Exercise and Type 1 Diabetes: How to predict and manage blood sugars

 I have recently had a succession of patients with type 1 diabetes describe their confusion over blood sugars increasing rather than decreasing with exercise. In desperation, many have suggested that they drop exercise altogether, since it “doesn’t appear to be helpful.” The first point that I would like to make is that exercise should be encouraged in all patients with type 1 diabetes. While exercise certainly has the potential to destabilize blood sugar levels, it clearly helps to protect against heart attacks, strokes, osteoporosis and even some forms of cancer.

However,  questions linger on my patients’ minds including 1) Why does the blood sugar rise at times and lower at other times with exercise?, and 2) What can possibly be done to ameliorate this problem?

The effect of exercise on blood sugar is multifactorial of course, but it has a lot to do with the type of exercise being performed.

Aerobic activity consists of exercises that are continuous and rhythmic with repeat movements in the same muscle groups for at least 10 minutes at a time. These include walking, jogging, running, bicycling and cross country skiing.  Generally speaking, aerobic activities reduce blood glucose levels because the muscles take up glucose the blood stream to use as fuel. Individuals with well-controlled type 1 diabetes will usually experience a decrease in blood glucose to a greater degree than non-diabetic individuals. This is because patients receiving insulin (rather than producing their own insulin) are unable to automatically decrease the insulin levels to adjust for dropping sugars.

In addition, the increased body temperature and blood flow from exercise may speed up insulin absorption from subcutaneous depots. This effect may be most marked if the insulin was recently injected or was delivered into an arm or leg that is involved in the exercise. Further complicating matters, there may also be a diminished response to exercise-induced hypoglycemia in patients with type 1 diabetes.

This may be due to low levels of liver glycogen, a storage form of glucose, which can be broken down into glucose and difficulty producing adequate hormones which raise sugar levels such as epinephrine, cortisol and GH. Since exercise increases the body’s sensitivity to insulin, exercise increases risk for hypoglycemia for up to 12 hours after exercise. It appears that late-day exercise causes an increase in insulin sensitivity first at the time of exercise and then 7-11 hours later. In contrast, exercise early in the day appears to cause sustained insulin sensitivity over an 11 hour period following the exercise.

Conversely, movements that are resistance-based, such as lifting weights, fast sprinting, gymnastics, and wrestling, are anaerobic in nature. Anaerobic activities may actually raise blood glucose levels. The proposed mechanism of action is an increase in catecholamine levels. Catecholamines are hormones which are released as part of a stress response and ultimately raise blood glucose levels by increasing the production of glucose by the liver.

When the production of glucose by the liver exceeds the ability of muscle to use that glucose, hyperglycemia occurs.

Some activities include both aerobic and anaerobic components, and these include basketball, soccer, tennis, lacrosse, field hockey, downhill skiing, golf, and skateboarding to name a few. Of course, it is hard to predict exactly how the blood glucose levels will be affected in this setting.

In addition to anaerobic activity, hyperglycemia can be induced by exercise in individuals with poor metabolic control. When blood sugars are elevated at the onset of exercise, there is often not enough insulin in circulation to allow muscles to properly take up blood glucose to burn as fuel. This leads to rising blood glucose levels since the exercising muscles are sending continuous signals to the liver to create glucose despite the inability of the muscle to subsequently take up the glucose from the blood stream. Another reason for rising blood glucose during exercise is that the psychological stress of sports may induce the production of catecholamines.

Regulating blood glucose during exercise is difficult and not an exact science, but the following tips may help stabilize blood sugar levels.

Blood glucose levels should always be checked prior to exercise for a target range of approximately 90-250mg/dl.  Low blood sugars should be treated with 15 grams of fast-acting carbohydrate, and sugars should be rechecked at regular intervals. If blood glucose levels are greater than 250mg/dl, exercise should be avoided until the hyperglycemia is corrected.

If prolonged exercise is to be performed within three hours of a  meal, reductions in premeal insulin should be considered. These reductions may be 25%, 50% or 75% for low, moderate, and high intensity exercise respectively. For patient on pumps, the basal rate can be reduced between 20% and 100% approximately 60-90 minutes prior to onset of exercise. In addition, a 6- hour 20% reduction in bedtime basal rate may help prevent nocturnal hypoglycemia.

As a precaution, blood glucose should be checked every 30-45 minutes during exercise. Hyperglycemia that occurs during exercise must be managed cautiously since a full insulin correction bolus may lead to hypoglycemia. If the blood glucose level exceeds 250mg/dl, 50% of the usual correction bolus may be considered.

Multiple factors lead to exercise- induced alterations in blood glucose for patients with type 1 diabetes, and the management is a combination of art and science. However, patients should be encouraged to engage in regular exercise, since the benefits are clearly proven time and again.

Chimen M, Kennedy A, Nirantharakumar K, et al. What are the health benefits of physical activity in type 1 diabetes mellitus? A literature review. Diabetologia 2012; 55:542.

Davey RJ, Howe W, Paramalingam N, et al. The effect of midday moderate-intensity exercise on postexercise hypoglycemia risk in individuals with type 1 diabetes. J Clin Endocrinol Metab 2013; 98:2908.

Delvecchio M, Zecchino C, Salzano G, et al. Effects of moderate-severe exercise on blood glucose in Type 1 diabetic adolescents treated with insulin pump or glargine insulin. J Endocrinol Invest 2009; 32:519.

Rabasa-Lhoret R, Bourque J, Ducros F, Chiasson JL. Guidelines for premeal insulin dose reduction for postprandial exercise of different intensities and durations in type 1 diabetic subjects treated intensively with a basal-bolus insulin regimen (ultralente-lispro). Diabetes Care 2001; 24:625.

Mitchell TH, Abraham G, Schiffrin A, et al. Hyperglycemia after intense exercise in IDDM subjects during continuous subcutaneous insulin infusion. Diabetes Care 1988; 11:311.

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