Type 3 diabetes · Brain insulin resistance

Type 3 Diabetes: What a Normal Blood Sugar Test Can Miss

Type 3 diabetes is a proposed name, not a chart diagnosis, for a brain that stops responding to insulin. A normal blood sugar test misses it because insulin resistance runs for years before fasting glucose moves.

A normal fasting glucose reports on the last system to fail. The insulin problem underneath it, including the one in the brain, can run for years before that number moves.

Type 3 diabetes is a name, not a diagnosis your doctor can put in your chart, and the researchers who proposed it were careful to say so. It grew out of a 2005 Brown University study of brain tissue from people who died with Alzheimer’s disease, where the insulin machinery was badly damaged: fewer receptors, weaker signaling, and less of the brain’s own insulin. That idea anchors Chapter 3 of The Starved Brain by Dr. Gurpreet Singh Padda, MD, MBA, MHP, and in the chapter video he puts the consequence bluntly: a brain can run short of fuel while a blood sugar test reads perfect. For Measura [Cardiometabolic and Autonomic Health Analysis], that is a measurement problem before it is anything else.

What does type 3 diabetes mean?

Most people know insulin as the hormone that moves sugar out of the blood. In the brain it has other work. Its receptors cluster in the hippocampus, where new memories form, and in the cortex. There, insulin helps the connections between nerve cells strengthen, helps those cells stay alive, and takes part in keeping two troublesome proteins, amyloid and tau, under control. The enzyme that breaks down insulin also breaks down amyloid, so a brain bathed in insulin all day may have less of that enzyme available for cleanup. In animals, the shared enzyme is shown; in the human brain, the competition is still a hypothesis.

The evidence tier, stated once: type 3 diabetes is a hypothesis with substantial support, not settled science. Some researchers see brain insulin resistance as a cause of Alzheimer’s disease; others see it as a result. What few serious people dispute is that the Alzheimer’s brain has a fuel problem and that the problem begins early.

Why does a normal blood sugar test miss insulin resistance?

Think of insulin as a message that should arrive after a meal, get heard, and go quiet. Feed a body carbohydrate from breakfast to bedtime and the message never stops. Cells react the way you would to a smoke detector that will not shut off: they stop listening. The pancreas answers by sending more insulin, which keeps glucose in range and makes the deafness worse.

That is why the number most people are handed is a late one. In the Whitehall II study of 6,538 British civil servants, the 505 who later developed diabetes lost insulin sensitivity steeply across the five years before their diagnosis, while fasting glucose rose sharply only in the final three. Sensitivity failed first, then the pancreas strained, then the sugar broke. A glucose test sits at the far end of that line.

Dr. Padda has said he spent years telling patients with borderline results that their A1C was not too bad. The physiology changed his mind. A reassuring glucose describes how hard the pancreas is still working to hide a problem, not whether a problem exists.

What does a standard checkup test, and what does it miss?

A routine visit usually checks fasting glucose, sometimes hemoglobin A1c, and a cholesterol panel. None of them asks how much insulin it takes to hold the glucose where it is. In 2,439 adults from the Framingham Offspring Study who did not have diabetes, fasting insulin tracked smaller brain volume, and hemoglobin A1c did not. Among 683 older adults in Manhattan, people with high insulin levels had about twice the risk of Alzheimer’s disease, and that risk was highest in those who had no diabetes diagnosis at all.

The brain side is harder to see. The scan that shows regional glucose uptake falling is a PET scan with a glucose tracer, and it is a research tool. It has found the pattern in young carriers of the APOE4 risk gene still in their twenties and thirties, and in families with inherited Alzheimer’s disease fourteen to nineteen years before symptoms were expected. Measura does not perform PET imaging, and routine care does not order it for someone whose memory is fine. So the practical question becomes: what can be measured in the same person, now, that shows whether the insulin problem is already running?

Part of the gap is structural. The food supply delivers carbohydrate around the clock, visits are short, and medicine is organized by organ. Insulin resistance gets called diabetes in the blood, fatty liver in the liver, polycystic ovary syndrome in the ovary, and memory trouble in the neurology office. One upstream failure collects four names and four waiting rooms, and nobody is assigned to the upstream part.

What can be measured instead of blood sugar alone?

Measura measures; it does not treat, and findings go to your physician. For a brain-fuel question, the useful measurements describe the terrain that insulin resistance builds:

• Laboratory panels: blood work that can describe how your body is handling fuel, beyond a single glucose value. It is worth asking your physician whether fasting insulin has ever appeared in your results.
• Bioimpedance body composition: how much of you is muscle and how much is fat, which a bathroom scale and a BMI chart cannot separate.
• Arterial stiffness and endothelial function: how stiff your arteries are and how well the lining of the vessels responds. The link between diabetes and dementia is strongest for vascular dementia, so the vessels belong in the brain’s story.
• Cognitive assessment: a structured record of memory, attention and thinking speed today, so a later change is measured against you instead of guessed at.

None of these is a brain scan, and none diagnoses Alzheimer’s disease. Together they answer what a glucose test cannot: is the terrain that produces insulin resistance already in place? Type 2 diabetes carries roughly 1.5 to 2 times the risk of dementia, and metabolic health is rarer than most people assume. Fewer than 12.2% of US adults were metabolically healthy on NHANES 2009–2016, and under 7% on the tighter criteria applied after 2021.

