Why Your Client's Glucose Rises During High-Intensity Exercise (and Why That's Not a Problem)
A client brings you their CGM data from last Tuesday's training session. Their glucose was sitting around 88 mg/dL at the start of the workout. Forty minutes into a high-intensity interval training session, it had climbed to 145 mg/dL. They want to know if something is wrong.
Nothing is wrong. What they are seeing is a well-documented, normal physiological response to high-intensity exercise. Understanding why this happens and explaining it clearly are among the most practically useful things a personal trainer, physical therapist, or health coach can do for a CGM-wearing client.
The Physiology of Glucose During Exercise
Working muscles need fuel. During moderate-intensity aerobic exercise, muscles primarily use fat and glucose, and the glucose demand is met through a combination of hepatic glucose release (the liver sending glucose into the bloodstream) and the uptake of circulating glucose by insulin-sensitive muscle tissue. This balance typically results in a gradual decrease in blood glucose during an aerobic session, which is why low-intensity, longer-duration exercise is often associated with glucose-lowering effects.
High-intensity exercise changes this dynamic significantly. At higher intensities, the sympathetic nervous system activates, triggering the release of catecholamines, primarily epinephrine and norepinephrine. These hormones stimulate the liver to release large amounts of stored glucose through glycogenolysis and gluconeogenesis. Simultaneously, glucagon rises, and insulin secretion is suppressed. The net result is a rapid increase in circulating glucose that can exceed the rate at which exercising muscles are consuming it, producing the visible spike on a CGM trace. (Marliss EB & Vranic M. Intense exercise has unique effects on both insulin release and its roles in glucoregulation. Diabetes. 2002;51(Suppl 1):S271-S283.)
This is not pathology. It is the body's fuel delivery system doing exactly what it is designed to do during high-demand activity.
What the CGM Trace Looks Like During and After High-Intensity Training
During a high-intensity training session, glucose often rises noticeably within the first 15 to 30 minutes and may continue to climb throughout the session. The peak typically occurs near the end of the session or shortly after. Following the training session, glucose gradually declines as the liver stops overproducing glucose, insulin sensitivity increases (a well-established post-exercise effect), and muscle glycogen replenishment draws glucose out of circulation.
Post-exercise glucose decline can be significant and prolonged, particularly in people with higher fitness levels. This is one reason why post-exercise snacking and recovery nutrition timing matter for clients managing both athletic performance and metabolic health. It is also one reason why clients on insulin or certain diabetes medications require specific monitoring protocols around exercise, a clinical consideration that is outside the scope of a fitness coach but important to be aware of for referral purposes.
Why This Is the Most Commonly Misread Exercise Pattern
The glucose rise during high-intensity exercise is the most commonly misread exercise pattern on a CGM trace because it looks alarming to someone without a systematic framework. A client or coach who sees a line climbing from 88 to 145 mg/dL during a workout and does not understand the mechanism will often assume something is wrong with the client's metabolic health.
This misread has real consequences. Some clients respond by reducing exercise intensity to avoid the spike, which actually removes one of the most metabolically beneficial forms of training. Others become anxious about a normal physiological response. And some coaches, not wanting to say the wrong thing, avoid the conversation entirely, which leaves the client without the information they need to understand their own data.
A fitness professional with a systematic understanding of exercise-related glucose patterns can short-circuit all of these misreads in one conversation.
How to Have This Conversation with a Client
The client conversation for this pattern is not complicated. It requires explaining that the glucose rise during high-intensity exercise is the liver's normal response to high-demand activity, that the body is doing exactly what it should, that post-exercise glucose will naturally decline as recovery proceeds, and that the magnitude of the rise is not the same as the elevated fasting or post-meal glucose that warrants clinical concern.
The coaching conversation can then move to what the client can actually influence: recovery nutrition timing, workout structure, and how different session types produce different glucose signatures. That is a professional, informed, scope-appropriate conversation that adds real value to the coaching relationship.
When Exercise-Related Glucose Patterns Warrant Referral
Most exercise-related glucose responses in otherwise healthy clients are within the coaching conversation. There are specific patterns, however, that require physician attention rather than a coaching response. Glucose that drops to unsafe levels during or after exercise is one of them. Any pattern that suggests impaired glucose regulation requiring clinical management is another. A systematic CGM framework for fitness professionals clearly specifies these referral thresholds, allowing a trained professional to coach confidently within their scope while also knowing exactly when to step back.
Want a systematic framework for every exercise-related glucose pattern? Download the free CGM Cheat Sheet for Coaches with code CHEAT.
Sources: Marliss EB & Vranic M. Intense exercise has unique effects on both insulin release and its roles in glucoregulation. Diabetes. 2002;51(Suppl 1):S271-S283. Colberg SR et al. Exercise and type 2 diabetes: the American College of Sports Medicine and the American Diabetes Association joint position statement. Diabetes Care. 2010;33(12):e147-e167.
