Blood Sugar, Biking, and my Dexcom


I’m hoping to tap into the collective wisdom here regarding how biking affects blood glucose levels. I used to be an avid cyclist, but ever since my diagnoses last year, I’ve been a little hesitant to spend much time on the bike because I find nothing lowers my blood sugar faster than biking, even if it’s just my 15 morning ride into work. So now I find the idea of going for a 4-hour ride a little scary. I’ve never had an incident on the bike, but I see the sudden crashes on my Dexcom. And about a month ago I did a ride with my wife, and despite my constant ingesting a glucose tablets, my blood sugar hovered between 60 and 70 for the last hour of a 2-hour ride. I just couldn’t get it up.

So my question is this: is this real, or is this perhaps something happening inside my interstitial fluid that the Dexom is measuring? The reason I wonder if it’s not something in the interstitial fluid is that the response seems to be almost immediate, there is no 15 minute lag as you would expect. As soon as I start biking, the Dexcom starts falling. From what I understand there is typically a 15-minute lag between real blood glucose and the Dexcom reading. So how could it start falling immediately if it there is that lag?

I think the next time I do a long ride I will take my meter and measure myself every half hour or so to see what’s happening. But regardless, I was wondering if anyone else has problems keeping their blood sugars up while biking and what you do to fix it.

Thanks in advance.


Regular bike rider here, and I keep my iPhone open to the Dexcom screen while riding because managing the drops is a big issue for me too. Ultimately it all comes down to insulin on board, whether it’s your programmed basal (assuming you use a pump) or the overhang from a bolus. Either way, aerobic exercise can turbo-charge your insulin sensitivity, which can be really great when you need to knock down a spike but can be a PITA when you want to get out for a bike ride, even if it’s long enough after your meal that you think it should have cleared.

Without more info it’s hard to judge, but it could be either. I find my G5 shows more of a drop during a morning (commuting to work) ride than I see in a fingerstick, sometimes quite a bit more. If you bring a meter with you, you can always stop and test a blood sample and see. If it is real–if you really are dropping that fast–that sounds like the exercise+IOB effect. It can be really dramatic even just doing a brisk walk. If that’s the case, dialing back your basal and/or any meal boluses prior to your ride is the way to go.

It is hard to get it just right, though. If I know I’m going for an afternoon ride I’ll tolerate a higher post-prandial BG—actually I pretty much need it—so I don’t arrive home with the light-bulb-flash-in-the-eyes effect that signals I’m in the 50s and dropping. 60s-70s at the end of a ride are usually ok for me if exercise-induced because I find they’ll bounce back to 90-100 in half an hour without having to treat 'em.

I’ve been managing T1 for 34 years across 3 treatment regimes (R/NPH->Lantus/Novolog MDI->pump therapy) and from the beginning it has always really annoyed me having to eat carbs in order to burn carbs. If I eat a zero-carb, zero-bolus lunch I can get through a 60 minute ride with a pretty tolerable drop, say 120 down to 80, and not have any need to treat it. If I’ve had carbs for lunch and bolused accordingly it gets complicated, even when it seems like the bolus should have cleared by the time I’m riding. So much depends on your own specific insulin sensitivity and how long your particular body takes to process things. But in general less insulin means easier to deal.

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Avid cyclist and mountain biker here. I use a Medtronic CGM, but, I never trust my CGM and ALWAYS bring a finger stick test along.

Generally, I find that cycling centres out my glucose around 100 and it doesn’t drop further than about 100. But, that only happens if I am trained-up.

If, for any reason, I am off the bike for a period of two weeks or more, I am back to square one with my glucose control. My sugar will crash to 40 within the first 15-20 minutes of the ride.


You can do the long rides without issue. I was diagnosed in Jan of this year. I just did 40 miles on my weekend ride without issue and have done longer. Get “endurance fuel” that works for you, like gels, shot blocks, or liquids. I use real food as well, in the form of grapes and cubed/salted potatoes. The research I have seen shows that you need about 40 to 60 carbs an hour for a good ride. With my two bottles, I use one with water and the other with Tailwind endurance fuel. The Tailwind has more carbs per ounce than Gatorade, so it works well for me.

