Continuous Glucose Monitor (CGM) and Training

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  • #54088


    I recently completed a two week trial using a continuous glucose monitor, or CGM. These devices stick to your arm and take blood glucose measurements every 15 minutes around the clock. They have been around for a little while for diabetics, but have recently transitioned to the non-diabetic population as a tool to better understand personal metabolism and how you body reacts to certain foods. For example, I personally handle regular potatoes with a very small increase in blood glucose, while many people experience significant jumps after consuming them. I also learned that if I consume any significant amount of carbs within several hours of bed, my sugar remains high through the night.

    My question is related to training though. I noticed that during intense workouts, my blood sugar increased significantly, probably from the liver releasing it’s stored glycogen. This is expected. On my long workouts below aerobic threshold, though, there was no spike. Apparently the liver did not see it necessary to release it’s stored glucose. Interestingly, this number seemed to correlate very closely with my aerobic threshold heart rate. What is the connection between this and fat adaptation? Is it safe to assume that if there was no blood sugar spike during a workout, then the body was primarily using fat for fuel? Or consumed carbs? Is there any way to tell?

    Admittedly, two weeks wasn’t enough time to do a lot of experimenting, but it’s just too expensive to continue to do it long term right now. I do plan on making changes to my diet and then doing another two week trial several months down the road.

    I’d just like to start a conversation about these devices and see if there is a place for them in the endurance training community. I think there might be, but I’ll leave it up to the people that are much smarter than me to decide that.

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    Anonymous on #54098


    Very interesting questions. Good for you for taking the plunge and trying out a CGM. One of the runners I coach, Luke Nelson was part of a study group testing CGMs on endurance athletes. I can ask Luke to chime in with what he learned.

    I think some of you suppositions are very likely to be correct.

    Spikes in glucose doing intensity: My wife who is a Type 1 diabetic sees big spike in blood sugar doing intensity. That can have significant negative health consequences for Type ones so I try to discourage that type of activity.

    Low glucose levels probably indicates that fat is the dominant fuel source.

    It makes intuitive sense to me that this would correlate well with the aerobic threshold. Once above ones AeT you start recruiting more and more FT fibers that require more glucose for fuel.

    We just an endocrinologist that’s an ultra runner to come in here and set the record straight.


    dan.k on #54102

    Hi David,

    I have worn a Dexcom G5 and now G6 for a number of years purely for the reasons you suggest (health monitoring, reaction to foods etc.).

    As you say it is pretty interesting looking at how your body reacts and how that changes.

    Some of the things I have noticed:
    1. Sleep impacts how my body reacts to identical foods. I am far less tolerant of glucose if sleep deprived. I think this has also been shown in studies.

    2. If I eat relatively low carb for not much more than a day (which I do most of the time) I become much less tolerant of carbs if I add them back in. This appears to be a from of physiological insulin resistance and I think it is resistance in the muscles because they have been preserving glucose for the brain and relying more on fat. When I first did a 7 day water-only fast my blood glucose was around 55-60mg/dl (flatline) at the end and I ate a small amount of carbs via some sourdough toast (along with fat and protein) and my blood sugar went through the roof (250+mg/dl). I went out of my office and ran some sprints on a set of steps which helped :). I have seen this a number of times. If I have been quite low carb my muscles don’t want to take up the glucose but doing some very intense exercise (running up steps repeatedly) helps. This has also been shown in studies even over short periods of mild carb restriction but it seems to be very dependent on the person and doesn’t seem to happen to everyone.

    3. I also see the same as you. Long endurance runs I will not see any spikes although, depending on my glucose state before, I may see a slow trend lower over a number of hours but nothing huge. If you are so carb depleted that you are generating a lot of ketones whilst out running then I find that also surpresses glucose. Very intense efforts does also show up for me as a glucose spike but, as you say, that is expected from your liver dumping out glucose for more glycolytic workouts.

    4. I also find that what you eat when doing exercise if far less important than what you eat when you are just round the house at home. I don’t think you see the same large insulin spikes during exercise and I definitely don’t see the huge spikes in blood glucose even if I was to take something very high glycemic. I personally use UCAN a lot but also now use other more traditional gels when things are more intense for easier recovery and if I take them whilst exercising I don’t see a huge spike like you might expect. I wore a CGM whilst doing the Tor des Geants a few years ago and, as you can imagine, blood glucose levels stayed pretty low throughout regardless of what I was shovelling into my mouth :).

    I would agree that if your glucose level is stable it would probably suggest your liver is not dumping out glucose like you see when you are doing very intense efforts but just looking at the glucose number doesn’t really tell you anything about the flux (i.e. the amount the liver is putting into blood or the amount being removed). Although I do find that having the CGM overlay to a metabolic test is quite interesting. As you say, I think it likely makes sense that you see the change around your AeT but not sure how much you can definitively read into the single number through time if you didn’t already know what your AeT was (i.e. if the only number you have is your CGM number and you know nothing else whilst training I am not sure how much you can really assume from it). I definitely see variation in response depending on sleep, what I have eaten, when I ate it etc. as well as just the intensity.

    I think the big thing for me using a CGM around training was seeing how it was possible to eat much more carbs around exercise without spiking blood sugar (and therefore insulin). This has been helpful for me to more proactively replenish glycogen stores before, during and after more intense workouts whilst knowing that I haven’t triggered big moves in glucose. Taking extra carbs around exercise like this also helps me keep my muscles more glucose sensitive (at least that is what I find). For a long time I think I was running around with very glycogen depleted heavy legs because I wasn’t good enough at proactively getting extra carbs onboard before/during/after intense workouts.

