Type 3 Diabetes? How Insulin Resistance and Blood Sugar Levels Shape Your Brain Health: The Surprising Link Between Blood Sugar, Insulin, Cognitive Decline and Alzheimer's

Last year, my step-dad was diagnosed with Alzheimer’s. Because I knew his health, diet, and lifestyle history—and we’d seen the signs—this diagnosis didn’t come as a shock. As a seasoned functional nutrition practitioner, I brought a different education and understanding of neurodegenerative disease than my parents and my parents’ conventionally minded doctors.

What shocked and fired me up was not only the lack of recommendations, but the almost complete absence of meaningful care from the medical team. There were no targeted labs, no nutrition or lifestyle guidance, and no real attention to the root causes of my dad’s condition. There is urgency here—not only for my dad’s health, but for the health of so many Americans who are in a similar situation.

Because I’m a functional nutrition therapy practitioner. I have spent years studying exactly this territory: the relationship between metabolic health, the brain, the whole body, and the number of diseases we’ve been told are simply inevitable. Diseases we’ve been told are locked in our genes, sealed by age, beyond our reach, such as Alzheimer's, mental illness, and even cancer.

I don’t believe that. And the science is increasingly on my side.

Alzheimer’s disease doesn’t start in your seventies. It begins decades earlier, quietly, with metabolic dysfunction, which affects many American adults now. This means we have a long window for intervention. This is what functional medicine does amazingly: prevention over reaction, understanding the root of metabolic dysfunction, and recognizing early signs to change the trajectory of our health so we can age successfully.

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Insulin Resistance: The Brain’s Silent Enemy

To understand the Alzheimer’s connection, we need to start with insulin and with a condition that is extraordinarily common and underdiagnosed: insulin resistance.

Here’s how it develops. When we eat a diet high in refined carbohydrates and added sugars, blood glucose spikes repeatedly and sharply. The pancreas responds by releasing insulin to pull that glucose out of circulation. This system works for a while. But over time, with repeated flooding, our cells push back. They downregulate their insulin receptors. They become resistant to insulin’s instructions. The pancreas compensates by producing even more insulin. And so begins a slow-motion crisis.

This intermediate state, with high insulin and normal or slightly elevated blood sugar, is called hyperinsulinemia. It goes by other names, too: “prediabetes”, metabolic syndrome, and insulin resistance. They’re all describing the same underlying dysfunction, at different points along a spectrum that eventually leads to type 2 diabetes.

What makes this so dangerous is how invisible it is. Standard medical checkups measure fasting blood glucose. But glucose can look perfectly normal for years while insulin remains chronically elevated, silently damaging tissues throughout the body, including the brain.

Most people are not told to check their fasting insulin. Most conventional doctors don’t order it. And so this metabolic crisis progresses, often for a decade or more, before anyone notices.

Type 3 Diabetes: When the Brain Starves

Here is where Alzheimer’s enters the picture.

The brain is the most metabolically demanding organ in the body. It runs on ketones OR glucose, which depends on insulin signaling to efficiently use it.

But the brain handles insulin differently than the rest of the body. Insulin doesn’t simply diffuse in it is actively transported across the blood-brain barrier by specialized receptors.

And when blood insulin levels are persistently high, those transport receptors become resistant. They downregulate. They stop letting insulin in.

The result is a cruel paradox: the higher your blood insulin levels, the lower your brain insulin levels. A brain swimming in a sea of glucose, and still starving.

A brain swimming in a sea of glucose, and still starving to death. This is the hidden mechanism behind what researchers now call Type 3 diabetes.

Without adequate insulin, brain cells cannot process glucose and convert it into the energy they need to function. This is called cerebral glucose hypometabolism, sluggish, insufficient brain fuel metabolism.

But glucose toxicity doesn’t only starve the brain of useful fuel. When blood sugar is poorly managed, oxidative stress rises, energy pathways become increasingly inefficient, leading to a compromised blood-brain barrier. The blood-brain barrier is the brain’s selective gating system, tightly controlling what crosses from the bloodstream into neural tissue. Chronic hyperglycemia weakens this barrier, allowing inflammatory molecules, toxins, and pathogens to enter brain tissue that would normally be protected. Once that gate opens, the inflammatory burden in the brain rapidly compounds.

Chronically elevated blood sugar also restricts cerebrovascular blood flow, depriving neurons of oxygen and nutrients — a mechanism that contributes not only to Alzheimer’s but to vascular dementia as well. These are not separate diseases so much as different expressions of the same underlying metabolic dysfunction.

