One of my favorite comments to medical students has always been that "there are a hundred reasons for blood sugar levels to change, and we know about fifty of them." One of the ones that we do know is something called post-exercise delayed hypoglyce mia. And I now know from personal experience about this phenomenon.
Seems that Steph and I were on a trip recently where our group toured a waterfalls from a huge inflatable boat. The falls were the IguazuFalls between Argentina and Brazil. The rubber boats carry about twenty naÃ¯ve thrill-seeking tourists; they don't simply float, but have two huge outboard motors on the stern. And to get to the boats requires walking down about 100-plus slippery slimy steps from the overlook to the riverside, and after the ride, another 100-plus similar steps back up to the overlook.
My noontime blood sugar (well before the trip) was fine at 94; I ate a good-sized lunch and took a smallish dose of insulin, knowing that there'd be lots of exercise to help keep my sugar down. After clambering down the steps in a pouring rain squall, taking an equally-wet boat ride (and getting soaked under the falls by the grinning maniac piloting the boat), and re-climbing what now seemed like zillions more steps, I had the chance to recheck my blood sugar before we got into the jeeps that were to take us back to the hotel.
Surprisingly, my blood sugar had climbed instead of crashing - to 139. Well, I'd underdosed on purpose, so maybe it was just the excitement of the trip and the size of the noontime meal overbalancing the insulin dose and the exercise effect. All-in-all, I was feeling pretty good (although still extremely damp), and decided not to give any supplemental insulin dose to knock the 139 down.
And it was a good thing that I didn't give any supplemental insulin for the 139 - I checked my blood sugar again before dinner about three hours later, and the meter read 62
How did this happen? The blood sugar after exercise was totally misleading - it takes a while to drain the sugar level down; probably something to do with stress-induced hyperglycemia and/or gradually using up glycogen stores over a period of hours. Whatever the scientific reason might be, I now knew of the frustration of post-exercise delayed hypoglycemia - something I'd always warned patients about: that you can be high after exercise, and then go low later" for example, heavy exercise can have an increased risk of middle-of-the-night hypos unless you eat a big snack at bedtime.
Or if you climb hundreds of steps in the tropical rain forest from your boat to your jeep.
So, if you do exercise, check your sugar before, during, and after exercise, and again several hours later. And don't be surprised if the sugar several hours after exercise is low, even if the sugar immediately after exercise is okay. You too can have post-exercise delayed hypoglycemia.