What should a blood glucose curve look like during a workout or race?

I’m going to try a GCM for a month and get a better idea how my body is responding to different foods and exercise. It does occur to me that beyond watching for unexpected spikes or drops, I’m not sure what a blood glucose curve should look like for maximum performance during a ride.
Can someone direct me to a good intro to idealized glucose curves?

Without diabetes, it should remain reasonably constant, assuming you’re eating properly. With diabetes, it’s a crap shoot and your goal is 80-120mg/dl

If you don’t have diabetes, there’s probably not a whole lot to learn here. I’ve wanted to try the same thing, but it’s not like it’s really actionable data.

To clarify: I understand ordinary blood glucose numbers and desired pre- and post-prandial ranges. What I’m looking for is help determining ideal numbers during a hard workout or a race. I believe there is a lot of actionable information there given the range of foods/supplements available and the range of individual rates of glucose processing.

You may find you’re not eating enough and your glucose drops a little, but that’s about all I imagine you’re gonna learn. Almost zero chance you’re eating too much in a race. Our bodies are pretty damn efficient at getting sugar to the blood when it needs it and storing it when it’s not. When that doesn’t work, it’s called diabetes haha.

Please prove me wrong, I’m interested in this too.

Surely there is useful information to be gleaned from a CGM during a long race and pre-race.

I am not a doctor but I do have some experience with blood glucose monitoring (see below) and I think there could be two potentially useful ways to use it on and ahead of race day:

  1. How do your pre-race meal(s) and snacks affect your blood sugar levels? Excessive simple CHO (sugars) without enough complex starches etc. might cause an upswing in blood sugar, followed by a bit of a low. You may be able to tune what you eat to avoid the pendulum swinging back towards hypoglycemia after your pre-race meal. (After all some sports gels say “5 minutes before and every 45 min during”–if it helped performance to consume them even earlier it would also certainly help their sales!)

  2. You might (maybe) be able to spot a bonk coming during the race (although you may also be too late once you see it in your blood glucose). If you have some experience during training with fueling and a GCM perhaps you could get a better sense of how your body is affected by fueling while exerting yourself and tailor your nutrition based on watching how you respond to different sports nutrition. (e.g. fructose to glucose ratios etc.)

As for my experience with blood glucose monitoring, I have lost a lot of weight and made huge body composition gains going from a pound or two away from obese by BMI to healthy. I did this using a combination of dietary ketosis, time restricted eating and fasting, calorie/macro tracking and blood glucose and ketone meter. Now, perhaps I was flirting with pre-diabetes, but I absolutely saw changes in my blood glucose that could be correlated to how I felt and what I did and ate. Your body is a pretty complex thing so it’s important to realize that changes you see may be due to unanticipated factors. For example most people see elevated glucose levels even when fasting and in ketosis in the morning right after waking up. Apparently this is due to the dawn effect: a release of cortisol that happens towards the end of your typical sleep window to help you wake up. The cortisol signals your physiology to also raise your blood glucose through gluconeogenesis. Despite some pitfalls like the dawn effect, eventually with enough research and experience you start to see patterns in the way that foods, sleep, training and stress effect your blood sugar. This might let you make tweaks to your diet and habits to ensure your training occurs in a more optimal state and/or you fuel properly for a race.

So do I think a GCM is a required tool to optimize your training and race-day nutrition? Certainly not. But it will provide you with more data, and those data might elucidate some nutrition and nutrition timing strategies or issues that could otherwise take longer to tease out or that you might not be seeing unless you rigorously analyze your nutrition and performance data together.


Well, I can’t tell you what a glucose curve should look like, but I’m wearing a Supersapiens CGM and I can show you what my glucose actually looks like during a workout, or rather several…

The first workout was this past Tuesday, 2.5 hrs, consisting of 1hr 45min Endurance, followed by eight anaerobic capacity intervals ranging from 50 seconds to 40 seconds.

During this ride, I consumed 540 calories (115g carbs) of Skratch rice krispy snacks and sugary drinks. The spikes in BG pretty much correspond to this fuel ingestion. The 70 “glucose score” is not my blood glucose, but rather a “grade” created by Supersapiens to let me know how “good” this session was, from a BG perspective.
More details on how they score here.

Corresponding workout:

Next, yesterday’s workout was 3.5 hrs Endurance. I consumed 780 Skratch calories (165g carbs).

Supersapiens grade: 50, an F!

