The implications of this are immense in terms of exercise and type 1 diabetes. First of all, in type 1 diabetes this critical regulatory hormone is essentially absent yet is provided artificially by insulin injection. Hence decisions made regarding insulin injections associated with exercise must be done carefully. For example taking a normal dose of insulin just prior to exercising maintains suppression of liver glucose, inhibits fat tissue breakdown and would further increase glucose uptake in tissue that would normally reduce glucose uptake during exercise (i.e. Non-exercising tissue). All of these could contribute to a hypoglycemic condition and are counterproductive to successful exercising. The situation is further complicated by the fact that exercise will increase circulation and thereby increase the delivery of insulin to the peripheral tissues. This is most likely to occur if insulin is injected shortly before exercising and even more so if injected in a part of the body that is being exercised. Hence some degree of insulin reduction prior to exercise is always recommended.

How much should insulin be adjusted? There is no one correct answer since everyone is different and exercise goals also differ. What is provided here are guidelines that you can incorporate according to your fitness level and exercise intensity and duration. You should also document important information such as insulin dose, time and site of injection, time of day, prior food intake, exercise duration and intensity and pre and post-exercise blood glucose levels. This information can be used to learn and make adjustments where possible before, during and after exercise to safely maximize your performance and optimize your blood glucose control.

Some Guidelines As a rule, if blood glucose levels are below 5.6 mmoles/L then exercise should not be undertaken without first ingesting some carbohydrate. You should then make a record of the amount of carbohydrate and use the information for future adjustments of either food intake or insulin under similar conditions. An additional consideration is the rate of change in your BG. For example if a BG value of 9.7 is followed by a BG value of 6.3 one hour later just prior to exercise then you should recognize that your BG is rapidly dropping and the 5.6 rule does not apply. Take food before you exercise!

General Rules:

• Reduce your usual insulin dose
• Inject somewhere away from the exercising tissue if possible
• Inject mealtime insulin at least 2 hours prior to exercise
• If blood glucose is less than 5.6 mmoles/L supplemental feedings should be taken before and during exercise
• Take in carbohydrate during exercise if exercise is vigorous and of long duration (ie. Greater than 1 hour)
• Test blood sugars before, during and after exercise if needed and take in food when indicated to avoid low blood sugars
• Be sure to also take in adequate fluids while exercising
• Keep in mind that your blood glucose response will vary depending on the type, intensity and duration of exercise.
• Do your best to plan ahead if you know you will be exercising and incorpaorate as many of these suggestions as possible.

The timing of your injection as well as the type of insulin are factors to consider. It is safer to exercise immediately after eating when regular insulin has been injected since the insulin has a slower time of action. Fast-acting mealtime insulin has a faster and more intense onset of action. For this reason you should consider waiting at least 2 hours after eating before doing any exercise.

In terms of the insulin dose reduction, a short-acting mealtime injection can be decreased by approximately 30-50%. For the longer acting insulins, either individually or pre-mixed, you have to plan further in advance. For example you may want to reduce your morning injection of long-acting insulin if you plan to exercise that evening. During that day you may also consider moderating your food intake to accommodate the reduced insulin and still maintain reasonable BG control. You will have to determine what works best for you based on your pre and post-exercise blood sugar values.

The time of day is also critical. Studies have shown that the risk of hypoglycemia is reduced when exercise is done in the morning before the pre-breakfast insulin dose. This is because insulin levels are lowest at this time. In contrast, late afternoon or evening exercise can be trickier since insulin levels can vary depending on when lunch and/or dinner were eaten. Worse yet, since the post-exercise blood glucose lowering effects can last for 12 to 24 hours the risk of nocturnal hypoglycemic events is increased. This doesn't mean you shouldn't exercise in the afternoon or evening. Simply be aware of additional challenges associated with BG management and take precautions; frequent BG tests and snacks if needed.

Type of Carbohydrate. Carbohydrate in either a solid or liquid form work equally well. The liquid form has the advantage of providing fluid that is essential for optimum performance and recovery. You may also find the liquid form easier to digest especially if consumed while exercising. Depending on the nature of the exercise you may have to consume carbohydrate before, during and after. The type of carbohydrate can be chosen depending on the desired effect. For example, if you are trying to address a blood sugar you know is low and dropping you want a fast-acting carbohydrate. In this case you should choose a carbohydrate with a high glycemic index such as fruit juice,fruit, granola bar etc. There are energy drinks designed especially for this and in most cases you can safely dilute these sport beverages in half. As a reminder, glycemic index refers to the rate at a which a consumed carbohydrate appears as glucose in your blood (Quarterly Newsletter Vol.2 No.3). There are also tables listing the glycemic index for a wide variety of foods. For a meal prior to exercise that you know will be of long duration (e.g. a 2-3 hour hike or bicycle ride) you should consume a balanced meal with a relatively low glycemic index. This meal will continue to deliver glucose to your circulation over a prolonged period of time buffering the blood glucose lowering effect of exercise. Be sure your diet is balanced and nutritionally sound in order to optimize your performance and your recovery.

Post-Exercise Hyperglycemia

This is a common occurrence and is believed to result from the continued output of glucose from the liver and the absence of insulin. If you find this is an issue for you some people have success with a small (i.e. 2-4 units) post-exercise insulin injection, which can signal suppression of liver glucose production and enhance removal of glucose from the blood. Always test your BG and take appropriate action.

Conclusion

Everyone is different and will respond in a different way to food and exercise. Take this information and these suggestions and see what works for you. Always consider your safety first and be sure to enjoy whatever exercise activities you choose.

Eric Norman is a research scientist investigating diabetes and heart disease.

References

1. Exercise For the Patient With Type 1 Diabetes Mellitus. Horton, E.S. Chapter 46 in Diabetes Mellitus: A Fundamental and Clinical Text. Second Edition. Editors Derck LeRoith, Simeon I. Taylor, Jerrold M. Olefsky. Lippincott Williams and Wilkins. 2000.
2. Fuel Metabolism, Exercise and Nutritional Needs in Type 1 Diabetes. Franz, M.J... In Canadian Journal of Diabetes Care 22:4 pp 59-63.
3. Dietary Carbohydrate in the Management of Diabetes: Importance of Source and Amount. Woelver, T.M.S. In Endocrinology Rounds, St. Michael's Hospital, U. of Toronto. May, 2002.