Exercise and blood sugar when you take insulin

Exercise is good for almost everyone with diabetes. But if you take insulin, whether you have type 1 or type 2 diabetes, a run, a gym session or even a long walk can move your blood sugar in ways that feel unpredictable. Sometimes it drops, sometimes it rises, and sometimes nothing happens until the middle of the night. Here’s what’s going on.

Why does exercise lower blood sugar?

Two things happen when you move.

First, working muscles pull glucose out of your blood for fuel. The American Diabetes Association explains that contracting muscles can take up glucose whether insulin is available or not. According to the ADA’s position statement on exercise, aerobic exercise can increase the glucose your muscles take up by up to five times.

Second, exercise makes your body more sensitive to insulin, during the activity and afterwards. The same dose of insulin has a bigger effect than usual.

Exercising with insulin on board

This is where insulin on board comes in: the insulin from earlier doses that is still working.

Diabetes UK puts it simply: if you’ve taken your usual dose of insulin and then exercise, you’ll have less glucose than normal, and that can lead to a low. Rapid-acting insulin typically peaks 1 to 2 hours after a dose, according to the ADA, so a mealtime dose taken an hour before a workout may be close to its strongest. Exercise on top of that means two things are lowering your blood sugar at the same time.

So before you exercise, it helps to know not only your blood sugar, but roughly how much insulin is still active and when you last took it. A dose log, or an app that shows active insulin such as Insulin Pal, can make that easier to see at a glance.

Why does my blood sugar go up after exercise?

Not all exercise lowers blood sugar. Diabetes UK notes that short, powerful activities where you get out of breath quickly, such as sprinting and weightlifting, tend to push blood sugar up. The ADA’s position statement says very intense exercise can cause high blood sugar, especially if you start with a high level.

It surprises a lot of people, and it’s one reason the same “exercise lowers blood sugar” rule doesn’t always hold.

Low blood sugar after exercise: why it can come hours later

Exercise keeps working long after you stop. Your muscles keep taking up glucose to refill their energy stores, and you stay more sensitive to insulin.

• The ADA’s position statement says lows typically happen 6 to 15 hours after exercise, and the risk can last up to 48 hours.
• Diabetes UK says lows can happen up to 36 hours after a lot of activity.
• The CDC lists being active close to bedtime as a cause of low blood sugar during sleep.

So an afternoon workout can mean a low in the middle of the night. If you use a CGM, its low alerts can help here. See how a CGM works.

What to check before, during and after exercise

These are general habits the sources recommend. The specifics are for you and your care team to work out.

• Check your blood sugar before you start, and after. For longer sessions, check during too.
• Know your insulin on board. A recent dose is still working.
• Carry fast-acting carbs, such as glucose tablets or a sugary drink, whenever you’re active.
• Watch for signs of a low. The CDC lists a fast heartbeat, shaking, sweating, anxiety, irritability or confusion, dizziness and hunger.
• Keep an eye on the night after, especially after long or hard sessions.

Plan it with your care team

The ADA’s position statement notes that blood sugar responses to exercise in type 1 diabetes are highly variable. The same workout can affect two people, or the same person on two days, quite differently. The statement does describe insulin and carb adjustments that clinicians use, but those are for your diabetes team to tailor to you. They aren’t general rules. The ADA recommends talking to your diabetes care team about your risk of lows before changing your routine.

The short version

• Exercise usually lowers blood sugar: muscles use glucose and your body becomes more sensitive to insulin.
• Insulin still working from an earlier dose adds to that effect. Know your insulin on board before you start.
• Intense exercise like sprints or weightlifting can raise blood sugar for a while.
• Lows can come 6 to 15 hours later or more, including overnight.
• Check before, during and after, carry fast-acting carbs, and plan changes with your care team.