By Sheri Colberg, PhD
Whether you consider yourself an athlete or an occasional recreational sports participant, you benefit from any activity that you do, because all exercise can improve your body’s ability to use insulin. If you do not inject or infuse insulin to treat your diabetes, exercise should result in a decreased release of insulin by your pancreas and a more efficient uptake of blood glucose by muscle cells. If you use insulin, however, you may be concerned that exercise can complicate the normal maintenance of your blood glucose level.
Active people who use insulin need to make frequent adjustments in their diabetes regimen to maintain blood glucose in target range, especially when doing higher-intensity or longer-duration exercise. But the effort can pay off in improved blood glucose control, reduced nighttime hypoglycemia, improved cardiovascular fitness, and weight loss. When it comes to managing exercise effectively, continuous, subcutaneous insulin-infusion therapy (or more simply, insulin pump therapy) is an option to consider.
Scientists attempting to mimic the insulin delivery of a normal, healthy pancreas first developed insulin pumps in the late 1970’s. Today’s cell-phone–size pumps are much more sophisticated and give both basal insulin doses (background insulin delivered every few minutes in small increments to cover your body’s general need for insulin) and boluses (larger doses given to cover meals and snacks or to lower elevated blood glucose at any time).
Currently, roughly 500,000 individuals with diabetes worldwide wear portable insulin pumps, and this number has been increasing each year. Most pump users have Type 1 diabetes, but some people with Type 2 diabetes who require insulin are choosing to pump insulin as well. A main advantage of insulin delivery via an insulin pump is more reliable insulin action through a constant infusion of short-acting insulin, along with precise dosing and timing of insulin to cover food intake. Many insulin pump users experience improved overall blood glucose control, reduced risk of nighttime low blood glucose, and improved awareness of low blood glucose (hypoglycemia).
Insulin pumps contain a reservoir or cartridge filled with insulin, either Regular or one of the rapid-acting insulin analogs, insulin lispro (brand name Humalog), insulin aspart (NovoLog), or insulin glulisine (Apidra). Compared to Regular insulin, these insulin analogs have a more rapid onset of activity (5 to 15 minutes versus 20 to 30 for Regular) and an earlier peak in activity (90 minutes versus 150 minutes). Their use allows for a blood glucose response following carbohydrate intake that is closer to that of a person without diabetes and for a more rapid correction of hyperglycemia (above-normal blood glucose levels).
All insulin pumps currently on the market deliver insulin subcutaneously (under the skin) in the abdomen, buttocks, legs, or upper arms, either through a needle or through a plastic infusion catheter. Pump users replace the needle or catheter infusion set every two to three days with a new set at a new site.
Pump users with active lifestyles can experience a metabolic response to exercise that is similar to that of people who do not have diabetes. In part, this is simply because of how an insulin pump works, by delivering small amounts of fast-acting or rapid-acting insulin continually. But it also depends on the user monitoring his blood glucose level frequently, adjusting basal and bolus doses to fit the exercise, and learning from experience. (Click here to see some sample basal profiles.)
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:
Some possible drawbacks of insulin pump use with exercise are important to know about. Excessive sweating can cause your subcutaneous infusion set to dislodge, which may result in elevated blood glucose or ketoacidosis if you fail to notice the displacement for some time. To prevent infusion set displacement due to sweating, try using a liquid skin preparation such as Skin-Tac-H to anchor the set more firmly to your skin, or apply additional tape below the infusion set. You can also apply antiperspirant to your skin around the infusion site to minimize sweating beneath it. Or you can try wearing the infusion set in an area of the body that experiences less movement with activity, such as the buttocks. A potential problem for those using metal needles is irritation of the infusion site as a result of movement or contact. Flexible plastic infusion sets do not cause this problem.
Especially when you exercise vigorously, you must adhere to the recommendation to replace your insulin infusion set every two to three days to prevent displacement or irritation at the insertion site. It is also recommended that you check the integrity of your infusion site following vigorous exercise, sweating, or water contact.
Another potential problem is that insulin is temperature-sensitive. Exercise in hot or cold environments can potentially cause insulin to degrade and lose effectiveness. When an insulin pump is placed close to your body during exercise in the heat, the insulin may become overheated; if the pump is placed outside your clothing in the cold, the insulin may freeze. If unexpected high blood glucose arises following such conditions, replace both the infusion set and the insulin in the reservoir as a precaution.
