Large spikes after workout

I am newly diagnosed with T2. I use Metformin but no other meds or insulin. I have been using a Dexcom G6 for several months.

I work out 8-9x per week (4 “Zone 2” Cardio sessions in mornings, 4-5 lifting sessions with a Zone 5 warmup in afternoons). Today is Day 2 of a 72-hour water-only fast.

I notice that during a workout, my BG descends slowly (eg. this morning from a 129 to a 103). Then, about 5-10 minutes after I stop, I get a significant spike (today to 158) before slowly coming back down.

Should I be alarmed by this? Is this just another sign of being unregulated?

Thanks in advance!


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Welcome to TuD, Steve! I don’t have an answer to your question but want you to know that I am impressed that you are using a CGM. It’s an incredible tool to help you understand how your glucose metabolism works. Was it your idea to get the CGM or did you have a doctor who encouraged you to use one?

Do you use a fingerstick meter, too? Will you share what your glucose level is when you get up in the morning? CGM and/or fingerstick reading.

While spiking to 158 is less than optimal, it is not a very high number. It’s possible that your liver is releasing some glycogen (glucose) and causing this spike. What time of day did you workout?

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Thanks for responding. The CGM is part of my strategy and I requested the prescription from my doctor. I learned about CGMs (and especially Dexcom) from a podcast that I subscribe to Peter is a physician and an expert on aging who believes that insulin resistance is at the root of a large number of health problems today. He prescribes CGMs to all of his patients. Having the CGM has allowed me to significantly adjust my behavior to keep my BG down.

I do have a fingerstick meter as well, but have to admit that I haven’t used it much lately (I think I’ll do some testing).

I appreciate your comment on the 158 reading. I set my CGM to alarm whenever BG goes above 140 so it kind of freaks me out when I get an alarm.

I generally do my “Zone 2” cardio first thing in the morning (today I used the treadmill in the basement) and generally lift at about 4pm in the afternoon. For lifting workouts, I warm up with a hard 12 min run on the treadmill before doing supersets. The pattern of gently decreasing during the workout and then spiking afterward seems to affect both.

I’ve been following Peter Attia’s work for the last few years. I’m in agreement with most of his recommendations but confess that I don’t fully understand all the scientific and biochemical mechanisms that he brings into his arguments.

But I do believe, given the poor metabolic health in our culture, that it’s entirely fitting that Attia encourages all people to wear a CGM, even if it’s just intermittent. You were smart to follow through getting a CGM. It will teach you things about your glucose metabolism that you could not learn any other way. These are things your doctor just could not know, much less use to guide your treatment.

Be aware that some diabetics consider his glucose control ideals as extreme. I concede that his ideas are extreme when compared to the poor metabolic health of modern society.

There are members here who are much more adept than me about aggressive exercise and blood glucose. @Eric2 might be able to help with your question. Understand that the glucose dynamics of T2D are different than T1D. There is a great deal of overlap but some significant differences remain.

As a T1D, I don’t seem adversely affected by large and inappropriate glycogen releases from the liver. I’ve read many accounts from T2Ds who report this.

Your ongoing awareness of your glucose numbers combined with your motivation to exercise bodes well for your success in regaining your health.

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Thanks Terry and hi Steve!

Just to make sure we are using the same terminology, is your Zone 2 the easy aerobic zone?

How much have you been exercising in the previous days?

If your body does not have muscle glycogen and fuel from food you are eating, your body will respond in whatever way it can to fuel your workout. In this case the spike you see is most likely from your liver releasing stored glucose (glycogen).
(Exercising during a fast means that most likely the only fuel source you have is whatever is left in your liver. )

As you become more depleted, your body has to work even harder. This results in more hormones contributing to a BG spike.

Another thing to add - Zone 2 is pretty easy and would not have much impact. But after fasting, Zone 2 is actually harder, and can become more like Zone 3 or even 4 if you keep becoming depleted.

@Terry4 Once again… I’m grateful for the response.

I couldn’t agree more with your point about the CGM. I follow a keto diet for the most part, but in just 2 months wearing it, I have been able to identify foods that spike BG (eg. 0-cal soft drinks, my brand of whey protein (!), certain condiments, etc.) and start to get a feel for the different impacts. I’ve been able to figure out a rough threshold for certain types of fiberous carbs before BG goes up (I avoid nearly all simple carbs). I’ve been able to get my meal plans to the point that, with the MetFormin prescribed by my doctor, I experience little or no BG bump from meals.

