Why does weightlifting raise blood sugar when cardio lowers it?

Hey Marcus, it is completely understandable to be frustrated by this! You are doing everything right by hitting the gym, only to check your CGM and see an arrow pointing straight up. It feels completely backward, especially when a simple walk or jog makes your numbers drop.

Here is the science behind what is happening: different types of exercise affect your body in very different ways. Cardio (aerobic exercise) uses a steady stream of energy, which makes your cells more sensitive to insulin and pulls glucose out of your bloodstream.

Weightlifting, sprinting, or CrossFit, however, are anaerobic exercises. They are intense physical stressors. When you lift heavy weights, your body goes into a mild "fight or flight" mode and releases stress hormones like adrenaline and cortisol. These hormones send an urgent signal to your liver, telling it to dump its stored glucose (glycogen) directly into your bloodstream to give your muscles quick, explosive energy to lift that heavy barbell.

Because you have diabetes, your body either doesn't produce the insulin needed to counteract that sudden liver dump, or it can't use it efficiently enough in that short window. The result? A frustrating post-workout spike.

Many lifters in the community manage this by experimenting with their routines, such as doing 15 minutes of light cardio at the end of their weightlifting session to help bring that adrenaline-induced spike back down. Keep at it—building muscle is an incredible tool for improving your long-term insulin sensitivity!