Glucose versus insulin: what a CGM does not show you
Updated 2026-08-07
Short answer
Does a CGM measure insulin resistance?
No. A CGM measures glucose in interstitial fluid, not insulin. Because rising insulin can hold glucose in the normal range for years, a completely unremarkable CGM trace is compatible with meaningful insulin resistance. A CGM can show you that something is wrong; it cannot show you that nothing is.
How to read the evidence labels
Every claim on this page carries a label saying how much weight it can hold. The label describes the strength of the finding, not the prestige of the journal it appeared in.
- Strong evidenceA large randomised trial, a long cohort study, or agreement between several of them.
- Emerging evidenceReal, but thin — small, short, observational, or not yet replicated.
- ContestedCompetent researchers actively disagree. We are not picking a side for 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.
What the evidence says
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.
What the evidence says
Sensor accuracy is reported as MARD, an average deviation from a laboratory reference, and current sensors do not read identically to blood.3
Strong evidence
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.
Common questions
- If my CGM week looked normal, am I fine?
- It means your glucose control is currently holding. Because insulin rises to keep glucose normal long before glucose itself moves, a normal week does not rule out insulin resistance. If you have risk factors, a fasting insulin test answers a question the sensor cannot.
- Why does my sensor disagree with my fingerstick?
- Partly because it is reading a different fluid with a short lag, and partly because no sensor is exact. Disagreement is largest when glucose is moving quickly. Manufacturers and clinicians both advise confirming with a fingerstick when a reading does not match how you feel.
References
- 1.Tabák AG, Herder C, Rathmann W, Brunner EJ, Kivimäki M (2012). Prediabetes: a high-risk state for diabetes development. The Lancet. 10.1016/S0140-6736(12)60283-9
- 2.Tabák AG, Jokela M, Akbaraly TN, Brunner EJ, Kivimäki M, Witte DR (2009). Trajectories of glycaemia, insulin sensitivity, and insulin secretion before diagnosis of type 2 diabetes: an analysis from the Whitehall II study. The Lancet. 10.1016/S0140-6736(09)60619-X
- 3.Almurashi AM, Rodriguez E, Garg SK (2023). Emerging Diabetes Technologies: Continuous Glucose Monitors/Artificial Pancreases. Journal of the Indian Institute of Science. 10.1007/s41745-022-00348-3