Four in ten Americans aged 18 to 44 are insulin-resistant. Almost half of them are not overweight (Parcha et al., 2022). They look fine. They feel fine. Their morning blood sugar is fine. And their pancreas is quietly working overtime to keep it that way.
This is the gap a continuous glucose monitor cannot show you.
A continuous glucose monitor, or CGM, is a small sensor worn on the back of the upper arm. A tiny filament under the skin reads your blood sugar every few minutes and sends the data to your phone. You see your glucose curve in real time. You watch what bread does. What an evening of wine does. What a walk after dinner does.
If you have spent any time on the glucose-flattening side of the internet, you know the drill. Veggies before pasta. Vinegar before meals. A walk after dinner. The curves on your sensor look better. Something feels like progress.
It might be progress. It might also not be. To know which, you need two markers your CGM does not measure: HbA1c and Insulin.
The CGM tells you what happened today. The blood test tells you whether anything is changing.
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HbA1c: the receipt your body cannot edit
Glucose in your blood slowly attaches itself to haemoglobin, the protein inside your red blood cells. The reaction is one-way and lasts the life of the cell, about three to four months. So when a lab measures the percentage of haemoglobin that has been glycated, they are reading a summary of your average glucose over that whole window (Nathan et al., 2008).
You cannot fast the night before. You cannot have a clean week and shift the number. The receipt is already written.
That is the value of HbA1c. A CGM week shows you what happened this week. HbA1c tells you whether this week was typical, or a blip.
Take-away: HbA1c integrates the past three months. Spike-flattening tactics either show up in it three months later, or they did not really do much.
Insulin: the marker no one talks about
Here is what the glucose Instagram crowd almost never mentions. Before your blood sugar starts to creep up, your insulin does. Years before. Sometimes more than a decade.
The mechanism is simple. Insulin is the hormone your pancreas makes to move sugar out of the blood and into your cells. When your cells respond less well to it, your pancreas pushes harder, producing more insulin to keep blood sugar in range. For a long time it works. Glucose stays normal. Your GP nods and sends you home.
Behind that normal number, your pancreas is on the gas pedal.
You will not see this on a sensor. A CGM measures sugar. It does not measure the hormonal effort it takes to keep that sugar looking fine.
The two markers most useful here, Glucose and Insulin, are both measured from a morning blood draw after at least twelve hours without food. That fasted state is what makes the numbers comparable from one test to the next.
This is why the opening statistic matters. Twenty percent of normal-weight adults carry the same hidden pattern, and their cardiovascular and mortality risk runs more than three times higher than lean peers who are metabolically healthy (Stefan et al., 2017). Body weight does not catch this. Glucose alone does not catch this. Insulin does.
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The combination to watch is normal Glucose with elevated Insulin. The standard GP panel does not flag it because it does not include Insulin.
The four markers that complete the picture
HbA1c and Insulin are the two stars. Glucose anchors the calculation. The HOMA-Index combines Glucose and Insulin into a single read-out for insulin resistance, validated against the research gold standard four decades ago (Matthews et al., 1985).
Together they answer the question your CGM cannot: is anything actually changing?
None of these four can be gamed by yesterday's dinner if you follow the fasting rules. That is what makes them honest before-and-after measurements for any glucose strategy.
What to do until your bloodwork is back
The CGM is still useful. It teaches you what your body does with different foods, how stress shows up in real time, how a walk after dinner feels. That education is real. The blood test is what tells you it is sticking.
Test before. Test after.
If you are going to commit to flattening your curve, get baseline bloodwork first. Not because the protocol "might not work." Because you want to know where you started.
Three months is the right re-test window. That is how long your red blood cells live, and that is how long HbA1c needs to shift. Test sooner and you catch only a partial picture. Wait much longer and the feedback loop weakens.
If you want to keep reading, 10 tips for flattening your glucose curve and the mitochondrial side of metabolic health are useful next steps.
Can a CGM replace an HbA1c test?
A CGM estimates average glucose over the wear period, usually one to two weeks. HbA1c reflects glucose exposure over roughly three months through a biological process inside your red blood cells. The two measurements often disagree, especially for people whose individual glycation rate sits above or below average. HbA1c remains the standard long-term reference and is not replaced by sensor estimates.
Why does my GP only check glucose?
Standard GP panels are designed to detect overt diabetes, defined primarily by elevated blood sugar or HbA1c. They are not designed to catch earlier insulin resistance. Insulin and HOMA-Index can flag metabolic shifts years before blood sugar rises, which is why they sit in preventive bloodwork rather than routine screening.
I am slim and feel fine. Do I really need Insulin and HOMA?
Normal weight does not rule out insulin resistance. Large US population data shows that a meaningful proportion of insulin-resistant young adults are not obese, and that normal-weight adults with metabolic abnormalities face substantially higher cardiovascular and mortality risk than metabolically healthy lean peers. Insulin and HOMA are the markers that detect this hidden pattern.
How often should I retest HbA1c?
Three months is the biologically meaningful interval because it matches the lifespan of your red blood cells. Testing sooner captures only a partial change and can be misleading. For people tracking long-term metabolic health without specific medical direction, every three to six months is a common rhythm.
Do I need to fast before an HbA1c test?
HbA1c itself does not require fasting because it reflects a three-month average. Glucose, Insulin and HOMA-Index do require at least twelve hours of fasting. Since the four markers are usually measured together, the panel is taken in a fasted state.
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