The 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-on.
The 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 a continuous glucose monitor (CGM) and normal fingerstick tests.
What do you think they found out? Figure 1 shows when the hypoglycemic events happened. 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.
What was Interesting about this study was that 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.
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—just when you don’t want to go low!). When people exercised before breakfast, no one got low during or immediately after the exercise.
Details about the 4 PM, late afternoon exercise: They had 322 hypos after the afternoon sessions (10.7 per participant during 36 hours), 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 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.
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. However, if you exercise later in the day, you may have to eat more or take less insulin to prevent later-onset hypoglycemia.
One more thing to keep in mind, though, is that this study only tested moderate activity. 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!
If you're a die-hard morning exerciser but get frustrated with the rise in glucose that comes with it, you can also get around that by simply "breaking" your fast before you exercise, or making your 7 AM bout of activity after you eat something instead of before. If you have type 1, you'll have to take a small amount of insulin (albeit much less than normal) to cover it, and if you have type 2, your pancreas should release what you need to lower the insulin resistance associated with early mornings. You don't have to eat a full breakfast to make this work. So, you still have lots of options, but all come with their own issues that you'll have to figure out to prevent exercise-induced lows.
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.