Timing of pre-workout carb consumption

Wanted to share this. There is variability for everyone, but in general, the principle applies.

If I am flat and about to go run, depending on circumstances and the workout, sometimes I will take a few carbs as insurance, to prevent a drop.

  • If I am flat, if I take the carbs, and immediately start the run, I can go the entire time and never get above 100.

  • But in identical circumstances, same flat BG and same amount of carbs, if I take the carbs and then get delayed a bit before starting, as little as 15 minutes, I can spike all the way to 150 during the same type of workout.

The difference a few minutes makes with the same number of carbs is amazing.

When you are exercising, your digestion can get slowed (speaking of strenuous type cardio exercise here, not passive exercise such as walking). Exercise slows down the digestion system in order to conserve energy and blood for the muscles. So the same amount of carbs can have a totally different effect depending on how soon you take them before starting.

Additionally, the more intense the exercise, the more the slowing affect.

Just something to think about for pre-workout carb consumption. Time it according to what you want and need for your BG and the workout you are about to do.

According to the BBC exercise slows down the digestion system in order to conserve energy for the muscles.

Exercise and digestion are mutually exclusive. When you exercise, your body isn’t using its energy for digestion. Instead, it slows any digestion currently taking place so it can divert as much blood as it can to feed your muscles and your lungs.


Just wondering, if digestion slows during exercise, then do you find that once exercise ends, digestion of thise few carbs eaten immediately before the run begin to digest? Could this potentially then cause a high post run? I ask because I often eat a few carbs prior to a run if I’m trending down. For example, today I ate 4 cherries which is 8g. I was trending down, but as I ran, I began to go high. Eventually from 95 to 120 at the high, and then it slowly moved back down to 95 by the end of the run. This is just one example, of course. I took 2u at that point for the missed basal and as a pre bolus. Meanwhile, my BG began moving higher again, eventually back to 120! Coincidental that the numbers worked out so even like this, which is rare or perhaps a first for me. I’m just wondering the effect of the pre run carbs. Maybe I should try to hold off on them to prevent post run highs…

Your digestion absolutely does take a back-seat during exercise and slows down. It isn’t just diabetics, it affects everyone. Your body prioritizes getting blood to your legs and you just can’t process as many carbs when you are exercising, or process them as fast.

I don’t know if what you saw is specifically the result of this, but it can certainly cause a difference. But usually it takes a little while before you see that effect.

I suspect the other issue you saw when you started to rise when you started running is that you probably had cut your basal off prior to running. Whenever you have carbs on top of a zero basal, you see a much more pronounced affect.

Yes, that is exactly what I did! Did a ZB a half hour before running. It worked out well in any case :slight_smile: I was wondering if the carbs one eats immediately before running is used for fuel immediately, or is it stored, and then may effect post run BGs? I’m thinking it is used immediately upon running. Of course, I’d rather not eat carbs beforehand.

Yes. Glucose in the blood is taken into the cells and takes one of several paths. If your body is currently undergoing physical activity, it will be used to fuel that activity immediately, it will not be stored for later.

The bigger effect on your post-run BG is your ZB. If you see a consistent rise when you finish, you can try turning off the ZB a little bit before you finish, or go with a low number like 20% basal instead of zero.

Thanks, @Eric, for clarification! It really helps to understand the workings behind bg tends. For some reason, I have a hard time grasping it, so I really appreciate being able to ask these questions! I’m still doing trial and error with ZB but so far I’m finding it is really helping with the lows during exercise. I need to fine tune it and be more diligent about the timing. Today I decided to go biking instead of running. Dur to a thunderstorm forecast, I had to go without any lead time for a ZB. So I suspended my basal for 1 hour thinking I would ride for an hour. My bg was 87 when I started, dropped to 71 an hour later then proceeded up to 91, on finish, 2 hours later (I was able to bike longer since the weather held out). It kept rising after I finished, too, to 120. These are cgm numbers. My fingerstick shows 106. All in all, I think it worked very well. No lows and no extreme highs:) I’m not sure why it went up but I guess it is also related to the basal timing. I didn’t have time to adjust basal properly today, but will try adjusting it as you describe to try to reduce the persistent high on finish.

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Your numbers sound great!

After 2 hours of zero insulin, BG going up is not unusual. A lot of it depends on how intense the workout is.

But if you find this happens a lot for you with long workouts, I think changing your basal to a smaller number like 15-20% instead of ZB would help.

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Thanks! The bike was an easy path with just short hills, not intense at all. Runs are a different story, they can be intense! Okay, that sounds like a good idea, inching up the basal a bit instead of ZB. I’ll try that tomorrow. I’m mostly concerned w/ the lows that I regularly experience which ZB has definitely helped. Thank goodness, I’m not going too high with ZB, but a bit higher than I’d like.

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I’ve been trying to exercise more consistently, 3+ times a week of tennis for half hour, followed by 20+ minutes of wimpy weight lifting (the two lightest weights in the fitness center, LOL). My BG’s have been consistently lower. I haven’t changed my I:C ratios; my post meal and pre meal numbers are in the lower range of acceptable with exercise, without exercise mid range of acceptable. On the days that I don’t exercise as much, I can tell the slight difference of about 10 mg/dL in BG. I do eat 5g extra carbs before I exercise, just to be safe.