Exercise is good for us for plenty of reasons, but when you have diabetes, its effect on blood sugar can sometimes feel anything but predictable. Different types of movement can send glucose in opposite directions, and even the same activity won't necessarily affect you the same way every time.
There is some science behind the chaos.
Why Exercise Often Lowers Blood Sugar
When you move, your muscles need energy.
Glucose is one of the fuels they can use, and working muscles can take up glucose during activity even without relying entirely on insulin. Exercise also increases insulin sensitivity, which means your cells can use available insulin more effectively during and after activity.
That's why activities like walking, jogging, cycling, swimming, and other forms of steady aerobic exercise often cause glucose to fall.
For someone who takes insulin, there's another important factor: insulin on board.
Your body can't automatically turn down injected insulin in the same way a functioning pancreas can reduce its own insulin secretion when you start moving. If you exercise while a significant amount of insulin is active, you may suddenly have both insulin and muscle activity working to move glucose out of your bloodstream.
That's when the downward arrow can appear fast.
So Why Can Exercise Make Blood Sugar Go Up?
This is where things get interesting.
Exercise is physical stress, and when that stress becomes intense, your body releases hormones such as adrenaline. Those hormones tell your liver to release stored glucose into your bloodstream so your muscles have quick access to energy.
Sometimes that glucose arrives faster than your body can use it, causing your blood sugar to rise even though you're exercising.
This is more likely during short, intense or anaerobic activities such as heavy weightlifting, sprinting, competitive sports, or high-intensity interval training. The ADA notes that intense activity can raise glucose rather than lower it, particularly when glucose is already elevated before exercise.
So that post-workout spike isn't necessarily evidence that your workout "didn't work." It's your body responding to the demands you're putting on it.
Cardio and Strength Training Can Look Very Different on a CGM
Think about two workouts.
During a 45-minute walk, your muscles are continuously using glucose for fuel. Your CGM might show a gradual downward trend as the workout continues.
Now imagine a heavy lifting session.
You're performing short bursts of intense work followed by rest. Your body may respond to that intensity by releasing stress hormones and making more glucose available. Instead of dropping, your CGM might stay relatively steady or start climbing.
Neither response is inherently better.
They're simply different physiological responses to different kinds of exercise.
And workouts don't always fit neatly into one category. A spin class, soccer game, circuit workout, or interval run might include both aerobic and anaerobic activity, which can create a much less predictable glucose response.
Insulin on Board Can Change Everything
Two identical workouts can produce completely different glucose graphs depending on when you last ate or took insulin.
Imagine going for a run first thing in the morning with very little active insulin.
Now imagine doing that same run 90 minutes after lunch with a meal bolus still working.
The exercise hasn't changed. Your insulin environment has.
For people who use insulin, physical activity can increase the risk of hypoglycemia when insulin or carbohydrate intake isn't adjusted appropriately. The amount of active insulin, your starting glucose, the duration and intensity of the workout, and the time of day can all influence what happens next.
This is also why someone else's perfect exercise strategy might not work for you.
Your Workout Can Affect Blood Sugar Hours Later
Sometimes the workout itself goes beautifully.
Then you go low six hours later.
Exercise can increase insulin sensitivity well after you've finished moving. The ADA notes that physical activity can lower glucose for 24 hours or more afterward because the body remains more sensitive to insulin.
That can be especially noticeable after longer or more strenuous activity.
For people taking insulin, delayed lows can happen several hours after exercise and may even show up overnight. Current ADA guidance specifically calls attention to the prolonged effects of activity and recommends learning how your glucose responds during and after different forms of exercise.
In other words, the CGM graph you see during your workout isn't necessarily the end of the story.
Why the Same Workout Can Hit Differently
This might be the most frustrating part.
You can do the same workout at the same gym at roughly the same intensity and still get a different glucose response.
That's because exercise isn't happening in isolation.
Your starting glucose matters. So does how much insulin you have on board, what and when you last ate, the length and intensity of the activity, and how your body is responding that day.
And then there are all the variables people with diabetes already know can complicate blood sugar: stress, hormones, illness, sleep, hydration, and more.
Your workout may be predictable.
Your physiology isn't always.
What About Automated Insulin Delivery?
Automated insulin delivery systems have made exercise easier for many people, but even the smartest algorithm can't know everything you're about to do.
Many systems include an exercise or activity setting that changes the system's glucose target or insulin delivery strategy. Timing matters, though, because insulin that's already been delivered doesn't disappear when you start a workout.
This is one reason exercise with an AID system can still require some experimentation.
Your diabetes care team can help you determine how and when to use your system's activity settings, adjust insulin around planned exercise, and handle activities that repeatedly send you high or low.
A High During Exercise Doesn't Always Need the Same Response
Seeing a rising arrow during a workout can make an insulin correction feel like the obvious next move.
But exercise-related highs can be different from a typical post-meal high.
If stress hormones are responsible for the rise, glucose may begin coming back down as those hormones settle. Add a correction dose while your insulin sensitivity is increasing from exercise, and there's potential for glucose to fall later.
There's also an important exception: high blood sugar accompanied by ketones.
The ADA recommends checking for ketones when glucose is high before exercise. If ketones are present, vigorous activity should be avoided because exercise can make the situation worse.
This is one of those areas where having an individualized plan from your diabetes care team matters much more than following a universal rule.
Learn Your Patterns, Not Someone Else's
Exercise with diabetes involves some trial and error.
Instead of expecting every workout to produce a certain CGM graph, it can be more useful to notice what tends to happen under similar circumstances.
What happens when you walk after dinner?
What does your glucose usually do during strength training?
Are you more likely to drop when you exercise with insulin on board?
Do lows tend to show up later after long workouts?
CGM data can be especially useful here because it allows you to look back at what happened before, during, and after different activities. The ADA recommends monitoring glucose around exercise specifically because responses can vary so much from person to person and from one type of activity to another.
Over time, those patterns can give you something much more useful than a universal exercise rule: information about your body.
Final Thoughts
Exercise can lower your blood sugar.
It can also raise it.
And occasionally, it can do both during the same workout.
That doesn't mean you're doing anything wrong. It reflects the complicated interaction between your muscles, insulin, stress hormones, food, and the type of activity you're doing.
Learning your own patterns can make those CGM arrows feel a little less random. And if exercise regularly causes significant highs or lows, your diabetes care team can help you develop an approach that fits your insulin regimen, technology, and favorite ways to move.
Exercise with diabetes may never be perfectly predictable, but understanding why your blood sugar responds the way it does can make it a lot less mysterious.