Right now, I’m trying to figure out how to address the “Dawn effect” spike upon waking as well as the spike after working out.

Ultimately, my strategy for “reversing” the insulin resistance is based around fasting. I have adopted the “Alternate Day Fasting” approach recommended by Dr. Jason Fung (3x 36 hour fasts per week). Hopefully, persistence will rule out.

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@Eric2 Thanks for the extremely knowledgeable reply.

Yes, Zone 2 is ‘easy aerobic’ as quantified by my heart rate monitor. For me, this means setting up a treadmill with an 8-degree slope and adjusting speed to make sure I am inside my estimated Zone 2 heart rate. I find that I am making progress – higher and higher speeds are required to keep me in that zone.

My exercise schedule is pretty consistent. I did not work out at all yesterday, but a day without at least one workout is generally rare. The effect appears consistently though.

I would assume that my glycogen is generally fairly low as I follow a keto lifestyle for the most part. While I don’t fully understand fuel partitioning, it seems like a possibility that the liver is releasing glycogen and it takes a little while after exercise stops for the liver to get the message. I thought that Zone 2 works by releasing and burning fat from storage (above the Zone 2 threshold the body starts burning glucose outside of the mitochondria, right?). I guess that it makes sense that the liver or something else would compensate for the exercise and take a few minutes to get the message that the exercise has stopped.

What I am worried about is what this indicates about my level of insulin resistance. Seems like it is pretty high or else the spikes would be awfully short lived, right?

Makes complete sense about the increased effort required to maintain Zone 2 when fasting. I hadn’t thought about that. I will do some research on adjusting HR levels to compensate.

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Your body is always using multiple fuel sources. It doesn’t switch from only using one or the other. The difference is in the rates of each, the amounts of each that you are using.

Yes, generally at the lower intensities your body will primarily use fats for fuel. But there always needs to be a background of carb metabolism for this to happen efficiently.

Here is why.

For fat metabolism to occur efficiently, we need oxaloacetate. We get oxaloacetate mainly from pyruvate, and we get pyruvate mainly from glycolysis.

Glycolysis primarily takes place from glucose that is picked up from the blood (from recent food eaten or from glucose the liver has released).

Without eating and with low stores of glycogen in the liver, glycolysis is greatly diminished. And that in turn will make it harder for our body to make oxaloacetate, which makes fat metabolism less efficient, because without sufficient oxaloacetate, it becomes harder to form citrate, and our ability to form ATP is slowed down.

The body hates being inefficient, so to counter this, when you do not eat, your body will release glucose from the liver.


@Eric2 Thanks for the explanation. This seems consistent with what I’m seeing.

I really appreciate the technical expertise here.

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@stevebro I may have missed it, but are you experiencing the BG spike during and/or after the strength workout? All stress causes a release of glucose from the liver, that includes the physical stress of lifting. When we lift, we stress and relax. Heart rate can rise and fall.

When I am on the bicycle doing a steady pace BG tends to fall over time. I can cause it to rise by doing a series of sprint intervals.

My go to exercise is road cycling. It is what I have loved since childhood. A few years ago I started doing resistance workouts twice a week because as we age muscle just disappears. When I do strength workouts I can see my BG rise. When I bicycle I see it drop.


@Luis3 No… BG slowly descends during the workout. Then, there’s a brief 5-7 minute pause and it spikes.

I’m trying to self manage. That one and the Dawn effect seem to happen no matter what I do.

@stevebro That makes sense. Short intensive exercise will cause an overall increase in blood glucose levels over time. Aerobic activities will decrease blood glucose over time. Activities that are a mixture of both tend to have no overall immediate effect on BG.

As to Dawn Phenomenon that is a problem many of us face along with feet on the floor. As it is caused by a release of glucose from the liver combined with insulin resistance in type 2 DM, it would seem that increasing insulin sensitivity is the ticket.

In my personal and anecdotal experience the best way to increase insulin sensitivity is long aerobic exercises such as running, brisk walking, hiking, swimming, rowing, skiing, skating or cycling for at least an hour, not miles but time.