My Dexcom works well only if I am WELL hydrated. I have found that if I am only OK on hydration the dexcom will go into the red and stay there for the duration of my ride. I will feel fine, have good levels on the finger sticks, but dexcom will still be in the red. I don’t know this for sure but it is my theory that the interstitial fluids aren’t available for a reading when you don’t have optimal hydration. I have had times where I fueled good but didn’t do well on my hydration. I felt great, with plenty of energy, but the dexcom was beeping lows at me for most of the ride, climbing right back to my normal level after I got off the bike.

I have found that as long as I keep my carb intake count around 50 to 70 an hour (I am a tall guy and weigh in around 210) and then down at least 20 oz an hour my dexcom won’t go into the red, that and I try to ensure I start the ride a bit high. Best of luck with your cycling efforts.

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From my perspective, the answer is ‘it depends.’ I used to see very big “drops” on my Dexcom when riding my bike - so much, that I also panicked and frequently over-treated the “lows” that were being reported. In addition, the meter I was using had a lot of trouble getting valid tests when I was sweaty, so I could not easily verify my BG during a ride.

Yesterday, I did a 40-mile ride in 90+ degree weather and not only did my Dexcom give me spot-on accurate numbers - confirmed by my meter, which also worked just fine throughout the ride - but my BG stayed at a virtually flat-line right at 88 nearly the entire ride. (I did snack twice during the ride, which bumped me up a little, but I returned to that 88 baseline before too long.)

My “solution” to the problem seems to have involved three strategies. First, I experimented with different locations for the sensor. For me, it seems that sensor sites on the sides of my chest are the least impacted by sweaty bouts of exercise. Stomach sites are the worst in that regard for me. Second, I started using a meter that requires a much smaller sample size, which seems to reduce the impact of being sweaty when testing. Finally, I’ve changed my diet to a much lower-carb approach, and in fact had no carbs, and therefore no bolus insulin on board, before the ride – so I did not get “surprised” by much stronger insulin action than normal when riding.

All-in-all, it made for a much safer and stress-free ride - well… except for , the heat and wind that were another matter altogether! :slight_smile:

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Thank you all for the great replies. I think they all contain some really good advice and examples of situations I can relate to. About a month ago I did a 30 mile ride and my Dexcom was reading around 65 the entire time, but I felt fine and finished the ride with no problems. If I remember correctly, the BG readings started going up on their own after the ride was over. It seems to make sense that it could be more related to hydration than anything. But I would hate to attribute it only to that while riding, ignore the values, and then actually have it crash.

A few times when I’ve gone on longer training runs I’ve filled my camel-back with and electrolyte and glucose solution, which maybe I need to start doing for biking.

Also, Michael, I just read your other post about your ride this weekend, which was fascinating. It’s hard enough trying to control BG values while being active, but then throw in Trulicity and the associated gastroparesis and things start to seem overwhelming. Maybe it’s time I took up the banjo or something.


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My experience is that there is not a lag on the way down, but there is a lag of at least 15 minutes once you have dropped below 65.

I have multiple strategies for the food that I eat before a ride, and like many strategies, some work better than others, some aren’t much more than seat of the pants guessing.

The first thing I do is drop my basal rate drastically. I typically run @ only 10% of my regular rate, and I like to get this going at least 1 hour before riding, frequently 2 hours before. I always do this, but I arrived at it after lots of trial and error using everything between 50% down to 0%…

Next, I like to eat something with some protein along with some carbs too - 2-3 eggs with cheese, a glass of milk, whole wheat bread or an english muffin. I will bolus at no more than 50% of my regular bolus, plus I will try and eat in less than 15 minutes after bolusing.

I always carry glucose tabs, at least two small boxes of raisins (22g carb, each), and at least two Clif Bars (41g carb, each). When I start dropping, I will normally start by grabbing 2-3 glucose tabs, but it all depends on how severely I am dropping, where I am on my ride, etc.

This past Sunday was a great example - I did everything described above, only to see my BG under 65 about 20 minutes into the ride - very weird! I ate two glucose tabs and a Clif Bar. I took about a 5 minute break from riding and started again because I really wasn’t feeling the low. My BG bottomed out around 53 and then started climbing back up. After about 2.5 hours into the ride, I was comfortably running in the 130 range, with some hills to climb as I turned towards home. When I finished a 4.5 hour ride, I was just under 100, with a slight downward trend. I went inside and drank a glass of milk (12g carb) and in about 30 minutes I was flat-lining in the mid-80s.

