Huge difference in insulin needs when exercising

Hi all! So I’ve noticed throughout the years that the days or periods of time where I am exercising more I will need literally 1/2 the amount of insulin to cover my food/carbs that I would need based on my “ratio” that is recommended by my doctor. This sounds like a happy problem until you factor in how hard it makes it to predict the correct insulin dosage as well as having to deal with more frequent/drastic lows. Has anyone else encountered this issue, any tips to share?

I also have this issue. Hoping others can chime in since I have no real tips. One thing I’ve observed myself is that when I’m unconditioned to an activity this problem is really bad but over a few weeks of regular exercise, the effect is milder. Eventually the crazy lows turn into crazy highs. :confused: Then I react and bring my TDD back up a bit. Finding that balance and timing… ugh

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Yes, it is trial and error. It is important to note which insulins(meal insulin or basal) are active when u exercise. I do not exercise around meals or late at night. I modify my basal rate.
via pump now and it has been trial and error.

This is where the cgm is really a nice tool to monitor your blood sugar.

If you do not have a cgm then test before exercise then every 15-20 until you get a pattern of how your sugar reacts to different levels of intensity and types of exercising(cardio vs lifting weights.) I sometimes get upward spike while weight lifting but eventually it goes down. Keep your testing things near you. Yes, it can be a drag to hop off treadmill while you are in the zone, but better that getting tow low. I keep hammer energy gel near me as well. Works better than the tabs for me-
Also your insulin sensitivity changes over time. exercise 5 days a week better sensitivity skip a week or two may lose insulin sensitivity and may have to build it up again. But everyone is different

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Thanks so much for the advice! I use a cgm and usually when I exercise it’ll drop a fair amount during and then go back up afterwards. I think I more so worry about the few hours/night after having exercised…where my sensitivity just seems to get higher and higher. But yes, knowing the patterns does prove helpful!

I play golf about twice a week. On the days I play, I reduce the amount of insulin I normally bolus for breakfast. Even so, I often still go low halfway through my game. I always bring granola bars and Gatorade to bring my BG back into normal range.

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Yes, When I exercise regularly I need 1 unit of insulin for 6 grams of carbs but when I stop exercising then require 3.5 grams of carbs per unit of humalog. Additionally need to pre-bolus 12-15 minutes in advance of eating during 1:6 ratio but that changes to 22-25 minutes during 1:3.5 ratio. Body is also much more sensitive to carbohydrates during non-exercise periods and can’t get sloppy on ratio or pre-bolus time or else quickly go out of control.

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Yes. This is a thing. For me, personally, my insulin requirements can more than double or I may need to cut them in half. Or, it can alter between the two extremes during the course of a day. My only advice is to have a really well adjusted basal and do your best to familiarize yourself with how the system behaves while exercising (and in the 24 hours after). If you can predict, then you are in pretty good shape. Often, I find that I cannot. Sometimes I exercise with a buddy if I’m worried about it.

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It doesn’t happen when I start exercising, it cuts in maybe 24 hours later. My current model of the process is that I break down lots of glycogen initially to get meet the immediate energy requirement then my body has to build it back up and, as it does so, it actually needs less insulin for a given amount of glucose. (Insulin is the hormone which promotes conversion of glucose to glycogen, glucagon promotes the reverse.)

It’s really difficult, but it only happens to me with serious amounts of exercise. A day skiing, no problem, my normal exercise routine (1/2 mile walk with a couple of hundred feet of ascent every day) no problem. Load 10 90lb bags of cement into the mixer and distribute the result, use a falling saw for a day, do that day trip from the floor of Yosemite valley to the top of Half Dome and back - big problems.

The break point is somewhere around the point where my muscles are in danger of cramping. Could this be it? Am I hitting the point some point in anaerobic exercise where my body switches to a different mode of processing glucose?

Anyway, pretty much every T1D I’ve heard talk about this has the same story - exercise increases insulin “sensitivity”.

John Bowler

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I think your reasoning is completely accurate as I notice the exact same things happen with me. I can walk on the treadmill and it makes no huge difference, but when I am really working my muscles to the point where they are sore afterwards are the days that I notice my insulin sensitivity goes waaaay up in a few hours thru the day after. I tend to think it has something to do with the amount of glucose needed in muscle recovery afterwards?

A couple things for this. Your body uses a combination of various fuel sources for any activity. These fuel sources are used in conjunction with each other, but your body uses what is easiest and best for the activity.

When you are walking, you are metabolizing more fat to fuel the activity than when you are exercising harder. More fat metabolism is used for less intense activities, because it provides for a longer duration of activity.

As you increase the intensity, your body uses a higher percentage of muscle glycogen rather than fat. Muscle glycogen is more limited in duration than fat, but it provides for a higher level of activity.

In addition to both muscle glycogen and fat metabolism, your body will use glucose from the blood to provide instant energy. This process is called glycolysis.

