The effect of exercise on blood sugar varies from one person to the next. Different people may need to prepare differently for the same type of exercise, and a person may have to prepare differently for different types of exercise.
The effect of exercise on blood sugar depends on many different factors. Aerobic endurance training increases insulin sensitivity and helps lower the blood sugar level. Anaerobic strength training, interval training and competitive exercise, on the other hand, are aimed at raising blood sugar. Furthermore, the intensity and duration of training, sugar balance and what you eat beforehand have an impact on how your blood sugar behaves during and after exercise. Checking your blood sugar and especially using a glucose sensor enable you to discover the effect of various forms of exercise on your system.
In other words, a change in the blood sugar level is the result of several factors:
The duration, timing and intensity of training
The blood insulin level during exercise and the duration of action of the insulin product you use
Circulation in the injection site as well as its condition and temperature
Blood sugar prior to exercise
The nutrition intake before and during exercise
Practice in the sport and muscular endurance
The type of exercise (endurance training, strength training, HIIT)
Stress hormone secretion accelerated by exercise or a competitive situation
More information
If you're interested in delving deeper into how type 1 diabetes insulin treatment can be adjusted with the aid of a glucose sensor during variable exercise, refer to the recommendation from 2020.
For most people, a good pre-exercise blood sugar level is 5–8–10 mmol/l.
If your blood sugar is below 5 mmol/l, consume 20 g of carbohydrates.
If your blood sugar is below 8 mmol/l before endurance training, consume 10 g of carbohydrates.
If your blood sugar is over 14 mmol/l for no apparent reason, measure the blood ketone level.
If your blood ketone level is over 1.5 mmol/l, take a small bolus insulin correction dose and drink some water.
If there is no ketone, you can do light exercise.
If your blood sugar is over 20 mmol/l, postpone the exercise session, drink some water and check your blood ketone level. Take bolus insulin to correct your blood sugar.
If you exercise within 2–3 hours of the bolus insulin jab, reduce the dose by 25% to 75% depending on the intensity of exercise.
If the exercise lasts longer than one hour, consider reducing the previous long-acting insulin dose by 20% to 50% depending on the strenuousness and duration of the exercise.
Check your blood sugar every 30–45 minutes during a long exercise session.
The risk of low blood sugar most commonly occurs approximately 45 minutes after the start of endurance training.
Keep on your person carbohydrates that are quick and easy to consume, such as a small fruit juice pack, glucose drops or glucose gel.
During a long exercise session, you typically need 20–40 g of carbohydrates per hour.
Good options include 5% sports drinks, 50% diluted fruit juice, glucose drops or a banana.
If your blood sugar drops below 4 mmol/l, consume 20 g of fast-absorbed carbohydrates.
If you exercise when your body is in insulin-deficiency, your insulin-antagonistic hormone level, blood sugar and blood ketone level will increase as a result.
Keep an eye on your glucose sensor or check your blood sugar 1–2 hours after the exercise and when you go to bed.
It is best to err on the side of caution with regard to correcting a rise in blood sugar resulting from exercise.
With experience and a glucose sensor, you can learn to administer the correct small correction bolus insulin dose.
Long-lasting exercise increases insulin sensitivity, and the sugar deposited in the muscles after exercise can lower the blood sugar until the following day.
Intense training which increases your heart rate and adrenalin levels may raise your blood sugar level during the exercise, but it tends to drop afterwards.
Following the exercise, you may need to slightly reduce the bolus insulin dose for the next meal.
For supper, you should consume some protein in addition to slow-acting carbohydrates.
After long, strenuous exercise, the basal insulin dose injected in the evening should be reduced by 20% to 30%.
