Most fitness and diabetes sources state that exercise can cause hypoglycemia in type I diabetics. Since I run and play hours of ultimate Frisbee without hardly ever go low, I like to think that I have a routine dialed in that prevents any exercise hypoglycemia. However, in the past few months, I have spent a lot of time hiking with my family in the mountains, and during these hikes, I have struggled quite a bit with going low. Even though I prepared and managed my insulin and fueling in exactly the same way, my body responded so differently. It didn't really make sense to me- why can I run for ninety minutes without any drop, but hiking for just twenty five minutes causes me to crash?
I did some research, looking for an explanation as to why different forms of aerobic exercise might affect my blood sugar differently. Once I dug deeper than the general articles that described how exercise ultimately lowers blood sugar and decreases the need for insulin, I found several studies that showed a different trend- exercise can also actually raise blood sugar! It turns out that there is a sort of threshold involving VO2 max; above a certain percentage exercise actually causes the sympathetic nervous system to release hormones that cause blood sugars to spike.
For example, DiabetesHealth cites one study in which subjects with Type I diabetics cycled for "alternating periods of maximal exertion for 4 seconds followed by 2 minutes of easier cycling, for a total of 30 min." The alternating pattern of intense and less intense exertion was designed to recreate the metabolism of a sport or game, such as soccer. After the workout, the subjects' blood sugar was actually elevated as compared to before the workout. In comparison, a 30-minute session consisting of steady-state aerobic exercise at only 40% of maximum intensity caused blood sugar to fall, although less total work was done in this time.
Although it is counterintuitive, it is true that especially vigorous exercise can cause hyperglycemia. Often, exercise intensity can be measured in terms of maximal oxygen consumption, also known as VO2 max. This measurement represents the maximum capacity of an individual's body to transport and utilize oxygen during exercise, and it also reflects the physical fitness of the individual. The studies recognized that there is a physiological different when intensity increases to the point where an individual is exercising at or above 80% of their VO2 max.
In this range, exercise is very strenuous, so the adrenal and other endocrine glands release hormones that are meant to help the body perform strenuous activity, causing blood sugar to rise. Essentially, the "fight or flight" response is initiated, exciting endocrine organs such as the adrenal gland. In order to meet the high energy requirements, the adrenal gland releases the stress hormones adrenaline and noradrenalin into the blood, which then stimulate the liver to release glucose at a faster rate than normal. The idea is that this extra sugar will provide working muscles more energy, but when this rate exceeds the rate at which glucose is absorbed by active muscle tissue, blood sugar levels spike.