by Dr. Sheri Colberg, Ph.D., FACSMThe biggest problem most insulin users face is the risk of their blood glucose going too low for up to two days after they exercise.
Given that there are no clear recommendations about the best time to exercise with diabetes, a recent study published in the Journal of Diabetes Science and Technology early in 2015 tackled this issue head-on1.
That study compared blood glucose levels and the number of lows during and following moderate exercise for 36 hours. A total of 35 adults with type 1 diabetes using insulin pumps undertook 60 minutes of moderate exercise on a treadmill at 7 AM (pre-breakfast) and 4 PM (pre-dinner) on different days. During exercise their insulin pumps were turned off completely and then restarted 45 minutes after they stopped working out. How frequently they developed hypoglycemia (defined as glucose values < 70 mg/dl, or3.9 mmol/l) for up to 36 hours post-workout was monitored using both continuous glucose monitor (CGM) and fingerstick tests.
Overall, hypos occurred significantly less often following 7 AM exercise compared to 4 PM (5.6 vs. 10.7 hypos per person). This is not a new finding, however, as other studies done previously in both type 1 and type 2 exercisers have found that you’re more likely to get low from exercise done later in the day. Most of the lows occurred 15 to 24 hours after exercise, regardless of the exercise timing, and over half of the adults in the study had at least one low in 36 hours. On days following morning exercise, there were 20% more CGM readings between 70 and 200 mg/dL (3.9 and 11.1 mmol/l) than on the day before they exercised in the morning, meaning that doing pre-breakfast exercise kept their blood glucose levels in a more normal range and improved their overall control, all with a lower risk of hypos.
Here are some additional details about the 7 AM, pre-breakfast exercise: Participants had 180 lows after the morning sessions (5.6 per participant during 36 hours), with most occurring 15 to 24 hours after the sessions (between 10 PM and 7 AM — or during the overnight hours, which is the time when most people really would like to avoid lows since they may be harder to detect and respond to while they’re asleep). Also, when they participants exercised before breakfast, no one got low during or immediately after the exercise.
As for the 4 PM, late afternoon exercise, participants had 322 hypos after the afternoon sessions (10.7 per participant during 36 hours, or almost twice as many as following the morning exercise), with most occurring 15 to 21 hours after exercise, which in this case was between 7 AM to noon. Six people got low during the exercise itself (compared to none during AM exercise).
Not surprisingly, this study reported fewer hypos when insulin users exercise at 7 AM, prior to breakfast and morning rapid-acting insulin. At that time of day, levels of the hormone cortisol are higher, which lowers insulin action and keeps blood glucose levels from dropping, and circulating insulin is lower then as well (prior to any insulin taken for breakfast). The afternoon exercise was done at 4 PM, which was four hours after lunch and use of rapid-acting insulin, but the “tail” of any insulin taken during the day may still lead to higher insulin levels overall in the afternoon compared to in the morning.
One more thing to keep in mind is that this study only tested moderate activity, and the exact nature of the exercise was not described (other than it being four 15-minute sessions on a treadmill). If you work out harder, your blood glucose is more likely to rise during the activity, which may result in hyperglycemia instead of hypos. In that case, doing intense afternoon activity may be preferable.
When it comes to preventing postprandial spikes in blood glucose, exercising after dinner is much more effective than doing it before2. Others have shown that for people with type 2 diabetes, a prior meal helps enhance the glucose-lowering effect of physical activity3. When it comes to blood glucose management, exercise done at any other time of day is likely better advice for the person with diabetes than exercising in a fasted state first thing in the morning.
For anyone with diabetes, engaging in pre-breakfast exercise can actually be detrimental to blood glucose management. Exercising first thing in the morning before eating prolongs the fasting period and promotes an even greater release of glucose-raising hormones like cortisol, which in particular heightens insulin resistance pre-breakfast. In fact, a usual response for anyone with diabetes who is active before eating breakfast is an increase in blood glucose levels, not the usual decline experienced during moderate physical activity. (I can’t tell you how many times I have advised people with diabetes to eat something first before engaging in early morning exercise to prevent elevations in their blood glucose levels!)
So, when should you exercise? I still say that the best time to exercise is when you have the time to do it on any given day. If you exercise early in the day before any meals, you may have to deal with elevations in your blood glucose that result from the activity, particularly when it’s more vigorous, and if you exercise later in the day, you may have to eat more or take less insulin to prevent later-onset hypoglycemia.
- Gomez, A. M., C. Gomez, P. Aschner, A. Veloza, O. Munoz, C. Rubio & S. Vallejo (2015) Effects of Performing Morning Versus Afternoon Exercise on Glycemic Control and Hypoglycemia Frequency in Type 1 Diabetes Patients on Sensor-Augmented Insulin Pump Therapy. J Diabetes Sci Technol.
- Colberg SR, Zarrabi L, Bennington L, Nakave A, Thomas Somma C, Swain DP, Sechrist SR: Postprandial walking is better for lowering the glycemic effect of dinner than pre-dinner exercise in type 2 diabetic individuals. J Am Med Dir Assoc 2009;10:394-397
- Poirier P, Mawhinney S, Grondin L, Tremblay A, Broderick T, Cleroux J, Catellier C, Tancrede G, Nadeau A: Prior meal enhances the plasma glucose lowering effect of exercise in type 2 diabetes. Med Sci Sports Exerc 2001;33:1259-1264
As one of the world’s leading experts on diabetes and exercise, I recently put my extensive knowledge to use in founding a new information website called Diabetes Motion (www.diabetesmotion.com), the mission of which is to provide practical guidance about blood glucose management to anyone who wants or needs to be active with diabetes as an added variable. Please visit that site and my own (www.shericolberg.com) for more useful information about being active with diabetes.
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