A spike during exercise, especially during high intensity exercise, is common and indicates your body is mobilizing fuel to your hard working muscles. Your body responds to the stress of the exercise with a surge of adrenaline and cortisol, resulting in your liver converting glycogen stores into glucose. As glucose is released into circulation more quickly than the muscle can absorb it, glucose levels rise.
High intensity exercise, especially prolonged high intensity exercise can result in glucose values >200 mg/dl (>11.1mmol/L) even in someone who is otherwise healthy. Such levels are often observed by elite athletes including cyclists and marathon runners who use a CGM to monitor their fuel stores and reach for high performance.
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The references below show a marked glucose response to high intensity and prolonged exercise.
Marliss EB, Vranic M. Intense exercise has unique effects on both insulin release and its roles in glucoregulation: implications for diabetes. Diabetes. 2002 Feb;51 Suppl 1:S271-83. doi: 10.2337/diabetes.51.2007.s271. PMID: 11815492. https://pubmed.ncbi.nlm.nih.gov/11815492/
Flockhart M, Larsen FJ. Continuous Glucose Monitoring in Endurance Athletes: Interpretation and Relevance of Measurements for Improving Performance and Health. Sports Med. 2023 Sep 2. doi: 10.1007/s40279-023-01910-4. Epub ahead of print. PMID: 37658967. https://pubmed.ncbi.nlm.nih.gov/37658967/
• This paper also observed a heightened glucose response to meals following high intensity exercise.
• Heightened glucose response to exercise can also be seen in the absence of high intensity if there is another associated stress such as performance anxiety or illness.