This insulin-deficient, insulin-resistant state of the brain is what we have termed Type 3 diabetes. It is distinct from but deeply connected to the insulin resistance occurring in the rest of the body.

It’s worth noting that Alzheimer’s is not a single uniform disease. Dr. Bredesen’s research identifies distinct subtypes with different primary drivers: Type 1 (inflammatory), Type 2 (atrophic), Type 3 (toxic), and Type 1.5 — which he calls Glycotoxic — driven specifically by chronically elevated blood sugar and insulin resistance. The glycotoxic subtype is precisely what this article is focused on. If your metabolic markers are off, this is your subtype to understand.

Why the Hippocampus Is Hit First

Not all brain regions suffer equally in this process. The hippocampus, a seahorse-shaped structure deep in the temporal lobe responsible for forming and consolidating new memories, is particularly vulnerable.

Why? Because the hippocampus is extraordinarily metabolically demanding. Forming a new memory requires intense, rapid glucose utilization — essentially a surge of cellular energy. That surge requires insulin. When insulin signaling in the brain breaks down, the hippocampus is the first structure to feel it.

Over time, hippocampal cells begin to shrink and die. This is called hippocampal atrophy, and it is one of the earliest and most consistent findings in Alzheimer’s disease.

By the time a person first notices memory problems, their hippocampus has typically already shrunk by 10% or more.

This is why early Alzheimer’s so often presents as difficulty learning new information and remembering recent events, while older memories can remain intact. The hippocampus, which encodes new experiences, is failing.

And this is why the timeline matters so much. The process that leads to hippocampal atrophy begins not at the moment of diagnosis, but potentially 10 to 20 years earlier, with metabolic dysfunction.

The Amyloid Question: Symptom, Not Cause

At this point, you may be wondering: what about amyloid plaques? Isn’t that what Alzheimer’s is — plaques and tangles?

Yes and no. Amyloid plaques are a hallmark of Alzheimer’s disease on autopsy and brain imaging. But the emerging understanding — one I find deeply compelling — is that amyloid is not the root cause of Alzheimer’s. It is more likely a downstream consequence of the metabolic dysfunction we’ve been discussing.

Here’s what the research suggests. When brain cells are under stress — from energy deprivation, inflammation, or toxic insults — they release small protein fragments called amyloid peptides as a kind of defense mechanism.

In other words, amyloid isn’t inherently villainous. It may actually serve a protective function under acute stress.

The problem arises when the brain is chronically stressed and insulin-deficient. Under those conditions, these amyloid peptides accumulate faster than they can be cleared — and they begin to clump together into the insoluble plaques that interfere with neuron-to-neuron signaling.

Amyloid may not be the villain it’s been cast as — but rather a distress signal from a brain that has been running on empty for years.

This is where insulin’s role becomes even more remarkable. Insulin stimulates the production of an enzyme called insulin-degrading enzyme, or IDE. As the name suggests, IDE breaks down used insulin molecules. But it also has a critical secondary function: it helps clear excess amyloid from the brain.

So when brain insulin is chronically low, as it is in the insulin-resistant brain I’ve been describing, IDE levels fall. And with fewer IDE available to do the housekeeping, amyloid accumulates more easily, clumps more readily, and plaques begin to form.

This helps explain something that has puzzled researchers for decades: why have nearly every drug designed to target and remove amyloid plaques failed to meaningfully reverse or halt Alzheimer’s disease? Because if the plaques are a symptom of an underlying metabolic problem, removing the plaques without addressing the root cause is like mopping the floor without turning off the faucet.

Dr. Dale Bredesen, one of the leading voices in Alzheimer’s research and prevention, puts it plainly: “The making of amyloid is actually a protective response to different metabolic and toxic perturbations. The idea of getting rid of the amyloid only makes sense if you first understand why it’s there and get rid of the cause or causes.”

This is a foundational principle of functional medicine: the body acts with biological intelligence. Pathological changes, such as amyloid production, don’t occur randomly; they are adaptive responses to an underlying imbalance. The goal, from a functional perspective, is not to suppress those responses but to uncover and address the imbalances that drive them. In the case of Alzheimer’s, that imbalance, more often than not, begins with chronically unstable blood sugar and the cascade of insulin dysfunction that follows.

Tau Tangles: When the Brain’s Scaffolding Collapses

Amyloid plaques are only half the story. The other defining feature of Alzheimer’s disease is neurofibrillary tangles — and once again, insulin is at the center of the picture.

Tau is a housekeeping protein. Its job is to maintain the structural infrastructure of neurons — specifically the microtubules, which act like a railway system inside the cell, transporting nutrients, proteins, and cellular cargo from one end of the neuron to the other.

