Glucose versus insulin: what a CGM does not show you

Two different quantities

Glucose is the fuel in your blood. Insulin is the instruction that moves it out. A sensor reads the first and is silent on the second, and for most of the run-up to type 2 diabetes it is the second that is changing.

This is the single most useful thing to understand before buying a sensor, because it sets the limit on what a good result means. A week of flat, tidy readings tells you your body is currently keeping glucose where it should be. It does not tell you how much insulin that is costing.

Qué dice la evidencia

• Insulin resistance and beta-cell dysfunction begin before glucose becomes abnormal, so glucose measurement alone cannot exclude them.^1,2

  Evidencia sólida

Interstitial fluid, not blood

The second gap is smaller but practical. The sensor filament sits under the skin in interstitial fluid, not in a blood vessel, and glucose reaches that fluid slightly after it reaches the blood. When your glucose is changing fast — the twenty minutes after a large meal, or during exercise — the number on screen is describing where you were, not where you are.

This is why a sensor reading that contradicts how you feel is checked with a fingerstick rather than acted on. It is also why the sensors that read every minute show a meal spike that the ones reading every fifteen minutes can flatten or miss.

Qué dice la evidencia

• Sensor accuracy is reported as MARD, an average deviation from a laboratory reference, and current sensors do not read identically to blood.³

  Evidencia sólida

What would actually measure insulin resistance

Fasting insulin measured alongside fasting glucose gives an index of resistance. An oral glucose tolerance test with insulin sampling gives a better one. Both are blood draws ordered by a clinician, and both cost a fraction of a year of sensors.

No consumer wearable on sale measures insulin. If a product implies otherwise, it is describing an inference from glucose, not a measurement.