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!
Summary:
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.
Awesome! Very much looking forward to that topic! btw, thanks a lot for all your recent biochemistry posts, which are very insightful and relatively easy to follow even for someone with no background in the area.
@Eric
Thanks for the info.
A couple questions if you don’t mind:
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.
Why would this not happen due to diabetes? I miss the link here.
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.
How would this work? In terms of the muscles being able to use glucose without insulin? I thought (based on earlier comments from yourself) that there was only a handful of body parts/components that had the ability to use glucose without insulin which I think you said was the brain, eyes, liver and pancreas (but correct me please if I have misrecalled on that). I don’t remember muscles being included so wondering how that happens. Or is this specifically what you describe as the “contraction mediated pathway” which immediately goes over my head. But perhaps the main point here is that it happens while the process is too complex to be easily understood.
And yes - your last sentence would be a natural followup question.
If this is useful, the next thing to discuss would be how exercise increases insulin sensitivity, even when you are not exercising.
Another question relates to high BG before starting exercise. We had been told (I don’t remember when - maybe a long time ago - potentially by our educators and doctors during initial diagnosis) not to start exercise with high BG. Now I know the concept of “high BG” can instantly spin its own heated thread. lol So to bypass that, assume BG of 300+ for the purposes of this question. First, is it accurate medical advice to not initiate exercise at BG of 300+ and Second, if so, why is that?
I was also told this, decades ago. I suspect it no longer applies in the era of rapid-acting insulins and pumps. In the old days of R/N, if you exercised when you were high, you’d get ketones. Nowadays, you can bolus an insulin that’s working within 10 minutes, and you won’t risk ketones. That’s my thinking, but I could be wrong.
For us, it takes about an hour to see the downwards curve on the cgm after a bolus via pump infusion set with Humalog or Novolog. Whether that is relevant or not towards the production or not of ketones, I have no idea. Certainly the two could be very different in terms of timings.
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.
Diabetics do not have alpha cells that automatically fix a low BG by releasing glucagon. With a non-D, the alpha cells release glucagon when BG is low, which signals the liver to release glycogen. That part is broken in a diabetic. Same with exercise.
Same reason you need to eat carbs to correct a low, the liver doesn’t automatically fix it with glycogen the way the non-diabetic does.
The skeletal muscle does not work without insulin in general. So I would not consider them to be included with the tissues that don’t need insulin under normal conditions. It is specifically “exercising muscles” that can do it. And it needs to be of sufficient intensity, i.e., it is not really sustainable (you can’t exercise your way through a large meal and not need insulin). And since muscles still use insulin and GLUT4, it is incorrect to say they are insulin-independent like the other tissues. But during exercise, studies have shown they can take up glucose via the method I described above. So I wanted to include it in this discussion. The “contraction mediated pathway” is not a super easy concept, but just wanted to include it so everything was covered.
(BTW, the tissues that don’t need insulin? Red blood cells, intestine, cornea, brain, kidneys, and liver!)
Your other question I will answer with a different post below because it is rather lengthy…
There are a number of reasons why it is a bad idea to exercise with high blood sugar. Some of them are health related, and the others are just related to problems associated with exercise performance and ability.
First the health reasons.
As you know, keytones can be a byproduct of high BG and can also lead to possible diabetic ketoacidosis (DKA). One of the recommendations for treating keytones is drinking a lot of water to flush the sugar and keytones out of your kidneys. High BG and keytones can cause dehydration. On top of that, higher amounts of sugar in the urine causes more water to be drawn into the urine, resulting in even more dehydration. Put the dehydration that comes with exercise on top of that, and you are making it even worse. This is a bad thing to do!
High BG increases your insulin correction factor. Many (most?) people find that there is usually a much lower correction factor for a BG of 150 than 300 (meaning you need more than twice as much to correct a 300 than a 150). It is non-linear as you get higher. Higher BG means the body becomes more insulin resistant, for numerous reasons. There are many reasons for the higher correction factor that comes with higher BG, such as the signaling that occurs with the protein tyrosine phosphatase 1B - which attenuates insulin action by dephosphorylating the insulin receptor proteins. The short of it - correcting a higher BG means more insulin is needed, and that is not what you want to do when exercising. (See reference here ===> Nonlinear Metabolic Effect of Insulin across the Blood Glucose Range in Patients with Type 1 Diabetes Mellitus - PMC)
Recent studies have shown that higher BG levels change the behavior of blood vessels, making them contract more than normal, and can possibly result in higher blood pressure. Again, not something to play around with. See reference here ===> https://bpspubs.onlinelibrary.wiley.com/doi/abs/10.1111/bph.13399)
Some of the non-health reasons, simply from a perspective of sports performance.
