Limiting Low and High Blood Sugars During and After Exercise


Exercise can be amazing, empowering, and uplifting, but it can also sometimes be a frustrating experience. If you live with diabetes and your blood sugars just won’t cooperate when working out, you know what I’m talking about.

Not cool! But can you imagine a world where you felt comfortable exercising, knowing that your exercise, food, and insulin choices lead to a limited risk of low or high blood sugar during and after your workout?

I’ve been living with type 1 diabetes since 1997, and I believe that we can achieve that if we have the right tools and the right knowledge and information. In this post, I’ll share some of the methods I use myself and with my clients to achieve optimal blood sugar control during and after exercise.

The Basics of Exercise and Blood Sugars

Knowing the basics of how different types of exercise affect your blood sugars is the first step to successfully mastering blood sugars and exercise.

If you have already started seeing patterns, and have a good idea about which types of exercise make your blood sugar drop and which make them soar, that’s great! That knowledge will set you up for success.

But maybe you are new to it all or haven’t seen any patterns yet, and need some pointers on where to start and what to be aware of. That’s what I will try to provide in this post.

In its most basic form, there are two types of exercise, aerobic and anaerobic.

  • Aerobic exercise is characterized by a steady elevated heartbeat that you’ll see from jogging, dancing, flat terrain biking and walking. Most people will see a drop in their blood sugar when engaging in this kind of activity.
  • Anaerobic exercise is characterized by bouts of intense movement that will have your heart rate fluctuating. Types of exercise that are labeled anaerobic are weight lifting and HIIT training (high-intensity interval training). As opposed to aerobic exercise, these types of workouts can make blood sugars skyrocket during exercise with the risk of it plummeting hours after.

Notice that I’m not saying this will always happen for everybody. There are individual differences and it also depends on the intensity of your workout.

Strategies for Blood Sugar Management during Exercise

Now that we know the basics of exercise and blood sugar, the question is how we use that knowledge to successfully manage blood sugars and limit the risk of hypoglycemia.

There are two ways to manage blood sugars during and after exercise: insulin tapering and eating carbohydrates. That sounds simple enough, but it can, of course, be hard to get right.

Aerobic Strategies

When it comes to aerobic exercise that lasts up to an hour, my approach is to adjust my insulin level, not to eat more carbs. I don’t see a need for “carbing up” unless you are doing serious endurance sports (like a half marathon).

Since insulin or insulin on board (IOB) is the determining factor for whether or not your blood sugars will drop, doing aerobic exercise first thing in the morning can be beneficial. That’s the time of day when you have the least IOB and you might have hormones raising your blood sugars as well, so this is the time of day you are least likely to experience exercise-induced hypoglycemia. You may still have to make a small adjustment to your insulin, but many people (myself included) can exercise without any adjustment if they are fasting.

If you exercise later in the day, you’ll have to adjust your insulin. If you use insulin pens, you’ll have to adjust any bolus you take in the period 3-4 hours prior to your workout. Same applies if you’re on an insulin pump, but with a pump, you can also do a temp basal rate before and/or after your workout. How much of an insulin reduction you’ll need will depend on you, but more on that later in the post.

Anaerobic Strategies

Since most people see an increase in blood sugars when doing anaerobic exercise and, potentially, a drop later, anaerobic exercise sessions require a different strategy for blood sugar management.

To prevent blood sugars from spiking, I recommend eating a small meal of carbs and protein before an anaerobic workout accompanied by a (potentially) reduced bolus. If eating before your workout is uncomfortable or interferes with your workout, you can just do a small bolus or basal increase without any food.

I recommend being proactive when it comes to preventing exercise-induced hyperglycemia since running high during a workout will affect your performance negatively, and the increased insulin sensitivity after a workout makes correction doses a little tricky.

Even if you nailed your blood sugars during your workout, be aware that exercise can affect your insulin sensitivity for up to 48 hours after you’re done exercising, so you may need to reduce your insulin for that whole period.

How to Find Your Formula for Food and Insulin around Workouts

We know that most of us will react to different kinds of exercise in a specific way (aerobic = blood sugar drop), but how much we’ll need to adjust our insulin or carbohydrate intake depends on the individual.

Figuring out how much of an adjustment to make is what I describe as “finding your formula”.

Although having access to a CGM will speed up the process of finding your formula, you don’t need more than a pen, paper and your meter to be successful. If you’re looking for a bit more structure, you can download a tracking sheet in this post on TheFitBlog.

My guidelines for finding your formula are:

  1. Reduce the number of variables: do the same exercise routine for the first week or so. If you can also keep your active insulin on board when exercising the same, even better, but it’s not a requirement
  2. Check, check, check: as a minimum, you will have to check your blood sugar before your workout, right after, and 60-90 min after
  3. Accept missteps: it’s most likely that you won’t get it right every time, and that’s OK. Keep glucose tabs around and learn from your mistakes

Write down all your findings, start looking for trends, and develop a hypothesis that you can test. You may only need a few data points to start seeing patterns. Then you can start being proactive and limit those exercise-induced highs and lows.

Have fun!!

Christel is a Los Angeles based blogger, certified personal trainer, and diabetes advocate. She has been living with type 1 diabetes since 1997 and at an early stage decided that it wasn’t going to slow her down. Her motto is “There is Nothing You Can’t do With Diabetes”. She writes about Health, Fitness and how to be Fit With Diabetes on She also trains people with diabetes from across the globe, online and in person, and supports them in meeting their health and fitness goals.

Christel holds a MBA in Finance & Strategy and an ISSA Personal Trainer certification with specialization in Fitness and Diabetes (Level 3 certified from the Diabetes Motion Academy).


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