Is it dangerous to exercise with high blood sugar levels?

Many people with diabetes are told to “just move more”, but could exercising with high BGLs do more harm than good?

Exercise is one of the most powerful tools for managing diabetes. It helps your muscles use glucose more effectively, improves insulin sensitivity, and supports overall health.

But there’s a catch… exercising with high blood glucose levels (BGLs), especially when insulin levels are low, can sometimes be unsafe. Instead of helping lower your sugar levels, certain types of exercise can actually make them climb even higher or lead to complications like dehydration or ketoacidosis.

So how high is too high? And what should you do if your BGLs aren’t in the ideal range before a workout?

1.  Know your numbers before you move

Before you start any moderate or vigorous exercise, it’s worth checking your BGLs.

Blood Glucose Level	What to Do
<5.5 mmol/L	Have a small carbohydrate snack before exercising (reduce risk of hypoglycaemia). Think “Under 5, stay alive”
5.5–13.9 mmol/L	Safe range for most moderate exercise. Think “You’re all fine under 13.9”
>13.9 mmol/L	Check for ketones before exercising. If ketones are present, delay exercise until levels are managed. “Over 14, check the scene”

2. Not all exercise affects blood sugar the same way

Different types of exercise can push blood sugar in opposite directions:

Aerobic exercise (like walking, cycling, swimming) usually lowers blood glucose because muscles use glucose for fuel.

Anaerobic or high-intensity exercise (like heavy weightlifting, sprints, HIIT) can potentially raise blood glucose temporarily due to stress hormones (adrenaline, cortisol).

Think of it like this: gentle to moderate exercise “burns off” glucose, while intense exercise gives your body a stress response that releases more of it. For people already starting high, intense workouts might tip you further upward, so pacing, monitoring, and knowing how your body responds is key.

3. Recognise the warning signs

Even with the best preparation, it’s important to listen to your body during exercise. Watch for symptoms that could indicate unsafe levels of blood glucose or dehydration:

Stop and check if you experience:

●      Nausea or vomiting

●      Extreme thirst or dry mouth

●      Fruity-smelling breath

●      Fatigue or confusion

●      Rapid breathing or pounding heart

If these appear, stop exercising, hydrate, and check your glucose and ketones. Seek medical advice if symptoms persist or readings stay high.

4. How to safely bring levels down before exercising.

You can absolutely exercise safely, it just takes some awareness and preparation.

Tips for safe, effective workouts:

1. Check your BGLs before and after workouts until you know your patterns or have it well managed.

   
   
   
   

2. Hydrate well. High BGLs cause fluid loss through urine.

   
   
   
   

3. Avoid exercise if ketones are present

   
   
   
   

4. Carry fast-acting carbs (like jellybeans, lollies or fruit juice) in case of sudden drops.

   
   
   
   

5. Start slow and gradually increase intensity to see how your body responds.

   
   
   
   

Exercise is a cornerstone of diabetes management, but timing and awareness matter. If your blood sugar is too high, your body might not be in a safe state to work out. By checking levels, staying hydrated, and knowing how different exercises affect you, you can move safely and confidently.