Mile by Mile: Managing Blood Sugar While You’re Actually Running (Part 3 of 12)

Mile by Mile: Managing Blood Sugar While You’re Actually Running (Part 3 of 12)

By Alicia Dahl | AliRunsOnInsulin.com | The Run Starts Before the Run Series

You did the systems check. You confirmed your number. You adjusted your basal, dialed back your bolus, packed your carbs, and made the mental decision to go.

Now you’re running.

And your blood sugar has its own agenda.

This is Part 3 of 12 in my series on running with Type 1 diabetes. We’ve covered the pre-run protocol and how to read your starting number. Today we’re talking about what happens once the miles actually start — and how to manage it in real time.

The First 15–20 Minutes: The Adjustment Phase

The first stretch of any run is where your body is figuring out what’s happening. For T1D runners, this window requires your closest attention.

Blood sugar can drop fast in the early miles as your muscles start pulling glucose and your active insulin gets amplified by the exercise. This is why starting in the upper half of your target range closer to 160–180 mg/dL gives you buffer room that a starting number of 120 doesn’t.

Watch your CGM trend arrow closely during this phase. If you’re dropping faster than expected, eat something now don’t wait until you hit 90. Catching a drop early takes 15 grams of carbs. Catching it at 65 on mile 3 with your legs turning to jelly takes a lot more.

 Your CGM Is Reading the Past, Not the Present

This is one of the most important things T1D runners need to understand about CGM technology — and most people don’t learn it until something goes wrong.

CGM devices measure glucose from interstitial fluid, not directly from your blood. Research shows that during aerobic exercise, the drop in CGM readings lags behind actual blood glucose by an average of about 12 minutes with a margin of error of up to 13% during activity.

What this means practically: your CGM might show 110 and a flat arrow while your actual blood glucose is already at 85 and dropping. The CGM is catching up. If hypoglycemia is suspected during exercise, the research recommendation is to confirm with a fingerstick.

This isn’t a reason to distrust your CGM it’s a reason to understand it. Use the CGM for trending and alerts. Use a fingerstick when something feels off, when your symptoms don’t match your number, or when you need to make a significant decision (like whether to eat, correct, or stop running).

Your CGM tells you the story. Your fingerstick tells you the current chapter.

The Fueling Formula: Short Runs vs. Long Runs

The carb math changes based on how long you’re out there.

Short runs (under 60 minutes)

For most T1D runners, a well-timed pre-run snack and a reduced basal rate are enough. You may not need to eat mid-run at all but I always carry fast carbs regardless. Like the same fruit snacks have been on so many training runs I think they’ve become one.

Medium runs (60–90 minutes)

This is where active mid-run fueling starts. One approach that works well for T1D endurance athletes is 15–30 grams of fast-acting carbs every 30–45 minutes for lower intensity runs under 2 hours. Adjust based on your CGM readings and how you feel.

Long runs and marathon training (90+ minutes)

For runs longer than 90 minutes, experts generally recommend consuming 30–60 grams of carbohydrates every hour during the run. That’s a gel or two, some chews, real food — whatever your gut tolerates and your pockets can carry.

The key principle: don’t take more than 24 grams of carbs at one time. Spreading your fueling throughout the run is more effective than front-loading before you go out.

What Dropping Fast Looks Like (And What to Do)

A double-down arrow on your CGM mid-run is not a “keep an eye on it” situation. That is a stop-and-eat situation.

The protocol when you’re dropping

1. Slow down or stop — exertion accelerates the drop

2. Take 15 fast-acting carbs immediately (glucose tabs, a gel, Gummy Nerds, whatever you’re carrying)

3. Wait 10–15 minutes before continuing — don’t just eat and keep pushing

4. Recheck your CGM; if still dropping, eat again before resuming

5. Never try to push through a low. Your safety comes first stop running until your blood sugar rises to a safe level before continuing.

The number at which you personally act is something you’ll dial in over time. My rule: anything below 100 with a downward arrow and I’m eating right now, not in a mile.

What Spiking Mid-Run Looks Like (And Why It Happens)

Not all blood sugar surprises go down. Some go up and this catches people off guard.

High-intensity running, sprint intervals, and race-pace efforts can trigger a stress hormone response that releases glucose into your bloodstream. Blood sugar may rise during strength training or short, intense bursts of aerobic exercise like intervals.

Adrenaline does the same thing, which is why race day numbers often behave completely differently from training runs at the same pace. (That’s a whole post on its own, it’s coming in this series.)

If you’re spiking during a run with no other explanation, it’s likely the intensity. Slowing your pace often brings it down without needing a correction dose. Be very conservative with mid-run corrections the combination of a correction dose and continued exercise is a fast route to a low 45 minutes later.

Hydration Is Not Optional

Dehydration concentrates the sugar in your blood, which increases your risk of hyperglycemia. For short runs, plain water works. For long runs, sports drinks can help you get fluids and some carbs at the same time but they typically only provide about 7 grams of carbs per cup, so you’ll likely need additional fueling on top.

Drink consistently throughout your run, not just when you’re thirsty. Thirst during a run often means you’re already behind on fluids, and behind on fluids means your CGM readings become even less predictable.

The Glance Rule

With a CGM, you don’t need to stop and check constantly. But you do need a system.

My glance schedule on a run, some times I’m more comfortable wearing my pump on my shoulder during races and log runs as it’s easier to grab:

Miles 1–2:** every 10–15 minutes, trend arrow check only

Miles 3+:** every 20–30 minutes or when I feel something shift

Any time I feel off:** check immediately, don’t talk myself out of it

“Feeling off” during a run with T1D is data. Fatigue that doesn’t match the effort, sudden mood shift, legs that feel wrong, lightheadedness these are symptoms before the CGM has caught up. Trust your body first.

The One Rule That Never Changes

Whatever your number, whatever your arrow, whatever your plan — you carry fast carbs on your body on every single run. Not in your car, not in your bag, not at the next water stop.

On your body. Accessible without stopping.

I carry a variety of SiS Gels, Clif Shot Bloks, fruit snacks or Nerds Gummy Clusters. These are faster-acting and easier to consume mid-run. 

Your running belt is not optional gear. It is safety gear.

The Bottom Line

Running with T1D means you’re managing a moving target while also moving. Your blood sugar during a run is influenced by your starting number, your pace, your intensity, your stress hormones, your hydration, your IOB, what you ate, what you didn’t eat, and factors that are sometimes genuinely unpredictable.

You will not get it perfect every run. The goal is to stay safe, stay fueled, and learn something from every mile.

That’s what the log is for. That’s what the next 9 posts in this series are for.

Always work with your diabetes care team on your personal exercise management. Every T1D responds differently — these protocols are starting points, not prescriptions.