I have worn a Dexcom G5 and now G6 for a number of years purely for the reasons you suggest (health monitoring, reaction to foods etc.).
As you say it is pretty interesting looking at how your body reacts and how that changes.
Some of the things I have noticed:
1. Sleep impacts how my body reacts to identical foods. I am far less tolerant of glucose if sleep deprived. I think this has also been shown in studies.
2. If I eat relatively low carb for not much more than a day (which I do most of the time) I become much less tolerant of carbs if I add them back in. This appears to be a from of physiological insulin resistance and I think it is resistance in the muscles because they have been preserving glucose for the brain and relying more on fat. When I first did a 7 day water-only fast my blood glucose was around 55-60mg/dl (flatline) at the end and I ate a small amount of carbs via some sourdough toast (along with fat and protein) and my blood sugar went through the roof (250+mg/dl). I went out of my office and ran some sprints on a set of steps which helped :). I have seen this a number of times. If I have been quite low carb my muscles don’t want to take up the glucose but doing some very intense exercise (running up steps repeatedly) helps. This has also been shown in studies even over short periods of mild carb restriction but it seems to be very dependent on the person and doesn’t seem to happen to everyone.
3. I also see the same as you. Long endurance runs I will not see any spikes although, depending on my glucose state before, I may see a slow trend lower over a number of hours but nothing huge. If you are so carb depleted that you are generating a lot of ketones whilst out running then I find that also surpresses glucose. Very intense efforts does also show up for me as a glucose spike but, as you say, that is expected from your liver dumping out glucose for more glycolytic workouts.
4. I also find that what you eat when doing exercise if far less important than what you eat when you are just round the house at home. I don’t think you see the same large insulin spikes during exercise and I definitely don’t see the huge spikes in blood glucose even if I was to take something very high glycemic. I personally use UCAN a lot but also now use other more traditional gels when things are more intense for easier recovery and if I take them whilst exercising I don’t see a huge spike like you might expect. I wore a CGM whilst doing the Tor des Geants a few years ago and, as you can imagine, blood glucose levels stayed pretty low throughout regardless of what I was shovelling into my mouth :).
I would agree that if your glucose level is stable it would probably suggest your liver is not dumping out glucose like you see when you are doing very intense efforts but just looking at the glucose number doesn’t really tell you anything about the flux (i.e. the amount the liver is putting into blood or the amount being removed). Although I do find that having the CGM overlay to a metabolic test is quite interesting. As you say, I think it likely makes sense that you see the change around your AeT but not sure how much you can definitively read into the single number through time if you didn’t already know what your AeT was (i.e. if the only number you have is your CGM number and you know nothing else whilst training I am not sure how much you can really assume from it). I definitely see variation in response depending on sleep, what I have eaten, when I ate it etc. as well as just the intensity.
I think the big thing for me using a CGM around training was seeing how it was possible to eat much more carbs around exercise without spiking blood sugar (and therefore insulin). This has been helpful for me to more proactively replenish glycogen stores before, during and after more intense workouts whilst knowing that I haven’t triggered big moves in glucose. Taking extra carbs around exercise like this also helps me keep my muscles more glucose sensitive (at least that is what I find). For a long time I think I was running around with very glycogen depleted heavy legs because I wasn’t good enough at proactively getting extra carbs onboard before/during/after intense workouts.
Obviously the N on these obervations is 1 although I have seen commentary from others like Peter Attia and studies showing similar observations in others.
Sorry if this is a bit of a random collection of thoughts. Definitely interested to hear others experience.