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• Blood sugar levels rise and fall in response to exercise for a variety of reasons. 
• Monitoring blood sugar with a continuous glucose monitor (CGM) during exercise can prevent hypoglycemia and hyperglycemia.
• Keeping an eye on a CGM’s trending arrow can help people complete their full workout.  

For some people, exercise is already numbers-focused: time on a treadmill, mileage on a bike, and pounds on dumbbells. Add reps, sets, and rest times to the mix, and there’s a mile-long list of numbers to keep an eye on. 

What happens when input from a continuous glucose monitor (CGM) joins the mix?

One Drop health coach, Lindsay Vettleson, registered dietitian (RD), certified diabetes care and education specialist (CDCES), believes it’s a great thing, especially for people with type 1 diabetes or type 2 diabetes who are on insulin. 

“There’s a huge fear of hypoglycemia [during exercise] among people who are on insulin,” says Vettleson. “So they avoid exercise or minimize their intensity. That means they may not push themselves to a level that they could. A CGM can not only give people their reading, but it allows them—thanks to trend arrows—to see where their blood sugars are going.” 

Factors that Impact Blood Sugar During Exercise

A person’s blood sugar may rise or fall in response to exercise depending on the following:

• Blood sugar level before starting exercise
• Length of exercise session
• Changes to insulin doses 
• Type of exercise (intensity of the activity)

For example, aerobic exercise such as walking, biking, and swimming usually decreases blood sugar levels. Anaerobic exercises such as sprinting and lifting weights often raise blood sugar so that it can match the high energy of your active muscles. Both aerobic and anaerobic exercises are safe to do as long as you are monitoring your blood sugar and making adjustments along the way. 

“It’s extremely important to know where your blood sugar levels are prior to exercise, and how they’re trending, which is where a CGM comes in. If your blood sugar is over 250 mg/dL, don’t start exercise,” cautions Vettleson. “Give yourself some insulin. If you’re below 90 mg/dL, make sure to have some carbs prior to starting. That’s because if you’re at 90 mg/dL and your CGM’s trend arrow is going down, you’re more than likely going to crash and burn during exercise without eating something.”  

A good rule of thumb for out of range numbers prior to exercise is to wait 15-30 minutes after taking insulin (in the case of a high) or eat something (in the case of a low) to make sure your blood sugar is in its target range in order to complete a workout safely. 

How Often to Check Your CGM During Exercise

As for using a CGM during exercise, Vettleson suggests checking it every 15-30 minutes. That’s because it can be hard for some people to distinguish between symptoms of a low (shakiness, sweating, and lightheadedness) and common feelings from a hard workout itself.

While watching blood sugar readings and trend arrows is important, Vettleson says there’s no substitute for checking in on how you actually feel. 

“Sometimes a CGM might not match how you’re feeling. Always check your glucose with a finger stick if you’re unsure,” says Vettleson. “There can be a lag time on CGMs with blood sugar readings in general. During exercise, there’s an intensified lag time due to an increase in blood flow and body temperature. If you’re feeling symptoms of a low, it might not register on a CGM until later, but if you check with a finger stick, it will register as low and you’ll know right away. Listen to your body. Not just what a CGM says. Then, check with a finger stick.” 

As for after exercise, Vettleson advises people to check their CGM regularly for 90 minutes following a workout. Because delayed onset hypoglycemia (DOH) can be fairly common, some people may need to adjust their long-acting basal. 

“That’s where a CGM is really helpful because it shows us trends, and can catch any surprising post exercise effects,” says Vettleson. “Then, when you see patterns over time with how your blood sugar responds before, during, and after certain types of exercise, it can help you prevent problems before they happen.”  

That’s what it’s all about: leaving the sweating in the gym. 

Curious to learn more about exercising with a CGM? Connect with your own dedicated certified diabetes care and education specialist (CDCES) through the One Drop Premium app. Get one-on-one health and wellness coaching from someone who really cares. 

This article has been clinically reviewed by Lisa Graham, RN, CDCES, clinical health coach and director of clinical operations at One Drop.