When you exercise more intensely, your body has used a higher amount of muscle glycogen, so your body will have to replenish the muscle glycogen. And the easiest way for your body to do that is from the carbs you eat afterwards. You gotta refill the tank when you are done.


That really does make so much sense and gives me a peace of mind knowing what to expect. I guess I didn’t think about how carbs after exercise are just as necessary as the carbs before, thanks for that!

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My DS cycles competitively and trains and competes at a very high level. The above post on what type of energy you are using is superb. Additionally after periods of intense exercise your body replenishes muscle and liver glycogen in 2 waves. Once directly after exercise and again 8-10 hours later. So if my son did an intense evening training ride, we would make sure he had a high carb and protein recovery shake with a low amount of insulin but he would still go super low at 4 am. That was the second wave of glycogen replacement. The fix for him is to take in glucose (a gel) every 45 minutes of exercise no matter what his bgl. It might send him a little high for 5 minutes but within 20 he is back where he was. This way he has more circulating available glucose and doesn’t go so deep into his glycogen stores. Understanding what source if energy you are using when you exercise, and how that differs with different types and duration is key to managing bgl’s up to 48 hours after.

Yes also, insulin sensitivity increases every consecutive day he exercises and starts to decrease after only 24 hours of no exercise and continues to decrease every consecutive day he doesn’t exercise to levels where we can’t seem to give him enough insulin to be in his target range.

Exercise can be difficult to manage with T1 but it is doable and do well worth it.


Try taking on carbs/sugar earlier than you think you need to and before your bgl starts to go low. You will keep higher circulating glucose available for exercise do you don’t have to dip into your muscle/liver stores so soon. Once those run out, it is very difficult to continue training.

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You can actually find out exactly how much fat versus muscle glycogen you are using at your current level of fitness, using indirect calorimetry, and oxygen and carbon dioxide analyzers.

For cycling, you would also use a cycle ergometer and go through various power outputs. For running, you would use a treadmill, with varying speeds or inclines.

If you know your RER (respiratory exchange ratio) at various speeds/power outputs, you know how much you are using of both fats and carbs

For example, at a RER of 0.85, you are at about 50% carbs and 50% fat. A RER of 0.70 is 100% fat. And a RER of 1.00 is 100% carbs.

You can get testing like this done for a few hundred dollars if you are interested.

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You sound like you might have a similar career path to me! I’m the Head Coach of a Swim Team, qualified at a pretty high level, also personal trainer, wellness coach etc. I train a number of triathletes in swimming and have competed in triathlons, half marathons, marathons etc. myself but my first love is swimming. My 16 year old daughter (non D) swims competitively and my 13 year old DS bicycle races. He’s apparently pretty good although it’s not my area so…humble Mum here.

I am hugely interested in the interaction of T1D and high performance training. Not only for my son but I also have a T1 adult swimmer I coach. I am a total sports science nerd!

We’d love to get Thomas (DS) a power meter to better pinpoint things but funds do not permit. Additionally the kind of exercise testing you refer to isn’t available where we live although in a few years he will be sent to the US by the National Team for that sort of testing. Currently I manage things with a massive spreadsheet comparing a number of training metrics (including heart rate, avg and max) and compare it with his Dexcom readings to try and predict training patterns. Of course then he has a growth spurt and it all goes to heck!


Currently not on a pump although we are traveling in a couple of weeks to the US to start the process as we can’t get one where we are. His endo and I both feel we will be able to manage his intense training better on a pump with changeable basal rates vs mdi and the very difficult 24 hour basal insulin.

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Yeah, I guess it is not everywhere yet. Many of the big cities have it, if you can find one that is somewhat close.

I hardly ever use my Dexcom on runs anymore. It was always about 3 miles behind. :grinning:

I made some stuff I use that let’s me do actual BG tests while running. I can PM you some pictures. It’s very helpful to know what pace causes the cortisol response and gives me a spike.


Also, Levemir would be a shorter acting basal and would let you have variable basal rates. But it would require a few shots a day instead of just one. Probably wouldn’t be a big deal for him to do several basal shots. I’d recommend that in the interim, if it will be a while before you can get a pump.

I found that when I go for a bike ride my blood sugar will rapidly drop, but then stay at a consistently low level throughout ride. However, I also discovered that the readings my Dexcom gives me while I bike aren’t always accurate. On a ride about a year ago I decided to stop every 30 minutes to take a finger stick reading to better understand what was going on. I found that the readings were very different from what my Dexcom was telling me (the Dexcom was reading lower). I never quite figured out what was going on, but suspect it has something to do with how much I was sweating during the ride and the fact that the Dexcom is actually measuring some property of your interstitial fluid, which may be impacted by sweating.

I guess my point here is if you’re CGM is telling you that you’re going low while exercising, it’s a good idea to stop and confirm that with a finger stick.

It is timing issue. The cgm is behind the actual meter. So if bg is going down say 70 the meter bg may say 60.
if bg is going up cgm says 150 the meter may say 180

At critical points also check meter for most accurate real time value,