Why does BG tend to drop during and after certain types of exercise? Several reasons.
From an issue of insulin absorption, exercise increases the rate of absorption. Muscles warm up, the blood pumps faster, and muscle contraction can increase the absorption rate from pump infusions or injected insulin. That one is pretty simple.
Activity increases the glucose uptake of muscles. Your body has several fuel sources it uses during activity - the ATP–CP system, liver glycogen, muscle glycogen, fat metabolism, and blood glucose. Your body uses any combination of fuel source depending on the availability and activity (it doesn’t just stick to one). Your body pulls glucose out of your blood and the glucose undergoes the process of glycolysis. Glycolysis takes place in the cytosol of cells, and is the process of breaking down glucose to form pyruvate. Pyruvate is then used to provide energy in one of two possible ways. In the non-diabetic, the blood glucose used in this process is replaced by the liver. In the diabetic, it is not replaced automatically, it needs to be done from carb intake.
After exercise, your body needs to replace the stored carbs the body used. When exercise has tapped into the fuel tank of stored carbs in the muscles - called muscle glycogen - it needs to be replaced. The replenishment of muscle glycogen occurs at a rate that correlates to the level of depletion. The more depleted your muscles have become after exercise, the faster the rate it is replenished. The muscles get replenished through a process of glycogenesis. It is the process of forming muscle glycogen from glucose. The important takeaway is that after exercise has reduced the fuel tank, that tank will be refilled. This process involves the body taking glucose out of the blood and this process will lower BG.
So far, pretty simple…
- Exercise can cause glucose uptake independent of insulin. Exercise of sufficient intensity promotes the uptake of glucose by muscles without insulin. The body is clever - exercising muscles need more fuel, so there needs to be a way to provide more fuel to them. This can happen without insulin, but only at sufficient intensity. This is where it kind of gets tricky, but basically there is one important note at the bottom that is the one thing worth hanging on to.
Not to get too deep in the weeds with it, but exercise increases uptake of glucose through what is called a “contraction mediated pathway”.
Adenylate kinase (ADK) converts 2 adenosine diphosphate (ADP) molecules into 1 adenosine triphosphate (ATP) molecule and 1 adenosine monophosphate (AMP) molecule. The ATP is hydrolyzed for energy, and the AMP attracts 5’-adenosine monophosphate-activated protein kinase (AMPK). AMPK acts in a way similar to how protein kinase B (Akt) recruits TBC1D1 (detailed in this post - Biochemistry - what does insulin actually do?!), which releases GLUT4 to the cell surface and allows glucose uptake. Additionally, calcium ions released from the sarcoplasmic reticulum during muscle contraction also promotes GLUT4. Other pathways include ROS signaling, and nitric oxide signaling.
Bottom line, (the important part) there are redundant and overlapping pathways for glucose uptake that occur during muscle contraction (exercise) that do not rely on insulin!
Exercise will increase the rate of insulin absorption
Exercise will increase the rate at which glucose is used
Exercise increases the amount of glucose your muscles need
Exercise can allow glucose uptake to happen without insulin
Post-exercise, the muscles will need to replace the muscle glycogen that was used. This can result in lower blood sugar as the muscles pull glucose from the blood and use it to replenish - a process called glycogenesis.
A lot of people have posted about issues with their BG during and after exercise. I wanted to expand on this a bit, not from a perspective of fixing the issue but just an explanation for some of the reasons involved. Fixing it depends on each individual’s circumstances - their fitness level, the intensity and duration of their exercise, timing of the exercise and meals, their carb intake, and many other things.
This thread was just meant as an explanation for “why”, not necessarily a way of fixing it for anyone, because the “fixes” are very individualized.
If this is useful, the next thing to discuss would be how exercise increases insulin sensitivity, even when you are not exercising.