Especially with the use of rapid-acting insulin analogs in your pump, DKA can begin as few as five hours following the displacement of your infusion set. When you become insulin-deficient due to any interruption of insulin delivery, exercise can exacerbate this situation and cause DKA to occur even more rapidly. Due to the severity of DKA, insulin pump users must be especially vigilant about maintaining the integrity of their infusion sites during exercise to prevent it from occurring.
In general, pump users should replace their entire cartridge and infusion set at the first sign of any unanticipated high blood glucose that does not respond to additional boluses.
To keep your blood glucose in the target range during and after exercise, follow some basic guidelines:
Understand the nature of the activity. Some activities, such as weight lifting, use mainly anaerobic energy sources and can have a markedly different effect on blood glucose level than prolonged or moderate-intensity aerobic exercise such as walking. Short-term, high-intensity anaerobic exercise can stimulate hormones that counteract the effects of insulin and often cause an increase in blood glucose level that can last for an hour or more after exercise. Mild to moderate aerobic exercise, on the other hand, tends to cause a decrease in blood glucose level.
Predict your blood glucose response and make regimen changes. Based on the type of activity you do, time of day you exercise, and amount and timing of your insulin delivery, you can often predict the effect of exercise on your blood glucose level and make appropriate changes. Of paramount importance are the length and intensity of the exercise and your training level.
Establish your pattern. Each time you try a new activity, check your blood glucose level frequently (before, during, and several times after an activity) to establish your usual response to the exercise, as well as variations from your predicted response.
Learn from the past. Make regimen changes (increased carbohydrate consumption and/or decreased insulin) based on your previous blood glucose response to an activity. You may also make regimen changes based on whether physical fitness or weight loss is a goal of the exercise. If you want to train hard and frequently for fitness, you will have to eat to replace the calories you are using. If weight loss is your goal, you may want to lower your insulin doses (instead of snacking) to maintain blood glucose control.
Prevent hypoglycemia. Learn to adjust your insulin doses to prevent hypoglycemia from occurring during exercise and for up to 24 hours afterward. Consider leaving your basal rate lowered for a longer period of time following more strenuous or prolonged activities.
Expect a training response. Once you have been doing an activity consistently for two to three weeks, you will experience a training response that will alter your blood glucose usage. Training increases your capacity for fat utilization, which has the potential effect of sparing blood glucose. For prolonged aerobic activities, training may result in your blood glucose level dropping less during an activity, resulting in the need for smaller regimen changes. In addition, as your muscle mass increases in response to training, your overall insulin sensitivity may increase, lowering your basal and bolus insulin needs.
The American College of Sports Medicine (ACSM) and the American Diabetes Association (ADA) have established general clinical practice recommendations for exercise and diabetes that apply to insulin pump users as well. According to these guidelines, people with diabetes should avoid exercising if their fasting blood glucose level before exercise is above 250 mg/dl and there is an elevated level of ketones present in the blood or urine. A blood glucose level above 300 mg/dl with no ketones indicates the need for “caution.” A third guideline is to ingest some carbohydrate if one’s blood glucose level before exercise is below 100 mg/dl.
The problem with these recommendations is their generality, so before beginning an exercise regimen, talk with your health-care provider about what you personally should do in these situations. It is a good idea for all people with diabetes to check their preexercise blood glucose level. However, contrary to the ACSM and ADA guidelines, pump users with a blood glucose level below 100 mg/dl before exercise may not require a carbohydrate snack since they can simply reduce or suspend basal insulin during an activity.
The insulin reductions and/or carbohydrate intake you require for aerobic activities will depend on the intensity and duration of your activity. You should be able to compensate for shorter, less intense activities with a change in either your insulin (basal and/or bolus doses) or carbohydrate intake. For short, intense activities such as weight training, you may not require any immediate regimen changes, but you will need to anticipate and prevent delayed-onset hypoglycemia from occurring later on. For longer, more intense exercise, you will generally require a combination of carbohydrate intake (15-60 grams per hour) and insulin reductions (25%-100%) to maintain normal blood glucose levels.
To effectively use your insulin pump during exercise, learn to program it according to your needs, understand the basic guiding principles for exercise, and follow the general recommendations for regimen changes presented here. If you have decided to get a pump, keep in mind that choosing one is mainly a matter of your preference: All of the current models can be used effectively for most types of physical activity.
The benefits of exercise far outweigh the risks, so don’t use your diabetes as an excuse to avoid exercise. Stay fit!
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