A low CGM reading during a run: causes, checks and what to do

Your phone flags a low glucose reading halfway through a run. Sometimes this comes with shaking, sweating or dizziness; sometimes you feel much as usual. First pause in a safe place, assess how you feel and follow the device’s procedure for checking the reading.^4,5

Understanding the warning starts with where the sensor measures glucose and how quickly glucose is changing during the activity.

Why sensor and finger-prick readings differ

A finger-prick meter measures glucose in capillary blood. A CGM sensor sits beneath the skin and measures glucose in interstitial fluid, with the device’s algorithm converting the signal into a displayed result. Glucose takes time to move between these compartments. During rapid changes, a difference in timing and concentration can develop.^1,4

Exercise, recent carbohydrate ingestion and treatment of a low can all create rapidly changing conditions. The size of the time difference depends on the situation and the device. Recording the timing of exercise, food and a confirmatory blood measurement makes the subsequent trace easier to understand.^1,4

Mason and colleagues tested FreeStyle Libre 2 in 13 runners without diabetes during running sessions lasting 30, 60 and 90 minutes at different intensities. Compared with finger-prick measurements, average absolute relative error was approximately 14% at the upper arm. Agreement was poorer at lower glucose concentrations, while the average pattern over time was captured more successfully.¹

Symptoms, the direction of change and a blood check therefore contribute different information. A low warning deserves a response even when its exact value needs confirmation.

Possible explanations for a low reading

Prolonged exercise increases muscle glucose use. In people using insulin or certain glucose-lowering medicines, the combination of medication, activity and food availability can produce genuine hypoglycaemia. Delayed meals and inadequate carbohydrate availability also belong in the history.⁵

Pressure around the sensor is another possible explanation for an unexpected reading. A sleeping-position experiment found abnormal sensor changes more often when participants lay on the sensor, with altered local blood flow proposed as a mechanism. A sudden overnight drop that coincides with pressure on the device is worth recording.³

Measurement agreement can also vary around strength exercise. Matzka and colleagues studied nine young adults across four squat sessions and different breakfast carbohydrate conditions. Differences between the sensor and blood measurements varied by testing period, including greater error after exercise.²

Check and treat the immediate situation first. Review the likely explanation once you are stable. The device instructions, symptoms and a blood measurement provide useful guidance while the sensor trend is still changing.^4,5

What to do during a run

Follow your individual hypoglycaemia plan if your clinical team has provided one. The following general steps distinguish someone who can safely swallow from an emergency requiring assistance.⁵

The American Diabetes Association commonly uses a value below 3.9 mmol/L (70 mg/dL) as an alert level in diabetes management. Your symptoms, prescribed treatment targets and device warnings should be interpreted together with your own management plan.^5,6

Chocolate and high-fat snacks are absorbed more slowly, so rapid carbohydrate is preferred for treating an acute low. After recovery, the need for a further snack depends on the interval until the next meal and the person’s treatment plan.⁵

A sensor can remain behind the blood response while glucose is recovering. Use the recommended confirmation method and reassessment interval to guide repeated treatment. This is particularly important when the displayed trend is still changing after carbohydrate has been taken.^4,5

Making the next session safer

Once stable, record the alert time, confirmatory glucose, symptoms, session details, last meal and response to treatment. People taking medication should also note its type, dose and timing. These records help the clinical team make specific adjustments.^4,5

If episodes recur late in long sessions, review pre-exercise food and the fuelling schedule. If the pattern occurs after demanding sessions, include recovery in the monitoring plan. Recurrent confirmed lows warrant assessment before increasing training demands.⁵

Practical preparation includes carrying a small supply of rapid carbohydrate and knowing the confirmation steps for your device. Someone using insulin should also tell a training partner how to recognise a low and obtain help. Planning this beforehand reduces the number of decisions needed during an episode.⁵

For a useful training record, keep sensor values and blood-confirmed values identifiable. The timing of the intervention is equally helpful: how long you had been exercising, whether you had just eaten, and when the symptoms improved. This makes a later consultation more informative than a screenshot of the lowest point alone.

When to seek assessment

Repeated blood-confirmed lows in someone without diabetes, especially with shaking, sweating, dizziness or altered awareness, deserve medical assessment. Evaluation considers glucose during symptoms, whether the symptoms improve after glucose is corrected, medication, food intake and other health circumstances.⁵

CGM supplies a continuous record that can make an episode easier to investigate. A prompt response, an appropriate blood check and a clear account of what happened are the most useful information to retain for the next training decision or clinical review.