Tau proteins are the ties that hold those tracks in place.

In a healthy, insulin-sufficient brain, insulin signaling helps keep tau proteins at their stations. It does this by suppressing an overactive enzyme called GSK-3β (glycogen synthase kinase 3 beta), which would otherwise add too many phosphate groups—a process known as hyperphosphorylation—to tau proteins, destabilizing them.

But when brain insulin is deficient, that enzyme runs unchecked. Tau becomes hyperphosphorylated — essentially overloaded with phosphate tags — and it detaches from the microtubules it was meant to support.

The tracks fall apart. And the abandoned tau proteins don’t simply drift away. They clump together inside the cell, forming the large, dense neurofibrillary tangles that are visible on autopsy in Alzheimer’s brains.

When insulin is in short supply, tau proteins abandon their posts. The scaffolding of the neuron collapses from the inside.

The consequences are severe. Without functional microtubules, neurons cannot transport the materials they need to survive. Synapses fail. Communication between cells breaks down.

Ultimately, the cell undergoes apoptosis — programmed cell death. This is not damage from outside the cell. It is the cell’s own architecture unraveling from within.

Together, amyloid plaques and tau tangles represent two distinct but insulin-connected mechanisms of neurodegeneration. One accumulates outside the cell, disrupting neuronal signaling. The other destroys the cell from the inside. Both are downstream consequences of the same root problem: a brain starved of insulin for too long.

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Functional & Restorative Health: The Tools I Lean On

My goal is to provide hope to my readers after reading this blog. I want you to know that when we learn and understand the underlying mechanism, which we aim to do in functional medicine, you can have solutions that work to correct the physiology and slow down the progression of neurodegenerative disease and restore health. We may not get to 100%, but even 25% is a win in my book.

Nutrition and the Glucose-to-Ketone Switch

The most powerful lever we have is what we eat. And the most important concept to understand is what I call the glucose-to-ketone switch.

The ability to shift from carbohydrate-based metabolism to fat-based metabolism is one of the most well-documented and therapeutically promising interventions in brain health research. When your body runs low on glucose — through dietary carbohydrate reduction, fasting, exercise, or calorie restriction — the liver begins converting fatty acids into ketones. The brain readily takes up these ketones without insulin. For an insulin-resistant brain, ketones are not just an alternative fuel. They are a rescue fuel.

But the benefits of flipping this switch go well beyond simply providing energy. Many cellular pathways suppressed by chronically high insulin levels begin to reactivate in a ketogenic state. Two of the most significant:

• Autophagy — the brain’s cellular self-cleaning process, in which damaged proteins and dysfunctional cell components are broken down and recycled. Chronically high insulin suppresses autophagy. Ketosis restores it. This matters enormously in Alzheimer’s, where the accumulation of misfolded proteins — amyloid and tau — is a defining feature.

• Neuroplasticity — the brain’s ability to grow new neurons, form new connections, and rewire itself in response to learning and experience. High insulin and chronic glucose dependence blunt this capacity. Ketosis, along with exercise, is one of the most potent stimulators of BDNF (brain-derived neurotrophic factor), the primary driver of neuroplasticity.

  
  

You don’t have to be strictly ketogenic to benefit. The goal is to spend some time in ketosis every day — to flip the switch regularly. This can be achieved through a low-carbohydrate diet, daily intermittent fasting (even an overnight fast of 12–16 hours), regular aerobic exercise, or calorie restriction. Each of these signals the body to shift toward fat metabolism, and each provides the brain with a daily dose of the metabolic environment it needs to repair and thrive.

• Reducing refined carbohydrates and added sugars lowers the chronic glucose and insulin burden that drives insulin resistance.

• A ketogenic or low-carbohydrate diet sustains ketone production throughout the day.

• MCT oil and coconut oil are rapidly converted to ketones by the liver and can be added to any dietary pattern, even without strict carbohydrate restriction.

• Intermittent fasting — even a simple 12–16 hour overnight window — naturally elevates ketones and activates autophagy.

  
  

Key Nutrients for Brain Support

The framework I use to organize supplementation is by therapeutic goal. In the context of blood sugar management and cognitive decline, I would start with Cell Membrane Support, Mitochondrial Support, General Cognition and Neuroplasticity Support, Anti-Inflammatory Support, Oxidative Stress and Cellular Damage Support, Leaky Brain / Blood-Brain Barrier Support, Cerebrovascular Circulation Support, Mood Support, and Nutrient Deficiency Support. Of course, all recommendations need to be individualized based on a person’s specific lab work, history, and presentation — this is not a universal protocol, but a map of the terrain.

