Diabetes and exercise: When to monitor your blood sugar

Date Updated: 01/09/2024

Exercise is a key part of managing diabetes. Exercise can help you:

Improve your blood sugar levels.
Boost your overall fitness.
Manage your weight.
Lower your risk of heart disease and stroke.
Improve your well-being.

But diabetes and exercise pose unique challenges. To exercise safely, some people with diabetes need to track their blood sugar before, during and after physical activity. This shows how the body responds to exercise. And it can help prevent blood sugar swings that could be dangerous.

Before exercise: Check your blood sugar before your workout

Before you start a new fitness program, talk with your healthcare professional. Ask if it's OK to do the type of exercise you want to try, especially if you have type 1 diabetes.

Exercise can cause blood sugar to become too low in people who take insulin. Blood sugar that's too low is called hypoglycemia. The risk also applies to people with type 2 diabetes who take insulin or other medicines linked with lower blood sugar. Your healthcare professional can teach you how to balance your medicine with exercise and diet.

Ask your healthcare professional:

How the activities you want to do might affect your blood sugar.
When is the best time of day for you to exercise.
How the diabetes medicines you take might affect your blood sugar as you become more active. Depending on your treatment, your healthcare professional may tell you to adjust your medicine dose or the food you eat before exercise.

For the best health benefits, adults should work up to at least 150 minutes a week of heart-pumping aerobic activity. The activity should be moderate to vigorous in intensity. Examples include:

Fast walking or hiking.
Lap swimming or a water aerobics class.
Bicycling.
Stair climbing.
Dancing.
Basketball, tennis or other sports.
Exercise classes.

Adults also should aim to do 2 to 3 strength-training activities per week. Give yourself at least a day to recover from a strength-training session.

Children and teens with diabetes should get at least 60 minutes of moderate to vigorous aerobic activity every day. They should do muscle- and bone-strengthening activities at least three days a week. Some examples of muscle-strengthening activities are games such as tug of war and exercises using body weight or resistance bands. Bone-strengthening activities include jumping rope and running.

When you talk with your healthcare professional about exercise, ask about your blood sugar testing needs. If you manage type 2 diabetes without medicines, you likely won't need to check your blood sugar before exercise.

But many people with diabetes do need to test their blood sugar levels before physical activity. If you take insulin or other medicines that can cause low blood sugar, test your blood sugar 15 to 30 minutes before exercising.

If you use a continuous glucose monitor to track your blood sugar, talk with your healthcare professional. You may be told to test your blood sugar with a finger stick before, during or after exercise. If you receive insulin through an automated insulin delivery system, talk with your healthcare professional about that. Ask how to keep your blood sugar in a healthy range for exercise. This is key if you usually don't notice symptoms when your blood sugar is low — a condition called hypoglycemia unawareness.

Do not exercise if you've needed help with recovering from serious low blood sugar in the past 24 hours.

Below are some general guidelines for blood sugar levels before exercise. The measurements are in milligrams per deciliter (mg/dL) or millimoles per liter (mmol/L).

Lower than 90 mg/dL (5.0 mmol/L). Your blood sugar may be too low to exercise safely. Before you work out, have a small snack that includes 15 to 30 grams of carbohydrates. Some examples are fruit juice, fruit and crackers. Or take 10 to 20 grams of glucose products, which come in forms such as gels, powders and tablets. After you exercise, check your blood sugar again to find out if it's about 90 mg/dL.
90-124 mg/dL (5-6.9 mmol/L). Take 10 grams of glucose before you exercise.
126-180 mg/dL (7-10 mmol/L). You're ready to exercise. But be aware that blood sugar may rise if you do strength training. Blood sugar also may rise if you do short bursts of hard aerobic exercise known as high-intensity interval training.
182-270 mg/dL (10.2-15 mmol/L). It's okay to exercise. Be aware that blood sugar may rise if you do strength training or high-intensity interval training.
Over 270 mg/dL (15 mmol/L). This is a caution zone. Your blood sugar may be too high to exercise safely. Before you work out, test your urine for substances called ketones. The body makes ketones when it breaks down fat for energy. The presence of ketones suggests that your body doesn't have enough insulin to control your blood sugar.

If you exercise when you have a high level of ketones, you risk a dangerous health problem called ketoacidosis. Ketoacidosis can be life-threatening. It requires urgent treatment. Ketoacidosis can happen to anyone with diabetes, but it is much more common with type 1 diabetes. Instead of exercising right away if you have ketones, take steps to lower high blood sugar. Then wait to exercise until your ketone test shows an absence of ketones in your urine.

During exercise: Watch for symptoms of low blood sugar

During exercise, low blood sugar is sometimes a concern. It's mainly a risk for people with diabetes who take insulin or other medicines linked to low blood sugar levels. If you're planning a long workout, check your blood sugar every 30 minutes. This is key if you're trying a new activity or increasing the intensity or length of your workout. Checking every half-hour tells you if your blood sugar level is stable, rising or falling. That way, you can get a sense of whether it's safe to keep exercising.

Checking every 30 minutes may be a challenge if you're doing outdoor activities or playing sports. But you need to take this safety measure until you know how your blood sugar responds to changes in your exercise habits.

Stop exercising if:

Your blood sugar is 70 mg/dL (3.9 mmol/L) or lower.
You feel shaky, weak, dizzy or confused.

Eat or drink something with about 15 grams of fast-acting carbohydrate to raise your blood sugar level, such as:

Glucose tablets or gel. Check the label to see how many grams of carbohydrate these contain.
1/2 cup (4 ounces/118 milliliters) of fruit juice.
1/2 cup (4 ounces/118 milliliters) of regular soda. Do not drink diet soda.
Hard candy, jelly beans or gumdrops. Check the label to see how many grams of carbohydrate these contain.
1 tablespoon of sugar, honey or corn syrup.

Check your blood sugar again 15 minutes later. If it's still too low, have another 15-gram carbohydrate serving. Then test again in 15 minutes.

Repeat as needed until your blood sugar reaches at least 70 mg/dL (3.9 mmol/L). If you haven't finished your workout, you can continue once your blood sugar returns to a safe level. You may need to have more snacks or a meal to raise it to that safe range.

After exercise: Check your blood sugar again

Check your blood sugar as soon as you finish exercising. Check it again throughout the next few hours. Exercise draws on reserve sugar stored in your muscles and liver. As your body rebuilds these stores, it takes sugar from your blood.

The tougher your workout, the longer it will affect your blood sugar. Low blood sugar can happen even 4 to 8 hours after exercise. Having a snack with slower-acting carbohydrates after your workout can help prevent a drop in your blood sugar. These types of snacks include a granola bar, trail mix and dried fruit.

If you do have low blood sugar after exercise, eat a small snack that has carbohydrates. For example, you could have fruit, crackers or glucose tablets. Or you could drink a half-cup (4 ounces/118 milliliters) of fruit juice.

Exercise is great for your health in many ways. But if you have diabetes, testing your blood sugar before, after and sometimes during exercise may be just as important.