I find a positive but limited effect on resistance training. I do it to prevent muscle loss and to actually increase strength in my 71 year old body. I limit it to 2 days per week. Usually do some form of aerobic exercise that same day.

Good luck, Steve, we all have to work it out for ourselves. Each of us responds a bit different to foods, activities and stress.

@Luis3 Thanks for this! I currently do 3.5-4 hours of Zone 2 cardio in 4 sessions over the week. The weights workouts are focused on keeping muscle despite my weight loss.

My current focus is stabilization – finding the steady state that I can live with without thinking about this stuff all the time.

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@stevebro I’m doing the same experience as you. I’m not using meds at all. While I’m using also a CGM, I’ve tried some “experiments” like doing some running in the morning without having breakfast to burn more fats.

My running routine is 45 mins to 1 hour and about 6/10 KM per day. When I run the BG level is about 110/120. The moment I stop running it spikes up to 180. Today I’ve tried to make the last running part decreasing in intensity for a longer time like 50 minutes run, 10 minutes very fast walk and 10/20 minutes walk and I’ve experienced a peak but not so extreme with peak at 145. It’s like slowing down in intensity makes your balancing easier while your muscles still uses glucose energy and your liver reduce glycogen production smoothly. I’m still experimenting but smooth workout end intensity, in my experience, could help.

My impression is also that eating just after avoid the normal peak that usually happens. I’ve tried eating more carbs in form of fruit just after the exercise BG peak and the BG goes just down like I’ve never eaten. I assume that this is due to the fact that I’ve reached high level of BG running and so the amount of insuline is already high so my breakfast is processed in a better way.

To be clear, it’s just my intuition. I’m not a specialist but I’m happy to share my experience with someone who is trying the same remission-like path.

Good luck!

I have struggled with this as part of a body building program as I’ve been lifting for months. I’ve tried everything with mixed results. I decided to move my weight lifting to the late afternoon to avoid the dawn issue with high intensity exercise. I leave steady state exercises like AB work and cycling for the am. I’m a morning person but the dawn issues are hard to manage and not very predictable even with a CGM. I use to try an extra bonus but I never really felt comfortable with it depending on the intensity. Sometimes in the mornings I will only eat very little because of my response to insulin and the fact dawn phenomenon is complicating management. I use a Cgm and pump.

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I use tandem pump with cgm and it eliminates my dawn phenomenon.
Lifting weights is difficult because it releases lactic acid which drives up blood sugar. It also causes some trauma to your muscles which causes inflammation and insulin resistance.

Not to say it’s not doable, but maybe if you try to map it out you can find a predictable pattern that you can dose for.

Yes. On a pump and Cgm. For me I avoid the morning heavy weight routine which I have moved to the late afternoon. More of a predictable response especially less risky if you are really exhausting v02 to the max. Just depends on how intense, duration etc. my newest challenge is as I become more muscular, and less body I’m finding less absorption.

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Hi, I have found that my body has changed over the years. I have always been a runner and found in the beginning I had to eat gels etc for anything over a 10 km run. I have now been able to run 21km without eating as I seem to release enough carbs for the running. I do have sort of carbo load before the long runs, about 2 days. I find the faster the pace the more the carbs are released. Once I stop running the body seems to take a while to stop releasing the carbs and I have to make a correction. Running slower than marathon pace seems to use all the cabs and there is no release and i have to watch the sugar levels carefully otherwise it drops very rapidly. Once I stop after a slow run I also have a spike but not anything like when the pace is faster. It is for me is all trial and error game and never two days will be exactly the same, regardless of eating the same foods and water intake been the same. I still run with a vest to have all my meds and gels as you just never know when you will need them. Gym does not effect me as much. Swimming has the same effect as running except once it starts to drop it is very rapid and I have to be very careful. I am now 60 and believe the training has been a huge help for me even if it gets to me once in while. You also get strange looks on short runs when you run with a pack I just smile and enjoy the run.


My body does the same. I’m T2, no meds yet. I have come to feel that as long as my number comes on down as I settle myself after a workout, then it’s the best I can do. I gotta have the exercise, after all—LOL…So do keep an eye on it, testing regularly—Test-Test-Test is my mantra!..Take care!