How to avoid lows when going for runs with TSlim

Hello everyone. I’m new to this forum, but have been Type 1 for 28 years. Last year, I switched from insulin pens and a Freestyle Libre to a Tandem TSlim pump paired with Dexcom G6.

My wife and I are distance runners, and we have run 4 half marathons together - all before I made this switch. I rarely had issues with lows before when running. However, we started getting back into it lately and I’m finding that no matter what my blood glucose is or how little/much I run, my blood sugar is dropping extremely fast. My tandem rep helped me set up an “exercise profile” on TSlim that I use when running, and it delivers less basal insulin when activated. However, none of this is working. Worse yet, I’m replacing all of the calories I’m burning while running with orange juice and granola bars after my runs.

Has anyone else run into this issue? I’d love to get some advice as I do not want this pump to prevent me from getting back into my passion for running. My wife and I had our first baby during the pandemic, and she has been just perfect (and exhausting)!

Thank you in advance for any help you can provide.


Hi @josephazimmer. Welcome to FUD! You are in the right place.

Before I get into pump or basal specifics, I think it’s helpful to have a little overview of how our bodies fuel exercise.

Our bodies have several different ways to “create” energy (ATP) which is used to allow the muscle to contract. (Of course we don’t actually “create” it, it simply changes form.)

The primary ones we talk about for exercise are glycolysis (carbohydrates) and the Krebs cycle (fat metabolism). There are others, but those are the main ones that keep us moving.

All of those systems are always working together, but in different amounts. You don’t completely switch from one to the other as you increase or decrease your intensity, but the percentage of the contribution of each source changes.

Carbohydrates are a very efficient source of fuel because the body requires less oxygen to burn carbohydrate compared to fat.

Fat is not as efficient, but is more abundant. Fat provides a bigger amount of available energy compared to carbs (9 calories per gram of fat versus 4 calories per gram of carbohydrate). It helps fuel lower intensity efforts, and we have much more of it compared to stored carbs, but it requires more oxygen. And it also takes longer for this fuel source to be ramped up.

An easy way to think of this is how you can walk without breathing hard, and you can walk farther than you can run, but you can run faster. And running means you will be breathing harder.

One other thing to mention about carbs - they can be stored in your muscles (muscle glycogen), but those stored carbs can never raise your blood sugar. Ever. Once they are stored as muscle glycogen, the only thing they can do is provide energy for the muscles where they are stored. (Muscle glycogen is never shared). But carbs that are recently eaten can either provide energy for exercise or raise your blood sugar.

So the most important thing to understand is that the fastest way for your body to get energy is from stored carbs (muscle glycogen) or from your blood sugar.

Okay, so all of that is just a little background on fueling. I can get to the fixes for your BG drops in my next post, but wanted to just give a little overview of that to start with. Let me know if all of that makes sense.


I don’t like exercise mode because it gives automatic boluses when going over 150 or so, which then works extra fast since I’m exercising. I just leave my pump in sleep mode 100% of the time, which gradually increases and decreases the basal rate, instead of giving boluses. This works well for me, but I am not doing long-distance runs/walks, so you may need to play around with it a bit.

Another option would be to turn of Control IQ altogether and just figure out what you need to eat/dose prior to exercise without the pump interfering.

One other resource that you may find helpful: Type One Run


I’m on the t:Slim and although I don’t run, I follow a fairly vigorous exercise regime that combines skipping/jump rope, HIIT and power yoga (vinyasa flow). I don’t find that there’s a single solution that works for everything as some workouts are aerobic and others more anaerobic and the latter, if anything, tends to push my blood sugar higher as it causes liver dumps. What I tend to do for aerobic activities, which distance running (rather than sprinting) is, is to purposely let my blood sugar run higher prior to the session, i.e. low double figures. It’s for a short period only and I’m well controlled the rest of the time, so I’m not overly concerned with long-term consequences. I generally achieve this by lowering my basal rate (or even setting it to 0%) about 90 minutes to 2 hours beforehand (depending on what the value is already). My workouts often tend to be short and sharp, so I invariably keep a low or zero basal rate for the duration of the session but I keep a close eye on what’s going on via a CGM-enabled smartwatch and act accordingly if I see thing going south. For longer periods of activity, which distance running would fit with, I would want at least some insulin on board if only a reduced rate. I’d agree with @allison that turning off Control or Basal IQ is a good idea. My feeling is that that technology just can’t take into account all the variables involved with endurance exercise.


I 100% agree. I have no clue why they designed exercise mode to work that way. The last thing I want when exercising is an automated bolus in addition it doesn’t take into account your insulin sensitivity has dropped way down. I think that was an absolutely terrible design decision.

Love everything else about ciq😉


Me too! I have had the best A1cs of my life!


Thank you all for the great advice so far. First, I have set myself up in sleep mode consistently to avoid the automatics bolus from occurring. I’ve stopped using exercise mode, and turned off ControlIQ before my run last night (a little over 2 miles).

Although my BG did go low when I got back, it was not as sharp as it had been when ControlIQ was on. As one person had commented, it seems like I’ll need to do this earlier moving forward.