    Obviously the N on these obervations is 1 although I have seen commentary from others like Peter Attia and studies showing similar observations in others.

    Sorry if this is a bit of a random collection of thoughts. Definitely interested to hear others experience.



    Chet on #54120

    to slightly amplify….Peter Attia, who Scott mentions above, is a physician who personally wears a CGM full time for the last few years….you can find stuff on his website …he has been a long distance swimmer and bicyclist but is most focused on longer quality of life issues personally and professionally….I think he is a pretty bright guy….double major in mechanical engineering and applied mathematics with MD from Stanford…he covers a lot of territory in his work/website though not specifically focused on endurance sports….he also very interested in various fasting/eating regimens…..anyway just some FYI to check out ….Chet

    Chet on #54121

    oops Dan above referenced Peter Attia, not Scott

    russes011 on #54147

    Thank you for these interesting posts–exploring the utility of CGM seems fascinating. I will check out the Peter Attia website.

    I would like to point out that the most likely driver of a glucose spike during higher intensity work may be mostly due to one’s natural uptick in both adrenaline and cortisol during these moments–both of these hormones increase blood glucose. My point is that it may be more hormonal than a correlation to fat adaption per se, but the body is so complex both are probably interrelated, and perhaps the hormones are just intermediaries between the inevitable ‘failure’ of fat adaption and it’s resultant blood glucose spike, that may occur with quite intense workloads. Who knows, a blood sugar spike during exercise may become a surrogate for AeT someday.

    Also of note, I believe although higher intensity work spikes blood glucose during the event, on average it lowers ones adrenaline and cortisol (and therefore blood glucose) for the rest of the day, on average–making exercise one of the best ways to control glucose in people with type II diabetes–it can often reverse it. Also, poor sleep increases cortisol, which would increase blood sugar and perhaps explain how sleep (and stress) may make food related glucose spikes variable.

    I also love David’s observation that fasting after dinner leads to much lower blood glucose overnight, and therefore likely insulin levels, and therefore likely more weight loss (or no weight gain). As mentioned in TFTNA no snacks after dinner is quite a potent way to stay fit from a dietary perspective.

    Once again, thanks for the interesting posts–looking forward to others posting with their experiences.

    — Steve

    Anonymous on #54194

    Interesting! Thanks, @davidkotula for bringing this up. And @dan-kefford for your detailed notes.

    : You mentioned that it’s too expensive to do continuously? How so? Does it require some type of subscription for the analysis?

    Reed on #54552

    This is great – seconding Scott’s thank you for the discussion and notes. Continuous blood glucose monitoring is awesome for managing diabetes. A family member has managed Type 1 diabetes since childhood and Dexcom’s CGM has been really helpful these past few years. I’m interested in using it myself, out of curiousity. But in addition to the system being a bit difficult to obtain without a prescription, the sensors only last a week or two. The costs are on the order of $300/mo., I believe.

    todd.struble on #54699

    This podcast from Dirk Friel at Training peaks just came out on this topic as well (which led me to search these forums!). The guest is an endurance athlete who is type-1 diabetic so has been deeply involved in glucose monitoring and performance. It looks he’s trying to bring Continuous Glucose Monitoring to the fitness industry. Their product looks like it’s only available in the EU right now though.

    Would love to hear what Luke Nelson’s experience was too.

    pkelly03 on #54713

    Hi all,

    First time post. I was really interested to use this also. However, after about 2 weeks of use, I picked up a blood infection. Cellulitis, required 10 days of strong antibiotics. You always run this risk with exposed skin / piercings.

    I do triathlons and continued training as normal. Perhaps swimming with the attachment was the final straw. It didn’t advise against though.

    I ditched it after that. Did enjoy using it. I would attach to my belly area in future. I had it on my upper arm when I picked up the infection.

    Curious Dan, did you clean in and around the attachment daily? I didn’t.

    I wouldn’t have thought you need to use it year in year out. Presumably after a month or two of use you would have a good idea how your body responds to different foods/sleep patterns and training blocks.

    Cheers all,


    dan.k on #54834

    Hi Paul,

    Interesting, I occasionally had some skin irritation from the sensor patches but I never had any issue with infection etc at the site. I always used on the belly because, until recently, G6 wasn’t certified for use on back of the arm so I just stuck with the standard placement.

    About a year or so ago I suddenly started getting a bad skin reaction. I couldn’t understand what was happening and then I got an email from Dexcom saying that they had switched to a different glue and it was causing some issues for some people. I think previously some people complained that the sensor would peel off so I think the new glue was stronger. Again, it was just skin irritation though and it would pass within a day or 2 of removing the patch.

    I never did any sort of cleaning of the area once it was inserted but I would always clean the skin with alcohol wipes before inserting a new sensor but that is just the standard recommended procedures.

    I also showered, sauna’d, trained, bathed with it in without issue. There are definitely not any restrictions on swimming etc. The only thing you will see is that it cant communicate with your phone while your swimming and so it will catch up all the data once you get out of the pool and are close to your phone again.

    In relation to the comments above about prescriptions I do think it is harder to get in US than in some European countries (e.g. in UK you can just call Dexcom and order them directly with no prescription required).

    To a certain degree I think Paul is correct that you can wear for a period and then stop after getting a general idea of how your body reacts. However I quite liked wearing it as I found it really interesting its also a great behavior modifier…and it was not that unusual to see a reaction that I wasn’t expecting based on past experience which would then cause me to learn something new about my body.

    It is pretty expensive and I am not convinced it is a huge benefit specifically for endurance training. I don’t think you would neccessarily have actionable things you would do differently in your training except perhaps around fueling/refueling, which was were I found it helpful.


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