Why does finding insulin resistance early matter?

The finding that shifts the prognosis is about a second fuel. In small dual-tracer PET studies of 29 people with mild Alzheimer’s disease, the brain took up 11 to 13 percent less glucose than in matched peers, yet no less of the ketone acetoacetate. Brain tissue that still absorbs an alternative fuel is not dead; it is short of the fuel it normally relies on. Whether supplying that fuel changes the disease is a separate question that clinical trials still have to answer.

For you, the point is timing. Insulin trouble is measurable years before glucose moves, and the brain’s fuel gauge falters years before memory does. A normal glucose means the last alarm has not sounded yet. It does not mean the building is sound. Unmeasured is unmanaged, and knowing your numbers earlier gives you and your physician more years to decide what to do with them. The evidence, study by study, is in the book companion for Chapter 3, and what insulin resistance looks like before diabetes walks through the body-wide version of the same story.

What to ask at your next visit

• My glucose is normal. Has my fasting insulin ever been measured?
• Does anything in my results suggest insulin resistance, even without diabetes?
• Do we have a baseline of my memory and thinking to compare against later?
• Diabetes or dementia runs in my family. Does that change what we should measure?

More questions worth bringing are collected in questions worth asking your doctor. What the brain tissue itself shows, and where the name came from, is set out in brain insulin resistance in Alzheimer disease.

Frequently asked questions

What is type 3 diabetes?

It is a proposed name for insulin resistance and insulin shortage inside the brain, suggested in 2005 after researchers found damaged insulin signaling in Alzheimer’s brain tissue. It is not an official diagnosis, and researchers still debate whether it drives Alzheimer’s disease or follows from it. The underlying observations have held up. What the 2005 brain-tissue findings showed is set out in brain insulin resistance in Alzheimer disease.

Can my brain be insulin resistant if my blood sugar is normal?

It can. Insulin resistance builds for years while the pancreas keeps glucose in range by making more insulin. In the Whitehall II study, glucose climbed sharply only in the last three years before diabetes was diagnosed, and studies of cognitively normal adults have linked higher insulin resistance to lower brain glucose uptake. See insulin resistance and metabolic health.

Does Measura do brain scans for type 3 diabetes?

No. The PET scans that show falling brain glucose uptake are research tools performed at imaging centers, and routine care rarely orders them for someone with normal memory. Measura measures the metabolic, vascular, body composition and cognitive picture in the same person, and your physician decides what the results mean for your care. The full list is in the Measura test library.

Is a cognitive assessment useful if my memory feels fine?

That is when it is most useful. A baseline records how you perform today, so any later change is compared with your own starting point rather than a population average. The brain fuel changes seen in Alzheimer’s research begin before symptoms, which is the argument for measuring before there is anything to explain. Read what a cognitive baseline is for.

How is this different from the statin and cholesterol question?

Fuel and building materials are separate problems. Insulin resistance concerns how the brain gets its energy. The next question is what the brain is built from, how it makes its own cholesterol, and why a blood cholesterol test cannot see that supply. That measurement conversation is covered in statins and memory loss.

What causes type 3 diabetes?

It traces back to insulin that never goes quiet. Feed a body carbohydrate from breakfast to bedtime and cells stop listening to insulin; the pancreas answers with more, which deepens the deafness. In brain tissue from people who died with Alzheimer’s disease, researchers found the same failure inside the brain: fewer insulin receptors, weaker signaling and less of the brain’s own insulin.

Is type 3 diabetes serious?

Yes, because of what it is tied to. Among 683 older adults in Manhattan, people with high insulin levels had about twice the risk of Alzheimer’s disease, and the risk was highest in those with no diabetes diagnosis. Type 2 diabetes carries roughly 1.5 to 2 times the risk of dementia. The brain’s fuel gauge falters years before memory does, so the early years are the ones worth measuring.

What are the symptoms of type 3 diabetes?

Early on, there may be nothing you can feel. On research brain scans, falling glucose uptake has appeared in APOE4 carriers in their twenties and thirties, and in families with inherited Alzheimer’s disease fourteen to nineteen years before symptoms were expected. Blood sugar can stay normal through those same years, which is why a cognitive baseline and a question about fasting insulin belong before memory changes, not after.

References

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• Croteau, E., Castellano, C. A., Fortier, M., Bocti, C., Fulop, T., Paquet, N., & Cunnane, S. C. (2018). A cross-sectional comparison of brain glucose and ketone metabolism in cognitively healthy older adults, mild cognitive impairment and early Alzheimer’s disease. Experimental Gerontology, 107, 18–26. https://doi.org/10.1016/j.exger.2017.07.004
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Related reading

• Ketosis vs Ketoacidosis: What the Numbers Actually Mean
• Statins and Memory Loss: What to Measure Before Anyone Guesses
• Laboratory Panels

Medically reviewed by Dr. Gurpreet Singh Padda, MD, MBA, MHP, medical director of Measura. Last reviewed September 12, 2026.