Has anyone just taken the pump off completely when exercising? I have not tried this yet but am speculating that it might be a simpler fix. And then putting it back on afterward. Would love to get your thoughts on this. Thanks again!



I do a zero basal but sometimes take very small boluses for fueling.


Pump reps have a hard time wrapping their heads around real workouts. I started the 670g in 2017 and the medtronic rep couldn’t understand why bumping me out of automode after 3 hours of zero basal was a problem. I didn’t need basal because I was 3 hours into an 80 mile bike race!

You’ve done plenty of running and I’m going to assume injections included some sort of carb count/sliding scale, so there shouldn’t be significant differences on a pump. There are a few nuances to pumps but the same basic principals apply. A couple questions: Are you running at the same time each day? When? Were you timing your long acting injections for workouts?

The big thing I notice with pumping is the long tail associated with boluses. If I exercise first thing in the morning, I can get by with a basal suspend and ~15g carbs. If I exercise in the evening I need to reduce basal 3 hrs ahead and consume a metric ton of carbs beforehand. It had not been my experience that injections reacted exactly the same way.


I totally agree about exercise mode and boluses. I also agree about basically liking CIQ.

I think what the rep is suggesting, however, is not using exercise mode but setting up an exercise profile i.e. a profile with a different (lower) base basal rate and a different insulin sensitivity (as in very sensitive to very small amounts of insulin.)
I don’t know if it would solve the whole problem of running for distance, but if it turned on well in advance of the run it could mean that you had relatively little insulin on board and any automatic corrections either in the form of higher basal rates or automatic boluses would be small because of the insulin sensitivity setting.


Reading all the posts in this thread, I thought to turn to what I think of as my bible for diabetic athletes, “The Athlete’s Guide to Diabetes” by Sheri Colberg PhD, a type 1 DM. In the chapter Endurance Sports she writes the way some 13 half and full marathoners managing blood glucose levels, bolus and basal and carbs. All cope in different ways, so that they can do their sport well.

If I know anything about diabetes, regardless of the type, we have to learn through careful experience how to get what works for us. I have no problem sharing what I do to be able to do long bike rides after I started MDI, but while that might be entertaining, it might not be helpful or it might.

The other thing I know is that if we have any natural ability in our chosen sport, diabetes cannot hold us back. We have body builders, basketball players, track and field, IronMan and more.

Just keep tweaking and tinkering with bolus, basal and carb intake until you find what works. As I don’t have a pump, I am not fully aware of the pros and cons of pumps for the diabetic athlete.

I don’t mean that we can’t learn from one another. That is one of the beauties of sites like this. Just remember we are not identical. I was going to say like laptop computers, but when I worked we would all receive laptops of the same lot numbers. Not one of those was exactly in function like another.

Keep moving, keep breathing and live long.


My T:Slim trainer called people who do that sleep walkers. I guess in Allison’s case it sleep runner. :smile_cat:


I am on my 3rd cartridge since training on January 6. My trainer suggested I turn on Exercise mode 40 minutes before a ride. On the 7th I did a 2:20 bicycle ride following her advice. The ride began 3 hours after a breakfast of 24g. When on MDI I would eat a Kind Nut bar,9g, just before the ride. This time I wanted to see how it went.

Nearly 2/3rds of the ride my BG was very stable, hovering around 105, dropped to 95 at our usual stopping place. After a toilet break, BG dropped to 89 so I ate the Kind bar as I hit the road for the return loop. BG went back to 105, dropping to 95 at the finish. I turned off Exercise, and was quite pleased with the results.

So far it seems that sleep, regular and exercise work well for me. I did a stationary bike ride on the trainer without using exercise to see what would happen. I got the warning at 80. I ate 4g of glucose and all was well.

This is my limited experience, but I think it would be different for others. I am a long time type 2 DM and my Basal/bolus ration is about 77%/23%, which is no way near the usual 50/50 for type 1 DMs.


I normally do bike rides in the morning. I let my sugar go to 180 or so before the ride and don’t want a lot of insulin onboard. That keeps me from going low on the ride when exercise jacks up the effect of the insulin. I can’t seem to remember to put my Tandem pump into exercise mode and take it out after the ride, so I just extended sleep mode until noon to keep the pump from giving me extra insulin when I don’t want it. That works for me whether I ride or not. Wish Tandem had a timed exercise mode.


I’m late to the party here, but have really appreciated all the perspectives and knowledge in this thread.

I’m not an expert here but when I’m doing long distance biking it’s easier for me to keep the pump off, bring a couple of sports bars and pay attention to the CGM. If I’m doing a lot of exercising, I can control my bg without insulin during the time I’m exercising.


Sounds like a good idea. I ride a Lemond Zurich weekly in exercise mode. Usually have to eat a kind bar mid-ride. I’ll try sleep mode to see how that works. Thanks for the suggestion.

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I do this (turn off control IQ) about an hour before any planned exercise, and turn my basal way down. It’s the only way I can avoid lows. That said, I sometimes have rebound highs. I have had the Tandem for a year now and haven’t figured out exercise perfectly.