I had an interesting response to intense exercise the other day. I was many days fasted and went to an introductory CrossFit class. I measured my blood sugar after I got home from the class and my blood sugar was around 80 points higher. Did some digging to try and find out why.
My first assumption was that the liver was dumping glycogen and that’s partly true. Turns out that the muscles store around 80% of our glycogen stores and the liver contains the rest. The muscles use the glycogen locally and they don’t dump glucose into the blood stream. All of the glucose does come from the liver which takes it out of stored glycogen. That would account for some of the rise. I usually see a ten point pop with some exercise like bike riding but not like that time. The CrossFit exercise was very hard compared to normal exercise.
Here’s where the the Cori Cycle comes into play. I was fasted during the exercise for more than 20 days. From the Wikipedia article:
The Cori cycle is a much more important source of substrate for gluconeogenesis than food. The contribution of Cori cycle lactate to overall glucose production increases with fasting duration. Specifically, after 12, 20, and 40 hours of fasting by human volunteers, the contribution of Cori cycle lactate to gluconeogenesis is 41%, 71%, and 92%, respectively.
This was a piece of data which I didn’t have before. This makes sense of the experience that I had with intense exercise. The muscles released a lot of lactate which at the end of the exercise gets converted in the liver through gluconeogenesis into glucose. Hence, the large pop in blood sugars.
[2018-06-29 – I think this is a pretty good explanation of the problem. I wish I knew more at the time about VO2max and Heart Rate Training. I could have picked the right mode of exercise (top of aerobic range) rather than the HIIT mode of Crossfit.]
Here is a great discussion of exercise and the Type 2 Diabetic with Dr Finney (Shelley. Low-Carb preserves Glycogen better than High Carb. Me and My Diabetes. April 14, 2011).