Zone 2 Training and Hypoglycemia

Type 1 & LADA
1

I’ve been changing my workout routine to add more Zone 2 training. Previously, I spent too much time working hard, with 3 days 4x4, 3 days Zone 3 (threshold), and 2 days devoted to strength using weights, kettlebells, and plyometrics. I’ve swapped the Zone 3 sessions for longer Zone 2 workouts, and although I am enjoying it, I am finding the hypoglycemic hit during and after hard to handle.

Recently, on a one (1) hour slow rowing erg workout, keeping it in zone, but about 45 minutes in my blood sugar dropped precipitously, even though I thought I adequately compensated with enough pre-workout sugar; it started at 230. I then grabbed two (2) glucose (maltodextrin) packets, and although that worked for a while, I had repeated lows all afternoon.

I prompted Gemini in an open-ended way, “T1 diabetic, type 1, zone 2 training, blood sugar goes low”, and it responded credibly, but I wondered about others’ experience.

I will break the hour into 2 halfs, and fuel part of the way through, but any suggestions are welcome.

Prompt

  • T1 diabetic, type 1, zone 2 training, blood sugar goes low

Gemini Response

Managing blood sugar during Zone 2 training can be a delicate balancing act, as sustained aerobic exercise significantly increases glucose uptake by the muscles. Since Zone 2 relies heavily on fat oxidation but still consumes circulating glucose, the “slow burn” often leads to gradual but persistent drops.

Here are several strategies to help maintain stability during these sessions:

1. Adjusting Basal Rates

For many Type 1 athletes, the most effective way to prevent a mid-workout drop is to reduce insulin on board (IOB) before starting.

  • Timed Reduction: If using an insulin pump, consider setting a Temp Basal (often a 30–50% reduction) or activating “Exercise Mode” 60 to 90 minutes before you begin. Reducing it right at the start of the workout often leaves too much active insulin in your system.

  • The MDI Approach: If on multiple daily injections, some find it helpful to perform Zone 2 sessions when their background insulin is not at its peak, or they may discuss basal dose adjustments with their endocrinologist if training occurs daily at the same time.

2. The “Pre-Flight” Snack

If your blood sugar is trending flat or downward before a session, a small amount of targeted carbohydrates can create a buffer.

  • Slow-Release Carbs: Foods with a lower glycemic index (like a small amount of oatmeal or an apple with peanut butter) can provide a more sustained glucose release during steady-state cardio.

  • The “Safety Floor”: Aim to start Zone 2 with a slightly higher blood sugar “ceiling” than your resting target—typically around 120–150 mg/dL—to allow for the inevitable drift downward.

3. Intra-Workout Fueling

Rather than waiting for a low alert, many find success with micro-bolusing carbs throughout the session.

  • Liquid Nutrition: Sipping on a sports drink or using glucose gels every 20–30 minutes can offset the glucose being burned by the muscles.

  • Rule of Thumb: Start with roughly 15 grams of carbs for every 30–45 minutes of Zone 2 work and adjust based on what your Continuous Glucose Monitor (CGM) shows.

4. Monitoring Trends

The direction of the arrow on your CGM is often more important than the number itself.

  • Lag Time: Remember that CGMs can have a 5–15 minute lag behind actual blood glucose levels during exercise. If you see a diagonal down arrow, it is usually time to consume fast-acting carbs immediately, even if the current reading is “in range.”

5. The Post-Exercise Effect

Zone 2 training improves insulin sensitivity for hours afterward.

  • Delayed Hypoglycemia: Be mindful of the “tail” of the workout. You may need to reduce your post-workout bolus or keep a close eye on your levels 2–6 hours after finishing, as your body works to replenish glycogen stores.
2

@Eric is our local expert on this. He has helped several folks. I’m thinking especially of the thread where he worked with Daisy Mae on how to maintain good BG while swimming. Daisy Mae's swimming BG thread A few of the basics I remember are (1) enter the exercise with a good BG (maybe like 120 or 140), not a hugely elevated BG like your 230, and (2) no IOB at the start of exercise, because the muscles have a different path to vacuum glucose out of the bloodstream and IOB will compound the BG drain, causing a low. And (3) after the exercise the glycogen has been used up, so the body will try hard to rebuild the glycogen stores by using up BG. Don’t try to fight this, work with it by taking lots of fast carbs and a modest amount of insulin. When the glycogen stores are reestablished the tendency to post-exercise lows will cease.

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3

I am T2, but the effects of physical activities varies on the type.

