Hypoglycemia after exercise can be a very frustrating experience. For individuals who use insulin and are active the likelihood of a low blood sugar is high if preventive measures are not taken. The problem is not so much that it can occur but it is so unpredictable. It is common to have low blood sugars after exercise. In fact, there is a high occurrence for people 4-6 hours after exercise but some experience it up to 24 hours later.
Once the body burns the food eaten prior to exercise it uses a stored form of glucose (called glycogen) in the liver and muscles. After that is depleted if food is not eaten the body will get glucose from the blood, which causes us to go low. This does not happen quickly but over a number of hours. There is a twofold reason for eating carbs after exercise. One is to help prevent a low, but probably just as important is to restore the glycogen lost during exercise. This will not only help with decreasing the chance of a low blood sugar but will help with performance for the next exercise bout.
Now that you know why it happens, the question becomes, what can be done to prevent this? There is no easy answer, but if you follow a few guidelines the risk of hypoglycemia will decrease. Some tips to follow for individuals who experience low blood sugars after exercise are:
- Aerobic exercise (running, cycling, etc.) will usually decrease blood sugars, so providing a buffer with a starting blood sugar higher than normal is recommended. Many athletes shoot for a starting blood sugar of approximately 150 mg/dl (8.3 mmol/l) prior to exercise but it may be lower or higher depending on the individual and intensity of exercise. When insulin is involved the guidelines are to have blood sugars above 100 mg/dl (5.5 mmol/l) but not higher than 240-300 mg/dl (13.3-16.6 mmol/l) with ketones or 300 mg/dl (16.6 mmol/l) (it does not matter whether ketones are present or not).
- Anaerobic exercise (weightlifting, sprinting) can cause blood sugars to go up during or immediately after the exercise due to the increased activity of hormones such as adrenaline. Even though there may be an increase, there is a likelihood blood sugars will go low hours after the activity. Once blood sugars are near normal, it is important to eat to replace glycogen stores. Drinking a lot of water and discussing with your healthcare team about using insulin to bring down high blood sugars immediately after exercise (i.e., 300+ mg/dl [16.6+ mmol/l]) is recommended.
- Decreasing the long acting insulin when there is a planned exercise event is one way to prevent low blood sugars. Increasing carbs prior to the activity and afterwards is another way. Finally, a combination of decreasing insulin and increasing carbs can work too. This is advantageous because it is important to eat carbs to help fuel the body for the activity but decreasing insulin along with it may help for individuals who do not want to eat too much food (i.e., carbs) prior to exercise if weight loss is a goal.
- Decrease basal rate for pump users after exercise – a 50% reduction rate is a good starting point over 4-6hrs but check frequently to see if more or less is needed and whether to decrease/increase the time of the decreased basal rate. Since most low blood sugars occur in individuals in the middle of the night (e.g., 1 a.m. – 3 a.m.) having a reduced rate overnight may help as well.
- A good rule is to eat 15 grams of carbs for every 30-60 minutes of exercise — this is individualized so checking blood sugars during exercise will help determine how many carbs are needed.
- Eat carbs with some protein or a good source of fat (e.g., peanut butter) within 20-30 minutes after exercise to restore glycogen stores. How much depends on blood sugars and intensity of exercise.
These are guidelines to follow but are not absolute. If a pattern develops look to make changes up or down depending on blood sugars and make sure to keep your healthcare team in the loop with any changes made.
A good book to read on exercise and diabetes is called The Diabetic Athlete by Sheri Colberg, PhD.
Rick Philbin, MED, ATC, CSCS
National Board Member, Diabetes, Exercise & Sports Association
National Presenter, Children with Diabetes
Certified Athletic Trainer
Strength and Conditioning Specialist
Northeast Regional Manager, Animas Corporation