High BG feels crappy. You just don’t have the same energy level and won’t feel as good. A negative feeling associated with exercise is not helpful.
I think #5 is important…
You can’t burn fuel for the exercise. You can’t even perform fat metabolism! In the fuel sources discussed above - obviously your body can’t use blood glucose for energy (no insulin to process it). It can’t use liver glycogen for the same reason. And it can’t even use fat metabolism. (The reason your body can’t use fat metabolism when you have extremely high BG - for fat metabolism to occur the complete oxidation of Acetyl CoA in the Krebs cycle requires a derivative of carbs - oxaloacetate. People often express this by saying, “fat burns in the flame of carbohydrate”. Without carb metabolism, there is no oxaloacetate, which means no acetyl CoA, which means no fat metabolism…) Exercising with high BG is like a big “screw you” to your body. Do all this work but I am not giving you any fuel for it…
Stressing your body with exercise - and causing even more stress by trying to perform it when feeling bad with high BG and without fuel - means your body may respond by dumping stress hormones out. The body says, “You are stressed. Here is some epinephrine, norepinephrine, and cortisol.” And your liver responds to the stress hormones by? You guessed it - dumping out more sugar! Ugh. A bad cycle.
From a performance perspective - you just can’t do the same thing with high BG. Your body will not fire the same way.
Anyway, just a few reasons I can think of. I would always advise people to aggressively correct a high BG before exercising. The actual BG number maybe depends on each person, but definitely not with any keytones in the urine.
I think we have touched on this in other posts and it seems the issue with alpha cells may become more significant over time such that someone newly diagnosed as T1 may not see a significant issue with alpha cells while another person diagnosed for much longer would be more likely to see significantly more of an issue with alpha cells.
I just read the following which give an overview of the alpha cells in this context but still appears that everything involved here (ie - alpha cells as related to T1) is not well understood. http://www.bjd-abcd.com/index.php/bjd/article/view/12
(Although it will take me more reading to fully understand the underlying concepts of your answers in points 2 & 3. ie - I did not follow through to your included links. Yet.)
The main point behind #2 is that when you have a big high, your body becomes more insulin resistant. You need more insulin to correct it - it is a nonlinear scale - which can mean more IOB for the correction. And of course more IOB is a bad thing when exercising.
For #3, it is more of a risk factor for people who have hypertension. These studies are not fully explored yet. But the idea is just that high BG can cause an increase in blood pressure.
I think that it’s ok to exercise for mild highs, BG >140 to bring it down. @Eric would that be correct? #1-#7 would not necessarily be applicable for mild hyperglycemia, would it?
#7, performance - When over 140 - at least for me personally - I can’t do the same as I can when under 100. So my workout isn’t as good.
And #5, can’t burn fuel for the exercise - I don’t know exactly at what point this happens. But at some point your high BG means you are not able to use any glucose, and that really screws everything up! If you read through my #5 above, it explains it a bit.
But performance considerations aside, perhaps your question is geared more toward safety?
Actually, it has been said by some health care professionals to take a walk or do some mild exercise to bring down high BG. So, my question is more related to that situation.
For regularly scheduled exercise routines, I can see that a WNL BG would be better.
Yep, exercise will definitely help bring it down faster, for all the reasons at the very top of the thread. And if that is the intent of the exercise, it can be done with high BG without any problems - as long as the BG isn’t high enough for keytones.
What really helps bring it down fast is an IM shot right in the exercising muscle, like in the calves when walking or bike riding.
I’ve been reading about insulin sensitivity and exercise lately and enjoyed reading what you’ve posted. I’ve been trying to use short pockets of exercise in order to give my insulin a boost. I do a regular 45 minute session daily, but I’ve been using short periods of sometimes just 3-10 minutes to get my insulin going. I like how it works, and now I’m trying to understand why.
When my trainer asked me why I couldn’t exercise daily for about an hour, I did reply that it would be easier for me to find 10 minutes here and there and maybe it would add up to a total of about 45 minutes daily; I can try to find 20 minutes daily, regularly. I do understand that 3 to 10 minutes may not be considered sufficiently aerobic, get the heart rate up…etc. However, something is better than nothing. I also did not want the exercise to become a chore that I “had to do”, but rather something that I would look forward to doing. It’s about motivation. When exercise was something that was presented to me as “Why don’t you exercise to bring down or manage your BG” instead of “Exercise is generally good for you, both physically and mentally, whether you are diabetic or not.” my reactions were quite different. It’s probably similar to those who complain about not being able to eat xyz now that they’ve been diagnosed with diabetes. I don’t miss many of the diabetic unfriendly foods because they are not terribly healthy for us regardless of whether we have diabetes or not.
