Some forms of exercise can cause an increase in your blood sugar because it releases glucose-raising hormones.

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Regular exercise is a cornerstone of healthy living and managing diabetes. Yet the conversation around exercise with diabetes is often filled with angst. This is especially true when exercise unexpectedly causes our blood glucose (BG) levels to spike.

“I thought exercise was supposed to bring my glucose levels down!” Is a common lament. Often followed by “What did I do wrong?”

This unanticipated outcome from exercise can be discouraging, particularly for people with insulin-treated type 1 diabetes (T1D). It might even leave you wondering if exercise is worth the effort to “get it right.”

So, what is going on when exercise causes your BG to rise, rather than fall? And how can you manage this, in order to benefit from and enjoy working out?

The short answer is that your body is doing what it is designed to do. But the mechanics behind that can be hard to understand.

The first-ever official guidelines for safe exercise were published in The Lancet journal in 2017. And more recently, in 2020, experts released an international position statement on glucose management for exercise using continuous glucose monitoring (CGM) in type 1 diabetes.

These guidelines note in particular that “weight lifting, sprinting, and intense aerobic exercise can promote an increase in BG that can last for hours,” but there’s little explanation of why this happens. And overall, the information can be overwhelming and hard to follow.

So, DiabetesMine turned to several experts in diabetes and exercise to help explain what’s going on here.

“It’s critical for your brain and your nervous system to have access to blood glucose at all times. For that reason, the body has redundant hormones that raise BG, like glucagon and adrenaline,” explained Sheri R. Colberg, PhD, professor emerita of exercise science at Old Dominion University in Norfolk, Virginia, and creator of “What happens with exercise is that glucose-raising hormones are released to help pump up the amount of BG being released to match what your active muscles are using.”

Colberg admits, “The system isn’t perfect, though, and doing intense activity causes an exaggerated release of these hormones. So, when someone does intense but short activities, BG often rises due to releasing too many hormones.”

Ginger Vieira, advocate, author of several instructional diabetes books, and current digital content manager at Beyond Type 1, called on her experience as a former health coach and competitive powerlifter and Ashtanga yoga instructor to describe the effect of some of the most common mechanisms for glucose spikes during intense exercise: lactic acid, adrenaline, and fasted exercise.

Lactic acid. The process of gluconeogenesis converts lactic acid into glucose and cycles that glucose back to your muscles for fuel,” said Vieira. “This is how the body provides your muscles with fuel when you’re working too hard to cycle oxygen and glucose to your cells as your body would during general aerobic [cardio] exercise.”

Adrenaline. As commonly happens when participating in competitive sports, “your body releases adrenaline for that ‘fight or flight’ burst of energy,” described Vieira. “Adrenaline tells your liver to release stored glucose in the form of glycogen to provide the extra fuel it needs for the ‘fight’… or the soccer game. This can easily spike your blood sugar over 100 points.”

Fasted exercise. Exercising on an empty stomach can lead to a glucose spike, especially right after waking up. That’s because exercise can further exaggerate what’s known as the dawn phenomenon, when in the early morning hours, “your liver is releasing stored glucose along with morning hormones, to give your brain the fuel it needs to function,” explained Vieira.

Clearly, many mechanisms can cause a spike in glucose levels during exercise. No wonder, it can be so difficult to know what to do to bring glucose levels back down.

One of the first things you might ask is if there are “good” and “bad” exercises for people with diabetes… as in “maybe I should just avoid the ‘bad’ ones.”

Christel Oerum, certified personal trainer and founder of Diabetes Strong and Diabetic Foodie, offered an alternative way to look at this question. “Think about it like this: Your body just wants to help you out, it wants you to be successful. So, when you do certain types of workouts, predominantly anaerobic exercises, your body tries to ensure that you have the energy to be successful. It does this by releasing hormones that allow energy, in the form of glucose, to be released into your bloodstream. And that can raise blood sugars.”

This response is not unique to people with diabetes. Vieira confirmed that “In a nondiabetic body, the exact same process is happening, but their bodies produce extra insulin to deal with the extra glucose.”

“Just because blood sugar levels rise during certain types of exercise doesn’t mean they are bad exercises or that the rise is happening for a bad reason,” Vieira added. “This is the body’s normal reaction to several factors that can occur mostly during anaerobic exercise — like weight lifting, sprinting, spinning classes, competitive moments, etc.”

Since it’s anaerobic exercise that causes BG spikes during activity, you might think that just avoiding sprints, resistance training, or other anaerobic activities could be the answer.

“But that would be a shame, as resistance training is fantastic for diabetes management,” noted Oerum. “Most people will see their insulin sensitivity increase afterward, and most often blood sugars will come down by themselves.”

Oerum suggests combining anaerobic with aerobic exercises. This approach will balance the effects and typically make BG levels come down soon after the exercise session is done.

Of course, if your exercise objective is to bring your BG levels down immediately, then aerobic exercise like walking, swimming, or skipping rope is going to be the effective choice.

Ultimately, it is the presence of insulin that determines when and how quickly BG levels come down.

So, try to assess the situation in terms of your insulin intake, or insulin on board (IOB). Maybe you didn’t take enough insulin to cover a meal prior to your exercise session, or maybe you are working out soon after waking up, when IOB is at its natural low point.

BG spikes caused by bursts of adrenaline can be hard to anticipate, as they happen most often smack in the middle of a an exercise session. This means that rather than treat the spike immediately, you most likely will need to wait and take additional insulin after the fact.

More insulin is also needed when the spike results from fasted exercise. Some additional insulin will be needed, but not so much that it leads to a hypoglycemic episode during or after exercise.

Unfortunately, there are no hard and fast rules for making these insulin dosing adjustments. Each situation for each person will require an individualized response. It’s best to work with your medical team to determine the best response for you.

That being said, both Vieira and Oerum suggest taking notes and tracking your experience so that you can learn from your experiences. You may find that for you personally, particular activities have a predictable BG spike effect. Over time you can develop a routine that allows you to both get the exercise you need and anticipate those frustrating spikes.

Many people who wear an insulin pump learn to use customized “temporary basal” settings to increase (or lower) background insulin during specific workout routines. This can help offset the spike so that you don’t have to treat with a huge bolus dose of insulin afterwards.

You can also experiment with your own ideal “starting glucose level” before kicking off exercise. The 2017 guidelines give the general recommendations of “at-target” levels of 126 to 180 mg/dL, and to consume 10 to 20 grams of fast-acting glucose before getting started. You’ll have to monitor your own experience to learn what’s ideal for you.

Once you understand why BG levels spike during exercise, and accept that this is not necessarily a bad thing, you will hopefully notice a mental shift, away from being frustrated and disappointed toward appreciating what you can do in response.

While there is no one-size-fits-all guidance, know that over time you can build an exercise routine that includes small amounts of glucose and insulin dosing that keeps your BG levels manageable.