Corresponding workout:

Lastly, today’s workout was just 75 min of easy Endurance/Recovery. I had one bottle of Skratch sugar water (80 calories).
Supersapiens grade: 37, FAIL

Corresponding workout:


Thank you! That’s the sort of info I was looking for, particularly the method they use to “score” results.
Do you know if the 110-180 range is unique to you, or if that’s a standard value they use?

I would guess it’s just a standard range. Are you trying Supersapiens, or another one?

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I’m going through Signos. Starting next week.

I wear a CGM sometimes to help me better understand how food and exercise impacts my glucose numbers. T2D here. I use a Dexcom G6 and it doesn’t give me any scoring, it just screams at me when numbers drop low.

I race crits in season and don’t consume anything other than water or water with LMNT during those races. I do the same when doing shorter, harder workouts. Here is an example of a recent workout where I had 1.5hr of SST - 2x30@88%. Tick marks are an hour. If you see this, you’ll feel it and it won’t be pleasant towards the end. Try to avoid this.


Here is a lighter ride of one hour of Z2. This might be closer to what you would expect if you were normal and also consuming sugars of some type at a low level. Ride started around 6.

Finally, just as an FYI, here’s a longer weekend ride that started around 12. I had a Quest cookie at 1hr and then a Clif bar and Super Coffee drink at 2.25hr.

Thank you. It’s good to have so something to compare results with.

@gergercha are the units on the left glucose mg/dL (milligrams per deciliter)? I think this is the standard unit of measurement for glucose in the US.

I’m using a Libre 2 for the same purpose as you intend. I think it’s made me smarter about my evening meals/snacks and helps me find combinations of foods that don’t spike my BS before bed. My very uneducated understanding is that it’s harder to lose weight if your blood sugar is spiked during your sleep even if you are operating at a caloric deficit. I’ve found a little apple cider vinegar in my nighttime supplement drink has a beneficial blunting effect on BS. I do notice hard efforts on the bike cause a rise in BS which makes sense to me as the body is trying to match the rate at which your muscles are utilizing glucose. My recovery drinks after a workout also cause a big spike which I expect since I’m slamming down a large high-carb drink. I’m not saying this is how it should be for everyone, it is just what I’m seeing in my experience monitoring blood glucose.
example from today’s workout, hard intervals, 1/2 bottle of Gatorade, 3 Swedish fish; Recovery drink with skim milk, whey protein, maltodextrin, and chocolate syrup.

If you get one, be sure to adhere it to a spot on your arm that you are unlikely to sleep on. I suffered through false low glucose alarms until I figured out the issue.

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Thanks for the tip. Just installed mine yesterday, and it woke me up at 3 Am with a low glucose alarm. Maybe I can avoid sleeping on that arm for the 10 days it’ll take for this session to end.

Correct, the left hand scale is mg/dL for glucose, and this is the standard presentation in the US. There is a way to switch it to mmol/L if you prefer those units.

ETA: Both Levels and Supersapiens give you a daily score, on a scale of 0 to 100, based on how well you managed your blood glucose during the period. IMO this is a really bad idea and confusing for US users because a normal average blood glucose reading is also in this range in mg/dL units. (Supersapiens, at least, can be excused as they are not officially available in the US, but Levels is US-based and this was a facepalm stupid design decision by them, IMO.)

I am pre-diabetic. My HbA1C is 5.9%. I have always fueled my rides at 60g of Maltodextrin per hour. I am concerned that I am doing harm with fueling at that level. Recently got a blood glucose meter from my doctor so that I can check my blood glucose levels. I want to see if I can improve my HbA1c. I did a test during an indoor 2 hour Trainer Road workout where I stopped and checked my blood glucose every half hour. 30 minutes into the workout, my glucose level jumped 20 points above my baseline reading at the start of the ride(120). At an hour, it peaked at 60 points above (180). From 1.5 hours to 2.5 hours (includes 1 half hour post ride) it was between 160-170. By hour 3, it returned to the starting baseline value.
I understand that during exercise the increasing blood sugar is absorbed by your working muscles. My question is: what is an allowable rise in blood sugar during exercise? Am I increasing my blood sugar beyond what my muscles can absorb? What should an athlete’s blood glucose profile look like during and post-exercise? I have searched the literature and cannot find an answer to this specific question. Does anyone know of any references or studies that can answer this question?
I plan on getting a Continuous Blood Glucose monitor so that I can run more tests and dial in my on-bike nutrition but I need to know what level of blood glucose is safe and acceptable for someone with pre-diabetes. Both the absolute level as well as the delta above baseline are important to know.