On other rides, I have really loaded up on carbs along with a good variety of protein and fat. Think banana, yogurt, peanut butter, bagel and milk all in one meal. Sometimes this runs my pre-ride BG to well over 200, but the riding brings it down. Last summer, when riding across the USA, I always carried some trail mix and I would have a handful about every hour as a kind of glucose basal rate.

Yes, I did always have my finger-stick meter with me, but these days I find my experience in reading the CGM “tea-leaves” is all I need.

I will also say that I have become very tolerant of lows, so I don’t often get the “light-bulb flash-in-the-eyes” effect that @DrBB describes. If you’re worried about that and wobbling around/literally crashing, I would try and run a little high during a ride.

I would experiment and get back out there. When I was on the dreaded NPH protocol, there was no way for me to safely ride, but pumping coupled with the near real time CGM feedback has enabled me to go places I never dreamed of.


I can only share my experience, but I do ride. My BG rises for the first 20 minutes (although no to unmanageably high levels), then starts to drop, and by 45m-1h I need carbs to keep stable. I use a 24g Clif gel between 45m and 1h into a ride and every hour afterwards, and that tends to keep me stable (above 65). I also have to double or triple my regular carb consumption post-ride if I’ve done anything 2 or more hours, or I just keep going low(ish) in the evening. Magic (or curse) of glycogen depletion.


So I went out for about a 20-mile ride yesterday and tested three times. Before the ride I ate a bar with about 30g of carbs in it. About 30 minutes into the ride my Dexcom was reading 75 with a steady downward trend, but a finger stick test showed I was at 113, a 38 point difference. At 75 and dropping I would need to take action. At 113 I don’t really need to do anything but keep riding. Needless to say, this confirmed some suspicions that what my Dexcom shows while riding isn’t always accurate.

At the half way point of the ride my Dexcom showed 86 and the finger sticks showed 94, which is much better. I tested once more at the end of the ride. Dexcome showed around 85 this time, but the finger stick showed 76, which is also not too far off and probably within the margin of error.

So I’m not quite sure what to make of this little experiment, which I will undoubtedly repeat the next time I go for a ride. But I think the ultimate take away for me is that I can’t always trust my Dexcom while riding, especially if I think I may not have drinking enough water.


In general I would guess a 20-mile ride isn’t going to leave you substantially dehydrated, unless you were already.

When I have been below 65, I find the Dex is pretty slow @ matching up with my finger-stick readings, especially after consuming some carbs. I frequently see a 30+ minute delay in getting back to reasonable agreement.

I also like to pay attention to the trending dots, sometimes giving the numbers far less significance them they would otherwise warrant. It’s a bit like reading the tea leaves, but with some data behind it.

Thanks for reporting on this experiment. Looking forward to seeing your next one.

Oh, did you do anything to replenish your glycogen stores post ride?


I have been an avid cyclist for many years. I am a pumper with a dexcom. When I go cycling if my bloodsugar is below 10 mmol/L I reduce my basal by 50-60%, sometimes even 70%. If I am higher than 10 mmol/L I only reduce by 30%. I usually ride 1-1/2 to 2 hours in the evening 4 times per week with a long ride of 3 hours on Saturday. I usually take a banana, gatorade and a couple of cans of coke with me packed in my water backpack. Things have been going pretty well this year while riding. I also do test 1/2 way through the ride.



Banjo isn’t the answer for you … or for me. :slight_smile:

I absolutely love cycling and am not willing to give it up.

I’ve had my confidence shaken and haven’t been on the bike since that ride.

I’ll be getting back on and riding tomorrow. I’ve registered for my first centurion this September, and I plan on finishing all 100 miles.

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I lower my basal rate 40%-50% an hour before the ride, and that is usually enough to prevent severe hypos. Here’s my question: I want to start cycling early in the morning (like 5 am), b/c that is when I have time during the week. Do I really have to wake up an hour before (4 am) to change my basal rate?? Is there any other solution?


Set up different basal patterns. I have three different patterns:

(1) No ride
(2) Morning ride
(3) Evening ride

I usually know in advance if I will be riding.