Why glucose rises during exercise—and how to plan fuelling and recovery
Hard exercise increases glucose supply. Understand the liver, working muscle and glycogen recovery before adjusting your fuelling.
You finish a hard interval session, open your glucose record and find a rising line through the hardest repetitions. Exercise uses glucose, so why did the concentration increase? The answer involves what the liver was supplying, what the muscles were using, and how quickly each process changed.
Why hard exercise can raise glucose
Blood glucose reflects the balance between glucose entering the circulation and glucose being taken up by tissues. As exercise becomes harder, working muscles need more energy. Adrenaline and noradrenaline rise, helping stimulate the liver to release glucose. When supply increases faster than uptake, the concentration rises.2
An early human experiment followed intense exercise to exhaustion in people with type 1 diabetes and healthy controls. Glucose production rose approximately sevenfold while utilisation rose approximately fourfold. Both processes accelerated, with the larger increase on the supply side. Some of the rise continued into early recovery, when insulin and other regulatory responses helped bring concentrations back down.2
Inside the muscle, exercise also increases access to circulating glucose. Human biopsy research showed that exercise moves the glucose transporter GLUT4 towards the muscle-cell membrane, facilitating uptake. Increased insulin responsiveness after exercise can further support glucose disposal.3
Recent athlete monitoring captures the resulting patterns in daily training. In Amatori and colleagues’ 2026 crossover study, 15 male cyclists completed two seven-day diets with equal energy but different carbohydrate and fat contents. Glucose was higher during exercise, and harder exercise was associated with higher readings under both diets. The lower-carbohydrate condition produced lower average glucose and variability.1
The intensity finding was an association within the training records. The dietary conditions were randomly assigned. Together with the physiology experiments, the results explain why the same activity that increases glucose use can also produce a higher circulating concentration.
What does that mean for the next session?
Increasing glucose availability supports the energy demand of hard exercise. In someone without diabetes, a brief rise coinciding with sprints, hills or demanding intervals can accompany this response. Its timing, the recovery pattern and any symptoms are useful parts of the interpretation.2
Training decisions still need the session’s actual results. Did you complete the planned work? Could you maintain pace or power? How demanding did the final repetitions feel, and how well did you recover? Nutrition records add the amount and timing of carbohydrate, fluids and gastrointestinal symptoms.6
A 2025 experiment by Poon and colleagues compared fixed-interval carbohydrate feeding with CGM-informed feeding in 12 men without diabetes. Each completed two 75-minute cycling trials. Sensor-guided timing reduced glucose variability, while heart rate, lactate and perceived exertion did not differ significantly.8
A useful application is to record fuelling and performance together over comparable sessions. For a long run, note carbohydrate per hour, late-session pace and whether your stomach tolerated the plan. This makes subsequent adjustments more specific: changing an amount that felt insufficient, or practising smaller, more frequent servings when larger feeds were uncomfortable. Established fuelling guidance provides the starting point.6
Liver and muscle glycogen recover at different rates
The body stores carbohydrate as glycogen in the liver and skeletal muscle. Liver glycogen contributes to circulating glucose availability, while muscle glycogen supplies the muscle in which it is stored. Prolonged exercise draws on these reserves, creating a recovery requirement afterwards.4,6
Gonzalez and colleagues studied 14 trained cyclists during three hours of exercise with continuous glucose or sucrose ingestion. Liver glycogen was maintained, while muscle glycogen declined. Working muscle was using its own stores alongside the carbohydrate supplied during the ride.4
A 2025 study by Fuchs and colleagues examined what happened afterwards. Twelve trained male cyclists completed glycogen-depleting exercise and underwent recovery conditions with carbohydrate feeding or fasting. Researchers measured liver glycogen with magnetic resonance spectroscopy and muscle glycogen using biopsies. In the feeding condition, total carbohydrate intake reached 10 g/kg body mass over 12 hours.5
Liver glycogen exceeded its pre-exercise level within six hours. Muscle glycogen had recovered to approximately 69% of its starting level after twelve hours. The difference matters when scheduling repeated high-demand sessions: restoration in the two tissues progresses on different timescales.5
For afternoon training after a demanding morning session, or consecutive days of long-distance competition, plan food availability and recovery time together. A short turnaround increases the importance of starting carbohydrate replacement promptly and continuing it through subsequent meals.5,6
Carbohydrate before, during and after exercise
The session’s duration and intensity, stomach tolerance and time until the next session determine the practical plan. The joint sports-nutrition position statement provides these starting points.6
| Timing | Practical starting point |
|---|---|
| Before | For longer or important sessions, eat familiar, easily tolerated carbohydrate-containing food approximately 1–4 hours beforehand. Adjust the amount to the session and your tolerance. |
| During | Extra carbohydrate is generally unnecessary below approximately 45 minutes. For endurance exercise lasting 1–2.5 hours, 30–60 g/hour is a common starting range. Beyond approximately 2.5–3 hours, intakes up to 90 g/hour may be appropriate, with prior tolerance practice. |
| After | When fewer than 8 hours separate glycogen-demanding sessions, approximately 1.0–1.2 g carbohydrate/kg/hour for the first 4 hours supports rapid replacement. With a longer recovery window, sufficient total daily intake becomes the main priority. |
Timing is particularly relevant to the early recovery period. Ivy and colleagues compared carbohydrate immediately after exercise with a two-hour delay. Immediate feeding produced faster muscle-glycogen storage over the first two hours.7
Preparing a recovery meal before training is therefore useful for athletes facing another session soon afterwards. With one session a day and more time to recover, carbohydrate can be distributed across the post-exercise meal and the rest of the day.6
Reading the trace in practice
Align glucose with the exercise and meal timeline. A rise during the final hard repetitions and a rise after the recovery meal have different immediate contexts. Record the accompanying pace, power and symptoms so that later comparisons are interpretable.1,8
CGM measures interstitial glucose. In a study of 13 runners wearing FreeStyle Libre 2, sensors captured changes over time, but individual readings differed from finger-prick blood glucose, with poorer accuracy at lower concentrations. Unexpected or symptom-inconsistent readings should be checked according to the device instructions.9
Repeated high readings away from exercise, or symptoms such as persistent thirst, frequent urination and unexplained weight loss, warrant clinical assessment. A clinician can interpret the history alongside fasting glucose, HbA1c or an oral glucose-tolerance test.10
People with diagnosed diabetes, particularly those using insulin, need an exercise plan that integrates medication, nutrition and monitoring through both the activity and recovery period.2
Study design & funding
Read the sources.
Amatori S, Radice R, Podlogar T, et al. (2026). Low-Versus High-Carbohydrate Isocaloric Diets on Continuous Glucose Monitoring Metrics in Healthy Trained Cyclists: A Randomized Crossover Trial.
Thorell A, et al. (1999). Exercise and insulin cause GLUT-4 translocation in human skeletal muscle.
Thomas DT, Erdman KA, Burke LM. (2016). Nutrition and Athletic Performance.
Ivy JL, et al. (1988). Muscle glycogen synthesis after exercise: effect of time of carbohydrate ingestion.
Poon ETC, Lau KW, Tsang JH, Zhang X, Wongpipit W. (2025). Comparative effects of continuous glucose monitoring-informed and traditional interval-based carbohydrate refueling protocols on endurance exercise responses: an exploratory study.
Mason LJ, Hartwig TB, Greene D. (2024). Validating the Use of Continuous Glucose Monitors With Nondiabetic Recreational Runners.
American Diabetes Association Professional Practice Committee for Diabetes. (2026). Diagnosis and Classification of Diabetes: Standards of Care in Diabetes—2026.