Some forms of exercise can release glucose-raising hormones, causing an increase in blood sugar. Measuring glucose levels, eating before exercising, and tracking changes can help prevent this spike.
Regular exercise is a cornerstone of healthy living and managing diabetes. Yet the conversation around exercise with diabetes is often filled with anxiety. 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
The short answer to why exercise raises BG is that your body is doing what it’s 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
These guidelines note, in particular, that “weightlifting, 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.
Overall, the information can be overwhelming and hard to follow.
Several experts in diabetes and exercise offered their guidance to help explain this issue.
“It’s critical for your brain and your nervous system to have
“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.”
Diabetes advocate and longtime type 1 Ginger Vieira, author of several instructional diabetes books, called on her experience as a former health coach, competitive powerlifter, and Ashtanga yoga instructor to describe the effects 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,” Vieira said. “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,” Vieira said.
Clearly, many mechanisms can cause glucose levels to spike during exercise. No wonder it can be so difficult to know what to do to bring them back down.
One of the first things you might ask is if there are “good” and “bad” exercises for people with diabetes.
Christel Oerum, a certified personal trainer and founder of Diabetes Strong, offered an alternative perspective on 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,” Oerum explains. “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 weightlifting, sprinting, spinning classes, competitive moments, etc.”
Since anaerobic exercise 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 and aerobic exercises. This approach will balance the effects and typically lower BG levels soon after the exercise session is over.
Of course, if your exercise objective is to bring your BG levels down immediately, aerobic exercise like walking, swimming, or skipping rope can be a more effective choice.
Ultimately, it’s the presence of insulin that determines when and how quickly BG levels come down.
This is the first key in addressing glucose changes tied to exercise.
Understand your insulin needs
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’re working out soon after waking up, which is 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 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 may be 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.
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.
Using an insulin pump
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 afterward.
You can also experiment with your ideal “starting glucose level” before kicking off exercise. Research from 2019 recommends “at-target” levels of 7.0 to 10.0 mmol/L before starting. You’ll have to monitor your own experience to learn what’s ideal for you.
Start with a lower blood sugar level
This may be a more challenging point, and you should always consult your diabetes care team to best understand how exercise may affect your glucose levels and what the best target range may be for you when exercising.
Some people with diabetes who take on exercises they know may raise their blood sugar levels, start at a point where they are a bit lower. This can be difficult, especially for those using insulin, because hypoglycemia is a danger whenever exercise is factored into the equation.
However, if you use a continuous glucose monitor (CGM) and keep close track of your glucose levels before and during exercise, you may be able to help avoid any higher blood sugar spikes from exercise.
Consult your healthcare teamYour doctor and diabetes care team is best equipped to guide you on the effects exercise could have on your blood sugar levels and diabetes management overall. This can include target blood sugar ranges during varying types of exercise, and how you may want to manage your condition at these times.
You may find that 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.
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’s 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 to keep your BG levels manageable.