Blood Sugars and Exercise
When you exercise, your body uses glucose for energy. During the first 15 minutes of your workout routine, your body converts the glycogen stored in your muscles back into glucose, and also uses the glucose circulating in your bloodstream for fuel. This action causes the natural blood-glucose-lowering effect of exercise.
After 15 minutes, your body turns to the liver to convert its glycogen stockpile into glucose energy. After about 30 minutes, your cells will also begin to burn free fatty acids (FFA) for fuel. Once the glycogen stores are used up, without a carb refueling in the form of food, hypoglycemia is a real danger, particularly in those with type 1 diabetes.
Before you get moving, test your glucose levels. If they are less than 100 mg/dl, don't start working out without a carbohydrate snack for fuel. A fast-acting carb should be available during and after your workout, in case you need it.
Snacking Savvy
Because of the risk of hypoglycemia during (and up to 24 hours after) exercise, especially in type 1 diabetes, a pre-exercise snack may be required. How much of a snack and when to eat it depends on your blood glucose readings and the planned intensity and duration of your workout. In general, people with type 2 diabetes who work out at a moderate level will probably not require a snack before exercise, particularly if they are trying to lose weight.
For people with type 1 diabetes, moderately intense exercise lasting longer than a half-hour will probably require a snack of at least 15 grams of carbohydrates. If you're planning on a high-energy game of racquetball that may last an hour or more, you will need a bigger carb boost of between 25 to 50 grams of carbohydrates. Adding protein and/or fat to the carbs can extend the glucose action over time if you're embarking on a long-distance bike ride or similar activity. A snack should be eaten about 15 to 30 minutes before exercise for the best results.
If you're going on a low-intensity, half-hour walk and your glucose readings are at least 100 mg/dl, you may not need any extra carbs (although you should always bring some with you just in case).
Alert
If you have type 1 diabetes and you have blood sugars greater than 250 mg/dl and ketones present on a blood or urine test, wait to exercise until the ketone test is negative. You should also use caution if glucose levels are greater than 300 mg/dl and no ketosis is present.
Never exercise immediately following an insulin injection. Give your body time — at least 90 minutes — to digest and process injected insulin. Don't plan exercise during the time your insulin is peaking. Exercise can also cause injected insulin to work much faster because your circulation speeds up as your heart pumps harder. In many cases, you may need to adjust your insulin dose downward for those times you will be exercising; again, your doctor can offer you further advice on what's right for you.
Lows (Hypoglycemia)
If you start to feel hypoglycemia coming on, don't panic. Stop exercising and test immediately. If your levels are too low, eat or drink 15 grams of carbohydrate right away. Don't resume your workout. Wait 15 minutes for the carbs to kick in and test again. If your levels are still too low, take 15 more grams and follow the same routine until blood glucose levels are back in a normal range.
A glucose gel or glucose tablet is a good choice for a fast-acting carb. Both are compact enough to carry along while you exercise and work quickly. Invest in a waterproof fanny pack to keep both your glucose gel/tablets and testing supplies in during your workout.
After your workout is finished, your body will recharge its energy stores by processing your blood glucose into glycogen. Depending on the level of exercise intensity and the amount of glycogen that needs to be replaced, this process could take anywhere from four to 24 hours. This is why post-exercise glucose testing is important, as hypoglycemia can occur well after you've finished exercising.
Essential
Keeping an exercise log along with your blood glucose readings can help you figure out what works best for you. Different sports and routines will have diverse effects on your blood sugar levels. Discuss your exercise plans with your diabetes care team to find out the best routine for preparing for exercise.
Highs (Hyperglycemia)
In some cases, when blood glucose levels are high and insulin levels are low to begin with, the adrenaline rush from heavy exercise can have the opposite effect, signaling the liver and muscles to break down glycogen into glucose. In these cases, ketoacidosis is a danger. If your blood sugars test high before exercise, always test for ketones before you start your workout. If your levels are high and you do choose to exercise, make sure you test glucose levels again about 15 minutes into your workout to ensure they are coming back down.
Essential
You should always wear or carry medical identification, but it's particularly important when you are exercising. Before you go for that run, make sure you are wearing your ID in a visible location. A shoelace tag or watchband ID are sports-friendly options that won't get in the way of your workout.
Take Precautions
To avoid dangerous highs and lows during exercise, take the following precautions:
Test first. Always take a blood glucose reading before your workout.
Warm up. Always stretch out before exercising to avoid injury, and start your routine with five to 10 minutes of low-intensity movement.
Wet down. Drink plenty of fluids before, during, and after your workout to prevent dehydration. If you're exercising in the heat, drink even more.
Cool off. If you're working out intensely, take it down a few notches at a time, and spend five to 10 minutes bringing your heart rate back down to its pre-workout level.
Carb load. Have a fast-acting carbohydrate on hand in case of hypoglycemia, and don't hesitate to use it.
Test last. Once your workout is over, test your blood glucose levels again to ensure they aren't dipping too low.
