HIIT and Type 1 Diabetes: My Science Experiment with Temp Basals

The HIIT Struggle is Real: Why My Gym Bag Looks Like a Lab Kit

Walking into a boutique HIIT (High-Intensity Interval Training) studio for the first time is an assault on the senses. The lights are dimmed to a neon glow, the bass from the speakers is vibrating in your chest, and the instructor is shouting words of encouragement that sound suspiciously like a drill sergeant’s commands. Everyone around me was adjusting their sweatbands and high-fiving. Me? I was staring at my phone, checking a fluctuating line on a graph, and making sure my bag had enough glucose tabs to revive a small army.

Since my Type 1 Diabetes (T1D) diagnosis, my relationship with the gym has changed. It’s no longer just about "getting a good workout." It’s a complex logistical operation. My gym bag doesn’t just hold sneakers and a water bottle; it’s a mobile laboratory. I’ve got my Continuous Glucose Monitor (CGM) receiver, my insulin pump, backup infusion sets, three different types of fast-acting sugar, and a notebook filled with scribbles that look like a mad scientist’s equations.

The struggle is real because, for those of us living with T1D, the standard rules of cardio simply don't apply to HIIT. We’re told that exercise lowers blood sugar, right? But then you finish a grueling set of mountain climbers only to see your sugar skyrocketing into the 200s. It’s frustrating, confusing, and enough to make anyone want to throw their barbell through a window.

I decided to stop looking at these gym sessions as a source of frustration and start looking at them as a Science Fair project. I am the lead researcher, the test subject, and the data analyst. Every workout is a trial, every spike is a data point, and every "temp basal" (temporary basal rate) is a variable to be tested.

The Science of the Spike: Why HIIT Isn't Your Average Cardio

To understand why my blood sugar behaves like a roller coaster during HIIT, I had to dive into the physiology of the "spike." Most people think of exercise as a way to burn glucose, which it is. When you go for a steady, 30-minute jog, your body is in an aerobic state. Your muscles are using oxygen and glucose at a steady rate, and your blood sugar typically trends downward.

But HIIT is a different beast. It’s anaerobic.

When you’re doing 30 seconds of all-out burpees or sprinting at 10 mph on a treadmill, your body enters a "fight or flight" mode. This triggers a massive release of stress hormones, specifically adrenaline and cortisol. These hormones tell your liver, "Hey! We’re in trouble! Dump all the stored glucose into the bloodstream right now so we can survive this lion attack!"

The result? You’re sweating, your heart is pounding, and your CGM is screaming at you because your blood sugar is climbing. It’s the ultimate irony: you’re working harder than ever to stay healthy, yet your glucose levels look like you just ate a stack of syrup-drenched pancakes. Distinguishing between the sugar-dropping effects of aerobic exercise and the sugar-spiking effects of anaerobic bursts is the first step in mastering the HIIT game.

Formulating the Hypothesis: My First Attempt at a Temp Basal

Every good experiment starts with a hypothesis. Mine was simple: "If I reduce my insulin, I won't go low." Oh, how naive I was.

Trial 1: The "Better Safe Than Sorry" Approach

For my first "official" HIIT experiment, I decided to be cautious. I set a temporary basal rate of 0% (essentially turning my pump off) thirty minutes before class. I figured since HIIT was so intense, I’d need zero insulin in my system to avoid a crash. The Result: I finished the 45-minute class at a staggering 345 mg/dL with "double up" arrows. I felt sluggish, my mouth was dry, and I spent the next four hours "rage bolusing" to get back into range.

Trial 2: The "Standard Basal" Approach

The next week, I tried the opposite. I kept my basal rate at 100%. I figured my body would need that insulin to combat the adrenaline spike I experienced in Trial 1. The Result: About 20 minutes into the intervals, the adrenaline hadn't kicked in yet, but the increased insulin sensitivity from the movement had. I plummeted. I had to stop mid-sprint to chug a juice box while my heart was still racing from the treadmill.

These failures taught me a vital lesson: a "one size fits all" approach to T1D and HIIT is a myth. My body didn't need no insulin, and it didn't need all the insulin. It needed a "Goldilocks" amount—just right.

The Data Points: Tracking the Pre-Workout, Peak, and Recovery

To find that Goldilocks setting, I had to get serious about my data. I stopped guessing and started recording. My CGM became my most important piece of research equipment, providing a constant stream of glucose data every five minutes.

But the CGM only tells half the story. I started logging a variable that many people overlook: Active Insulin on Board (IOB). If I have a large amount of insulin left over from a lunch bolus, no amount of temp basal adjustment is going to save me from a crash.

