Managing Blood Sugar During Exercise: Part 1

‼️DISCLAIMER: although I am a healthcare professional, this post is not intended to be medical advice. This is simply me sharing some of what I know, but your body may not respond in this way so please make sure you are chatting with your diabetes educator and/or doctor first ‼️ or bring me onto your care team 😘

Exercise is one of the most effective ways to improve overall health and blood sugar management, especially for those with Type 1 diabetes (T1D). However, for individuals managing T1D, physical activity can feel like walking a tightrope—balancing exercise intensity, insulin management, food intake, and blood sugar levels. Whether you’re hitting the gym for strength training or going for a jog to get your cardio in, understanding how to navigate blood sugar before, during, and after exercise is key to managing your diabetes safely and effectively.

In this guide, we will break down how to manage your blood sugar around exercise, with particular focus on the differences between cardio and strength training, and provide detailed steps for a safe workout routine that helps prevent high and low blood sugar levels during activity.

Why Exercise Affects Blood Sugar

When you engage in physical activity, your body undergoes a series of physiological changes that affect your blood sugar. Both anaerobic (strength training, HIIT) and aerobic (cardio) activities impact your blood glucose in different ways.

• Aerobic activities (e.g., walking, swimming, jogging): These activities tend to lower blood sugar because they use glucose for energy during the workout. Aerobic exercise can enhance insulin sensitivity, meaning your body becomes more efficient at using the insulin it has available to lower blood sugar levels.
  
  
• Anaerobic activities (e.g., weight lifting, HIIT): In contrast, strength training and intense interval exercises may initially raise blood sugar. This happens because these types of exercise trigger the release of adrenaline and other stress hormones that temporarily signal the liver to release more glucose into the bloodstream.
  
  

As you plan your workouts, understanding these differences will help you make adjustments to your insulin, food intake, and hydration strategy to keep your blood sugar in a healthy range.

Pre-Exercise Guidelines for Optimal Blood Sugar Control

1. Two Hours Before Exercise: Check Your Blood Sugar

Before you engage in any physical activity, it’s essential to check your blood sugar levels. Here’s what to look for:

• If your blood sugar is high: If your blood glucose is elevated (over 250 mg/dL or more), exercise might exacerbate the problem, especially if you have ketones present. Consider taking about half of your usual correction dose, but make sure you check for ketones first. If ketones are present, avoid exercising until the situation is under control.
  
  
• If your blood sugar is within range: If you’re in range, insulin may not be necessary. If you take insulin during the day, your insulin levels may still be working well in the background, especially for those with more stable blood sugar control.
  
  

2. One Hour Before Exercise: Snack or Adjust Insulin

• If you’re planning to eat a pre-workout snack: If you're in range and typically snack before exercise, a light snack that contains both carbs and protein can help stabilize blood sugar. Think of something like fruit and nuts, a granola bar with protein, or toast with peanut butter. A small snack can help maintain steady blood sugar levels during your workout without causing a spike or dip.
  
  
• If you’re fasting or usually spike during exercise: Consider taking a small dose of insulin 20 minutes before your workout to prevent a sharp rise during exercise. In some cases, reducing basal insulin by 30–50% can help avoid insulin stacking that leads to a low post-workout.
  
  

3. Hydration

Staying hydrated is crucial. Dehydration can cause blood sugar to rise, particularly when you're sweating a lot during exercise. Make sure to drink water, especially if you’re working out outdoors or for long durations. In cases of heavy exercise or sweat loss, electrolytes may help keep your hydration in check.

Adjusting Your Blood Sugar Strategy Based on Exercise Type

A. Managing Cardio (Aerobic Exercise)

Cardio exercise, such as running, cycling, or swimming, can cause a significant drop in blood sugar, particularly when insulin is on board. Here’s how to prepare for a cardio workout:

• Pre-Cardio Adjustments: About one hour before engaging in aerobic activity, you may want to reduce your basal insulin dose by 30–50%. This helps prevent a sudden drop in blood sugar during or after the exercise.
  
