How diabetes affects exercise, and vice versa, is a topic of its own. Sometimes, managing blood sugars around exercise and activity turns out to be more challenging than the workout itself.
On one hand, it’s hard because your body reacts completely differently depending on the time of day and how fit you are. On the other hand, it’s hard because your blood sugar behaviour varies depending on what kind of exercise or activity you’re doing.
Yikes! Talk about introducing a ton of variables into the mix. But exercise is so good for you that it’s worth the trouble. It’s a surprisingly powerful tool in your diabetes toolbox. People with diabetes can often find themselves spending a lot of time before, during, or even after exercise worrying about how to stay in range and avoid nasty lows. Then, in the end, your blood sugar does whatever it wants. This can be very frustrating.
Sometimes, situations sneak up on us too! It’s so easy to forget that everyday activities like gardening, pushing a stroller or cart, or even doing your shopping also count as physical activity. It’s not only physical activity that burns a lot of sugar. Your brain needs a lot of energy too, so deep thinking can also lower your blood sugar levels.
What Exactly Does Exercise Do to Your Body?
Power sports (anaerobic)
In anaerobic sports (for example strength sports, lifting weights, climbing, but also stop-and-go sports such as volleyball, tennis and sprinting), blood sugars may stay stable or even increase, and then fall again afterwards while you’re recovering.
Anaerobic exercises are usually brief, intense bursts of physical activity when oxygen demand surpasses the oxygen supply. The body cells can’t use their usual methods of gathering the energy required to perform whatever the action may be (lifting weights, sprinting, etc.).
If the muscles need a lot of energy very fast, the body uses another way of delivering that energy. In situations like these, adrenaline rushes in to help, which causes sugar stores in the muscles to dump their load. You can already guess what happens next; your blood sugar rises quickly. This is especially common in competitive situations. Some athletes with diabetes even take a little bit of fast-acting insulin to stay on track.
Endurance sports (aerobic)
In aerobic exercise (for example, jogging, walking, swimming, cycling), blood sugar usually drops both during the activity and for a long time afterwards. You can often do aerobic exercises for extended periods of time (10 minutes or more). They are typically rhythmic and repetitive, and will increase your heart rate and pace of breath. Here, your body is burning oxygen along with glucose stores or fat to supply the energy needed.
For exercise newbies and those getting back into shape, low-intensity, steady workouts are ideal since your body will use fats as the primary energy source and burn sugar reserves at a slower pace. This allows you to exercise with more stamina and also reduces the risk of hype.
With increased intensity and duration, more glucose is used, and your risk for low blood sugars also increases. The more often you exercise, the more efficient your muscles will become in terms of using energy. They will also get better at using fats for energy. Additionally, the glycogen stores in your muscles (200-400 g) grow bigger the more they are used.
At the beginning of any new physical activity, you’ll have to pay extra attention to planning and precautions until your body gets used to it.
What Happens to My Values After Exercising?
After exercising, while you’re recovering and basking in the glow of doing something great for yourself, your body is just getting started. Especially after any endurance exercise, your glucose stores will be empty and hungrily snatching up any sugar floating through your bloodstream for the next 7-11 hours. It may feel like your insulin is supercharged. In this case, you’ll need to stay mindful of lows.
For extreme events, like a marathon or a sports tournament, it may take several days until things are back to normal. This is partly due to small insulin-independent transporters that
help shovel sugar into the cells to replenish the sugar stores in your muscles and liver.
Did you catch that? Those special transporters work even without insulin! That’s part of what makes exercise so cool!
Diabetes and Exercise Tips and Tricks
- It can be hard to recognize lows during exercise because the symptoms are hard to detect. Check your blood sugar often and stay mindful about it until you get your routine figured out – especially if it’s a new exercise.
- Most athletes start their exercise session with an initial blood sugar of 150-180 mg/dL (8.3-10 mmol/L), depending on the current acting insulin dose, shape etc. For risk related sports (e.g. climbing, paragliding), you should aim for a higher starting value.
- Keep fast-acting glucose nearby and readily available to treat low blood sugars. For more extended duration activities, you may need to consume carbohydrates during your exercise to prevent lows. Consider choosing only fast carbs, such as juice and glucose tabs, right before starting or during your exercise.
- If you use an insulin pump, consider reducing your basal rate, or even disconnecting your pump for a while, depending on the type of exercise. Generally, you can program basal rate reductions for 4-5 hours or longer. Start your basal rate reductions on a pump 1-2 hours ahead of time if you are using an analog insulin. You’ll need at least that long before you will see any change in your blood sugar.
- Rapid drops in blood sugar during exercise often result from too much active insulin on board. Be extra mindful of this if you’re starting a workout with lots of insulin on board, and prepare if you know you’ll be working out soon after taking any insulin. For pre-breakfast exercise, it can be helpful to reduce the average basal rate the evening before, depending on insulin type, exercise intensity, and current fitness level.
- For endurance sports lasting more than 1-2 hours, you may only need 20-75% of the usual amount of insulin for the meal before exercise.
- Full-time (all day) activities often require lowering meal and basal insulin for the next 24 hours after the exercise. For medium intensity activities, like a cycling trip, reducing the basal requirement by ca. 30-50% can prevent night-time lows.
- For type 1 diabetes, eating at least 90 minutes after your workout may reduce the risk of overnight lows. Especially in the afternoons or evenings, you can reduce your insulin intake by up to 50% or eat small extra fat and protein snacks before you go to sleep without insulin. When correcting high values, the insulin intake should be reduced by at least 60 - 80%.Everyone, however, is unique and you’ll need to experiment a little bit to find the right balance for you.
- After the strength training, which can initially lead to higher blood sugars, insulin requirements may fall over the following 6 hours as the liver replenishes its reserves. You can compensate for this with less insulin at the next meal or additional snacks. For pump users, a basal rate reduction of about 30% might also be an excellent place to start.
- Take special care with alcohol after exercise – this will prevent your liver from releasing sugar. In this case, increased insulin sensitivity and reduced sugar release overlap. Watch out for low blood sugars, keep your wits about you and your glucose tabs ready!
What Else Is There to Consider?
If you start to experience symptoms of hypoglycemia, immediately stop your activity!
If you have type 1 diabetes and start your exercise with high blood sugars, i.e. above 250 mg/dL (14 mmol/L), be sure to check for ketones. If you have ketones, it’s best to sit this one out. Exercising may accelerate/encourage more ketones, potentially leading to diabetic ketoacidosis, and that’s something you want to avoid for sure.
Thurm,U., Gehr, B., Diabetes-Sportfibel, 4. Auflage (all suggestions for therapy adjustment are taken from this source)
This article has been updated in September 2020.
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