Cardio Vs Weights: A User’s Guide to Knowing What Will Happen to Your Blood Sugar

Diabetes and Exercise Effects

Oftentimes, exercise can be treacherous–inciting low blood sugars immediately after, then the resulting high blood sugars, and leaving us wondering why we signed up to exercise in the first place. It’s time to provide clarity to something we all need and crave in our lives: movement. Matching personal experience with research, I’ll provide a few generalizations that will help you traverse the exercise path with optimum, never perfect, glucose management.

What’s the rundown on cardio exercise?


Before we jump into the specifics, let’s point out the main trend involving cardiovascular activity and blood sugar: well, it makes it drop. Cardio is any activity that requires the body to keep the heart rate up for an extended period of time.

Let’s take biking and running for example. (Okay, I hear you. Walking counts too. I love walking. It does not have the same dramatic effects needed for this illustration. Certainly, it can lower blood sugar too.) You hop on your bike, ride at a good pace for an hour, and your blood sugar drops from 165 to 90, pre to post ride. You lace up your shoes, go for an hour long jog, subsequently dropping from 165 to 65.

This is typical. Without adjustments in insulin, medication, or pump settings, this will usually happen. Why? Your muscle cells need energy and glucose is readily available in the blood to pull from and your glucagon/liver system doesn’t work.

Preparation for going low during or after needs to be part of your plan. Don’t be surprised.

How long have I seen the effects of cardio last? Depending on the intensity and length of the workout, I would say anywhere between 2-6 hours.



What’s the rundown on lifting weights?

Some discount the effects of weights on blood glucose. I would argue that it has an even more complex effect on blood sugar. Anyone who knows me knows that I love running, cycling, and yoga. None of which fall in this category. Due to chronic hamstring issues, mainly because of less than diligent rehab on my part, I have been forced back to the gym, withholding cardio anything.

Tuesday was my first day back. I worked out, a relatively intense upper body workout, for roughly 45 minutes. My blood sugar remained unaffected for the next hour after. Then, it stabilized. I wasn’t having to take as much insulin. The effects lasted for the next 24 hours. Over that 24 hours, I took 10 units of insulin less than average.

Ready for the surprise? Weight lifting, especially high intensity weight lifting (think CrossFit), can actually cause a sharp blood sugar spike upwards. How is that possible? Research by the Canadian Journal of Diabetes indicates that anaerobic exercise starts up the stress response, causing high blood sugar due to epinephrine, norepinephrine, and our friend going way back, cortisol.

What’s the real takeaway here?


Cardio usually causes blood sugar to go down. Weight lifting has longer lasting effects, including stabilization. High intensity weight lifting can even cause blood sugar to go up.

3 thoughts on “Cardio Vs Weights: A User’s Guide to Knowing What Will Happen to Your Blood Sugar

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