  1. Resistance exercise generally is made up of series of repetitions if short intense effort and rests. This often causes a rise in BG during the exercise as a result of physical stress. The benefit of resistance exercise is increased muscle mass acts as a sink for glucose as it can hold more glycogen.
  2. Aerobic/cardiac endurance exercise in HR zones 2-3 uses up glucose and both muscle and liver glycogen at a slow but steady rate. This causes a regular drop in BG while exercising.

My favorite physical activity is bicycling. I have a Garmin bike computer that displays my Dexcom right on the screen. I use a Tandem T:Slim pump with control IQ. Because of the C-IQ my basal rate will drop to zero if the BG falls.

I can see BG on my Garmin clearly. Often I can raise it without consuming carbs by doing some interval sprints. That is resistance exercise.

  1. The 3rd type of physical activity is mixed. These are sports like basketball, soccer maybe even hockey which are typically a balance of resistance and aerobic.

The best information I have found on this is a book by Dr. Sheri Colberg, “The Athletes Guide to Diabetes.”Besides having a PHD in Exercise Physiology she is a T1. You might get more information at Diabetes Motion.

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4

@James.J.Igoe,
Can you tell me what your insulin regimen is?

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@Eric

I take 24 units of Tresiba in the morning, then as needed Humalog, usually about .5 units for 40 calories carb or 40 mg/dl. That said, the last Zone 2 workout was done mid afternoon without any prior Humalog that day. I walked off breakfast, so that after 60 to 90 minutes my blood sugar was dropping. When I had lunch to raise my blood sugar, the carb component was 1.5 cups of skim (0%) milk with a caramel sweetener, and a turkey sausage for protein. The caramel raises my blood sugar high enough so I can finish my workouts, but suggest away.

We usually go to sleep around 9:30 to 10 PM, and I have not experimented with injecting in the evening, but that would change my current peaks. Not sure it that would minimize the afternoon issue, without creating some other problems.

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Is that 40 calories of carbohydrate which would equate to 10 grams (4 calories per gram)? If so that would make your ICR 1unit for 20grams.

I am also curious as to what is your average total daily dose. The basal plus meal and correction boluses. It may be that your basal is too high, The nominal ratio is 50/50% but that doesn’t work for all. Mine tends to run 60% basal 40% bolus and I understand that this is fairly common. The opposite ratio sometimes works for others.

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7

Basal injections are tough for exercise because it doesn’t let you adjust for the exercise time. You can’t turn them off like you can for a pump.

So you have to either take the carbs before, and/or feed the basal throughout the workout.

Exact numbers are unique for everyone, you just have to try taking a certain amount of carbs before you start, and then adjust with more or less carbs until you figure out the right amount of carbs to start with. The before part is the tough part to figure out. The during part is much easier I think (as long as you can take in carbs while exercising).

Are you able to take carbs while exercising? I don’t know how hard it is with rowing. It’s pretty easy with running and cycling. If you can take carbs during, you don’t need to take as much up-front.

The other option would be to switch to a different basal insulin. For example, if you did Lantus instead of Tresiba, you could time your injection the day before to run out right when you are about to start your exercise. And could also take more or less from day-to-day depending on how hard you are plan on working, and you’d see that reflected on that day, instead of having it spread out over several days like it does with Tresiba.

As @bkh mentioned, no IOB is super helpful. Definitely shoot for having no IOB.

Sorry, I am probably not saying anything you don’t already know.

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8

Before and after starting MDI I was on Lantus and cycling 9-10 hours every week. I split the dose and would not take the AM dose until I did my ride.
I began this after a bad hypo on a short easy ride. Low BG makes my brain go stupid. I was swerving from edge to edge,thinking something
is wrong with my bike. I didn’t have a CGM , I stopped and my BG was 40mg/dl. I ate all my glucose tablets and other snack. It came up to 60, but I felt awful. I went in my cousin’s store and she was making a bacon sandwich for herself She knew I was in a bad way gave me hers . I after 30 minutes I was able to slowly mak it home. Splitting the dose stopped the exercise hypos and the ones in the middle of the night.

I love C-IQ

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9

Based on the response I got from Gemini, I was going to break my Zone 2 1-hour workout into two separate 30 minute workouts and then have one or two glucose packets at the separation point. During my workouts, I use a Polar heart rate monitor, its app, and the Concept2 ErgData app, and the app requires I set up the workout parameters beforehand, I wasn’t able to just stop midpoint. It’s a decision I have to make beforehand, but that is easy enough to change going forward.

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As both you and @CarlosLuis suggested, I could switch to Lantus, but I think that’s not really an option I want to consider.