High-intensity exercise can paradoxically raise blood glucose, even though moderate physical activity typically lowers it. This temporary spike occurs because intense effort activates the sympathetic nervous system, releasing catecholamines that stimulate the liver to rapidly produce and release glucose through glycogen breakdown and gluconeogenesis. During very hard exercise, hepatic glucose output can exceed the muscles’ capacity to use it, causing blood glucose to rise before it eventually falls as insulin sensitivity improves during recovery. In people without diabetes the spike is usually brief and self-correcting, but in individuals with Type 1 diabetes—who lack the endogenous insulin response that normally reins in the surge—the rise can be more pronounced and longer lasting. Understanding the physiology behind this response allows athletes, coaches, and clinicians to anticipate it and apply practical strategies in training and recovery to help moderate post-exercise glucose elevations.

https://brokenscience.org/the-paradox-of-working-hard-and-going-high-on-the-glucometer/