  
• During Cardio: For longer cardio sessions, consider having carbohydrate support during the workout. Start with about 8–12g of carbs every 20–40 minutes. This can be anything from a small handful of fruit to glucose tablets. Pay attention to how your body reacts and increase the carbs if your blood sugar starts to drop too low.
  
  
• Post-Cardio: After cardio, your blood sugar may continue to drop for a few hours, especially if the workout was intense. Be sure to consume a recovery meal or snack that includes carbs and protein (aim for 15–45g of carbs and 20–30g of protein) to help with muscle recovery and keep blood sugar stable. Some people find that reducing their insulin dose by 30-40% after cardio can help avoid low blood sugars during the next few hours.
  
  

B. Managing Strength Training (Anaerobic Exercise)

Strength training, which includes activities like weightlifting or high-intensity interval training (HIIT), tends to raise blood sugar temporarily due to the body’s natural response to stress.

• Pre-Strength Training Adjustments: Unlike cardio, you may not need to adjust your basal insulin before strength training. However, be mindful of the stress hormones released during anaerobic exercise that can elevate blood sugar. If you are insulin sensitive or know you tend to spike during HIIT, you may want to consider a small insulin dose (10–20%) about 20 minutes before the workout to keep your blood sugar from rising too high.
  
  
• During Strength Training: Anaerobic exercises typically stabilize blood sugar or even elevate it slightly. This is especially true if your body is using stored glucose from the liver to fuel your muscles. If your blood sugar starts high, try to keep an eye on it, but you may not need carbs unless the blood sugar is rapidly dropping.
  
  
• Post-Strength Training: After anaerobic exercise, check your blood sugar within 1–2 hours. If it’s rising, you might not need extra insulin, but be cautious and keep an eye on your trends. For those who tend to experience delayed highs after lifting, a small bolus insulin dose may be helpful.
  
  

Insulin Pump-Specific Guidance

If you use an insulin pump, the device can be a helpful tool for managing blood sugar around exercise. Many pumps have an "Activity Mode" or "Exercise Mode," which temporarily reduces your basal insulin delivery.

• Pre-Exercise: If you know that certain types of exercise (like cardio) tend to drop your blood sugar, you can enable the activity mode about an hour before your workout. This will help to lower your basal insulin and prevent a hypoglycemic event during exercise.
  
  
• Post-Exercise: If you find that your blood sugar remains low hours after a workout, leave the activity setting on your pump a bit longer. This may help prevent delayed lows and give your body a little extra time to stabilize.
  
  

The Importance of Tracking

Managing blood sugar around exercise can be a tricky balancing act. Keep a detailed record of your exercise, food intake, insulin adjustments, and blood sugar readings. Over time, you’ll start to see patterns emerge and can fine-tune your approach to your specific needs.

• Track your blood sugar before, during, and after exercise. This will help you spot trends and adjust your routine accordingly.
  
  
• Track your insulin doses. Experiment with adjusting your basal and bolus insulin based on the type and duration of exercise. For example, you might find that cardio requires a greater reduction in basal insulin than strength training does.
  
  
• Record your meals and snacks. Knowing how different foods affect your blood sugar during exercise can help you plan better. Avoid heavy, fatty meals before exercising, as they can slow down digestion and cause discomfort during the workout.
  
  

Final Thoughts: Experiment and Customize Your Routine

The key to successful blood sugar management during exercise is trial and error. Everyone’s body responds differently to insulin, food, and exercise. What works for one person may not work for another. Start by testing small adjustments and tracking everything carefully. Over time, you’ll learn how your body reacts and how to make exercise a consistent part of your routine without sacrificing blood sugar control.

Remember, exercise is one of the most powerful tools for improving overall health and blood sugar control. With some strategic planning, you can enjoy the benefits of physical activity without the added stress of unpredictable blood sugar swings.

You’ve got this.

Stay fun,

Madi Cheever, MPH, RD, LDN, CHES
Type One Type Fun

‼️DISCLAIMER: although I am a healthcare professional, this post is not intended to be medical advice. This is simply me sharing some of what I know, but your body may not respond in this way so please make sure you are chatting with your diabetes educator and/or doctor first ‼️ or bring me onto your care team 😘