My workout journal became a spreadsheet of:

• Starting Blood Sugar: Was I starting at 120 or 180?
• IOB: How many units were still active?
• Workout Intensity: Was today a "heavy lifting" day or a "sprint" day?
• The Temp Basal Setting: What percentage did I use?
• The Peak: How high did I go during the intervals?
• The Recovery: What happened two hours later?

Troubleshooting the Sugar Spike: When Adrenaline Takes Over

One of the hardest parts of HIIT is seeing that "high" arrow while you’re mid-workout. The instinct is to "rage bolus"—to deliver a large correction dose of insulin because you’re frustrated by the number.

Pro Tip: Don't do it.

Correcting a HIIT spike mid-workout is like trying to catch a falling knife. The adrenaline spike is temporary. Once the class ends and your heart rate settles, that adrenaline leaves your system. If you’ve injected a bunch of extra insulin to fight the spike, you will likely experience a massive, dangerous crash 60 to 90 minutes later when your muscles start soaking up glucose to recover.

Instead of a manual bolus, I started experimenting with small temp basal increases for the last 15 minutes of class if I was trending high, or simply waiting it out. Often, the spike would level off naturally once the "fight or flight" response subsided.

The 50% Rule: Finding My Personal Goldilocks Setting

After weeks of "Science Fair" trials, I finally found my baseline: The 50% Rule.

For most HIIT sessions, I’ve found that a 50% to 70% temp basal reduction is my sweet spot. It provides enough insulin to keep my body from entering a ketotic, high-sugar state, but not so much that the increased muscle sensitivity causes a crash.

However, the "Goldilocks" setting is a moving target.

• Weights vs. Sprints: If the HIIT class is heavy on lifting (anaerobic), I might only reduce to 80% basal because the spike will be higher. If it’s heavy on cardio (aerobic), I might drop to 40%.
• Time of Day: In the morning, I struggle with "Dawn Phenomenon" and insulin resistance, so I need more insulin. In the evening, I’m much more sensitive, so I need less.

Timing is Everything: The 30-Minute Pre-Class Window

Perhaps the biggest "Aha!" moment in my experiment was the realization that timing is more important than the percentage.

If I set my temp basal to 50% the moment the instructor says "Let’s go!", it’s already too late. Most rapid-acting insulin has a "tail" that lasts for hours. There is still a significant amount of insulin in my system from the previous hour's basal rate.

I discovered that I need to set my temp basal 30 to 60 minutes before the first interval. This gives the "insulin tail" time to clear out, ensuring that when my heart rate hits that target zone, my insulin levels are actually at the reduced rate I intended. This lead time is the secret weapon that prevents the mid-class low.

The Afterburn Effect: Managing Post-Workout Sensitivity

The experiment doesn't end when you wipe down your yoga mat. HIIT creates an "afterburn effect" (technically known as EPOC—Excess Post-exercise Oxygen Consumption). Your metabolism stays elevated, and your muscles are hungry to replenish the glycogen they just burned.

I noticed a pattern: I would be stable for two hours post-workout, and then at 2:00 AM, my CGM would start screaming. My "science experiment" had a delayed reaction.

To manage the "Delayed Drop," I’ve started implementing these post-lab notes:

1. The Post-Workout Protein/Carb Combo: I eat a snack with a 3:1 carb-to-protein ratio. The protein slows down the carb absorption, providing a steady trickle of glucose to my muscles.
2. The Overnight Temp Basal: If I do a late-afternoon HIIT session, I set a 90% temp basal for the first five hours of sleep.
3. Hydration: Dehydration makes me more insulin resistant, which masks the drop. Staying hydrated makes my glucose more predictable.

Final Lab Notes: Tips for Your Own Fitness Science Fair

If you’re a young person living with T1D, I want you to hear this: Do not let the fear of a spike or a crash keep you out of the gym. You are not a broken machine; you are a complex biological system that requires a little more calibration than others.

Embrace the "experimenter" mindset. When your blood sugar does something weird, don't get mad at it. Say, "That’s interesting data," and write it down.

Here are your starter instructions for your own Fitness Science Fair:

• Start Small: Don't change everything at once. Pick one variable (like temp basal timing) and test it for three workouts.
• Check Your IOB: Never start a HIIT class with a massive amount of active correction insulin.
• Pack the Essentials: Glucose tabs, a juice box, and a fast-acting carb like honey are non-negotiable.
• Consult the Pros: Before you start messing with your basal rates, sit down with your endocrinologist or a CDE (Certified Diabetes Care and Education Specialist). They can help you set safe boundaries for your experiments.

Every body reacts differently. What works for my "50% Rule" might be your "30% Rule." But with enough data points and a little bit of patience, you’ll find your Goldilocks setting. Now, get out there and start your own experiment!

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