise fast?

Use a simple plan: start around 90–180 mg/dL, reduce insulin on board (bolus reduction and/or Exercise/Activity Mode started early), and carry fast-acting carbohydrates. Treat a downward CGM trend early before it becomes a low.

What should my blood sugar be before I work out?

Most people perform best when starting around 90–180 mg/dL, with a steady or slightly rising trend. If you’re below 90 mg/dL or dropping, take 10–20g fast carbs and recheck before starting.

Why do I go low during a workout even when I start “normal”?

The most common reason is too much active insulin from a recent bolus (high insulin on board). Aerobic activity (walking, jogging, cycling) also makes muscles pull glucose faster, so glucose can fall quickly.

When should I turn on Exercise Mode or Activity Mode?

For planned aerobic exercise, many people need it 60–120 minutes before the workout so insulin has time to decrease. For unplanned exercise, turn it on right away and be ready to take carbs early.

How often should I check my CGM or blood sugar during exercise?

For most workouts, check every 30–60 minutes. Check more often if you’re doing long sessions, intense intervals, water sports, or if your CGM trend is dropping.

What carbs work best to treat or prevent a workout low?

Fast carbs work best: glucose tabs, juice, regular soda, sports drink, gummies, or candy. If you’re preventing a low (not treating one), smaller amounts taken early can stop a bigger crash later.

Why do I go low hours later or overnight after exercise?

Delayed lows happen because your body refills muscle and liver glycogen and stays more insulin-sensitive for up to 36 hours. That ongoing glucose “pull” can cause nighttime lows after long or new activity.

How can I prevent overnight lows after a hard workout?

Consider a slow-acting bedtime snack, a temporary basal reduction (pump/AID) based on your history, and slightly higher CGM low alerts overnight (example: 90 mg/dL) on heavy training days.

Is it safe to exercise when my blood sugar is high?

If you’re above ~250 mg/dL, check ketones (especially with type 1). Avoid vigorous exercise if ketones are present. If glucose is very high with moderate/large ketones, postpone intense activity and treat the insulin deficiency first.

What if my CGM says I’m fine but I feel low?

Treat your symptoms immediately, then confirm with a fingerstick if readings don’t match how you feel. CGM can lag behind rapid glucose changes during exercise.

Last Updated on January 15, 2026

Authors

Ruth Roberts, MA

Ruth Roberts, MA, is a medical writer, editor, and educational consultant on intensive self-management. She has been involved in diabetes support groups for over 20 years and has co-authored several books on the subject of diabetes. She is a professional member of the American Diabetes Association and has served on the Board of Directors for the International Diabetes Athletes Association. She manages business matters and product development for the company. Her expertise in presenting complex information in a clear, easy-to-read format benefits our publications. She worked for 20 years in academic teaching and corporate training before developing the company.
John Walsh, PA, CDTC

John Walsh, PA is a Physician Assistant and Diabetes Clinical Specialist with Advanced Metabolic Care and Research. He has provided clinical care for patients with diabetes for more than 30 years, including thousands of people on insulin pumps. He is the co-author of Pumping Insulin, now in its 6th edition, Using Insulin, STOP the Rollercoaster, and The Pocket Pancreas. President of the Diabetes Mall, he is also the webmaster of diabetesnet.com, a highly trafficked source of diabetes information and technology, and opensourcediabetes.org. John is a consultant and advisory board member for pharmaceutical and device corporations and a frequent speaker on diabetes, insulin pump therapy, bolus calculators, infusion set design, glucose management, and the future of smart pumps, meters, and continuous monitors. His recent research, published in the Journal of Diabetes Science and Technology, explores the dosing and behaviors that lead to successful glucose outcomes by pump wearers. John has worn nearly every insulin pump and CGM available and is a sub-investigator on numerous research studies on diabetes drugs and devices.