I track blood glucose and ketones before and after different activities both fasted and fed…
Remember that ketones are a source of energy similar to glucose. There are several sources of energy: creatine, glycogen, glucose, ketones, fatty acids…
Depending on the type of exercise or exertion, the body will work through each based on a priority… light weight, high reps will consume a different energy than high weight, low reps.
Also, depending on how you’ve trained your body, its ability to generate glucose or ketones will vary as well. So, you have things that use energy like exercise and stress - and you have things that make energy like making ketones or glucose, etc…
When I exert myself on low intensity cardio, sauna, UV, slow & low time under tension (TUT) weightlifting- my body has a preference for using ketones… so that fuel drops faster than I can make more ketones & usually glucose doesn’t change much… however, give it a couple of hours of rest and ketones come back even higher.
For example- ketones are the preferred fuel source for long distance marathon runners and those that train fasted get their bodies used to rapid lipolysis to make ketones and fatty acids. In this case, the body gets used to GNG (gluconeogenesis) from fat: it rips off the three fatty acids and converts some into ketones. The glycerol backbone is turned into glucose or glycogen to feed the obligate glucose using cells like blood, some brain and deep kidney cells. This is because those cells don’t have mitochondria and must use glucose only… every other cell can be trained to use ketones and its a question of training the liver to make enough fast enough.
But if I do HIIT or other fast paced high intensity activities, I see my glucose spike. This is because the body recognizes this as an emergency situation or stressor and responds by giving me a “boost” of fast burning energy. This can come from liver glycogen or GNG of protein (lean mass).
This doesn’t raise insulin because the body only makes what it absolutely must and will not overshoot… so this form of GNG is a controlled response. GNG caused by eating excess protein will cause both a glucose and an insulin spike…
Basically, GNG needed for surge activities like sprinting or HIIT is demand driven - no insulin spike. GNG due to eating excess protein is to protect the body from excess nitrogen and capture the energy on the backbone amino acid structures. That will cause glucose and insulin increase.
But back to high intensity exercise… glucose goes up and ketone production is halted. I usually see my ketones drop slightly too, but nowhere near the same as low intensity.
These are my results after a couple of years … and training during extended fasting. The rate of glucose or ketone production and the sources of those energy metabolites will change over time. You and your metabolism are a function of what you’ve demanded of your body and how you’ve trained it with your behavior.
The best way to think of it is this - eat and act in the way that you want your body to prepare. If you eat and act like a strongman weightlifter- it will work to optimize that. If you eat and train like a marathon runner, it will give you that too… their bodies are NOT the same… even if both are keto.