The amount of insulin circulating in the blood has an important effect on a person’s metabolic response to exercise. In general, people who don’t have diabetes have less insulin circulating in the blood during exercise. Likewise, research has shown that people who use an insulin pump can program it so that they have less insulin circulating in the blood during exercise than people who use multiple daily injections, putting them at lower risk for hypoglycemia.
The explanation for the difference in insulin levels is that exercise causes increased blood flow to the skin, and in people who use multiple daily injections, this causes subcutaneously injected long-acting insulin to enter the bloodstream earlier than it normally would, effectively raising the circulating insulin level. An insulin pump, on the other hand, does not deliver insulin more rapidly than usual when blood flow to the skin increases. In fact, if desired, an insulin pump can be programmed so that it delivers less insulin than normal prior to exercise or it can be disconnected ahead of time so that no additional insulin enters the bloodstream.
The time of day you exercise can also affect your circulating insulin levels. In the morning, blood glucose levels tend to decrease less with exercise because of naturally higher levels of certain counterregulatory hormones at that time of the day that help raise blood glucose levels. Pump users may choose to exercise before breakfast and before bolusing any insulin to ensure low circulating insulin levels. However, some may be able to perform 30 minutes of moderate-intensity exercise two to three hours after breakfast without a high risk of hypoglycemia, despite slightly higher insulin levels after eating.
Later in the day, levels of counterregulatory hormones fall, so you will likely need to reduce your pump’s basal insulin delivery for perhaps 60-90 minutes prior to moderate or strenuous activities lasting longer than 30-45 minutes. Basal rates may also need to be lowered during the exercise to reduce insulin levels sufficiently. To achieve optimal control, you will also need to make appropriate insulin-bolus reductions for any exercise closely following a bolus to minimize circulating free insulin levels and the risk of hypoglycemia during and following the activity.
Even with insulin pump use, your metabolic control with exercise can worsen under certain conditions. If your blood glucose level is elevated (250 mg/dl or higher) and you have ketones in your blood or urine, you are insulin-deficient and are at risk of developing diabetic ketoacidosis (DKA), a life-threatening condition that may require emergency treatment in a hospital. Exercising when you are insulin-deficient is likely to raise your blood glucose level even higher and hasten your progression toward DKA. It is therefore not recommended in this circumstance.
The type of exercise you do can also cause blood glucose level to rise (albeit temporarily). A short period of intense exercise, such as heavy weight lifting or sprinting, especially when done before breakfast, can cause your blood glucose level to rise during the activity and stay elevated for two or more hours afterward. High blood glucose after intense activity is found both in people who have diabetes and in those who don’t; it is attributed to an exaggerated hormonal response to intense activities. For such activities, pump users may need to elevate basal insulin rates during and for a period after the exercise, or give an additional small bolus of insulin.
The insulin pumps currently for sale in the United States provide a variety of features. With regard to exercise, the most important features to consider are the following:
- The number of basal profiles that can be programmed and saved in a pump’s computer memory. Being able to program several different profiles could be particularly useful for people who participate in several sports or who alternate intense workouts with less intense workouts.
- The degree to which basal and bolus doses can be fine-tuned. Some pumps allow the user to fine-tune doses to the nearest half unit, whereas others allow the user to fine-tune doses to the nearest one-twentieth of a unit. Being able to make very small changes in insulin delivery may be particularly important to insulin-sensitive individuals, such as those on a total daily insulin dose of 30 units or less.
- How a pump handles temporary basal settings. Pumps vary in the degree to which a temporary basal setting can be fine-tuned and the length of time a temporary setting can be maintained.
- The ability to deliver extended boluses. If insulin must be bolused shortly before exercise, giving a bolus over a period of time can help lower the risk of hypoglycemia during the activity by keeping circulating insulin levels lower.
- For swimmers, whether a pump is waterproof.