Among the most important:

• Omega-3 fatty acids (especially DHA) — essential for neuronal membrane integrity and anti-inflammatory signaling. Most Americans are significantly deficient due to consuming a standard American diet high in Omega-6s.

• Vitamin D — a neurosteroid as much as a vitamin, with receptors throughout the brain.

• B vitamins (especially B12, B6, and folate) — essential for methylation, homocysteine regulation, and neurological function.

• Magnesium threonate — a form of magnesium with a unique ability to cross the blood-brain barrier and support synaptic plasticity.

• Phosphatidylcholine and Alpha GPC — choline-based compounds that support the production of acetylcholine, one of Alzheimer’s most consistent neurochemical features: a dramatic reduction in acetylcholine, the neurotransmitter most essential for memory, attention, and learning. When acetylcholine-producing neurons die, communication across brain circuits breaks down.

• CoQ10 and NAD are mitochondrial support compounds that help restore cellular energy production in aging brain cells.

• Low-dose lithium — a remarkable and underappreciated mineral with strong evidence for neuroprotection and reducing tau pathology, at doses far below pharmaceutical levels.

• Peptides (Semax, Selank) — nootropic peptides with research-backed effects on BDNF, cognitive resilience, anxiety modulation, and neuroprotection; used under practitioner guidance.

• Peptides (BPC-157 and KPV) — peptides to support gut and blood-brain barrier repair, anti-inflammatory effects, and neuroprotection.

  
  

Exercise

Exercise is, without exaggeration, one of the most powerful interventions we have for brain health. It increases BDNF (brain-derived neurotrophic factor) — essentially fertilizer for new neuron growth. It improves insulin sensitivity. It increases cerebral blood flow. Even modest, consistent movement — daily walks, resistance training a few times a week — has been shown to slow neurodegeneration and improve cognitive function in people already showing decline.

Sleep and the Glymphatic System

Sleep is not passive recovery. For the brain, it is active maintenance, and it may be one of the most underappreciated levers in Alzheimer’s prevention.

During deep sleep, the brain activates what researchers call the glymphatic system,  a network of channels that surround the brain’s blood vessels and act as a dedicated waste-clearance system. During sleep, these channels expand significantly, and cerebrospinal fluid is pumped through brain tissue, flushing out the metabolic waste products that accumulate during the day’s activity. Among the waste products cleared by this nightly rinse are amyloid-beta and tau proteins.

In other words, sleep is when the brain takes out its trash. And when we consistently cut sleep short, or when sleep quality is poor, that waste accumulates. Every night of poor sleep is a missed opportunity for the glymphatic system to clear amyloid and tau. Studies have shown that even a single night of sleep deprivation produces a measurable increase in amyloid accumulation in the brain. Chronic poor sleep is now recognized as a significant independent risk factor for Alzheimer’s disease.

The sleep-insulin connection runs in both directions. Poor sleep worsens insulin resistance; even 2-3 nights of disrupted sleep can significantly impair insulin sensitivity and elevate fasting glucose. And insulin resistance, in turn, disrupts sleep architecture, reducing the deep, slow-wave sleep stages in which glymphatic clearance is most active. It is a self-reinforcing cycle, and breaking it, through sleep hygiene, circadian rhythm support, and metabolic optimization, is an essential part of any serious brain health protocol.

Practical priorities: consistent sleep and wake times (circadian rhythm is everything), a cool and dark sleep environment, avoiding screens and food in the two hours before bed, and addressing any underlying sleep disorders such as sleep apnea, which has its own strong independent association with cognitive decline.

Stress and Toxin Management

Chronic stress elevates cortisol, which directly raises blood glucose and worsens insulin resistance — another feedback loop connecting our mental state to our metabolic health. Stress management is not a soft recommendation. It is a metabolic intervention.

On the toxin side, the brain is uniquely vulnerable to environmental insults. Its high fat content makes it a magnet for fat-soluble toxins. Its immense oxygen demands make it highly susceptible to oxidative damage. And its dense network of neurons and glial cells makes disruption by inflammatory agents particularly consequential.

Dr. Bredesen coined the term “dementigens” to describe external substances that can drive cognitive decline over time. These include environmental pollutants such as heavy metals and pesticides, pharmaceuticals with anticholinergic or neurotoxic effects, mold mycotoxins, and food-related chemicals, including certain artificial additives and plasticizers. For people with the toxic subtype of Alzheimer’s (Type 3 in Bredesen’s framework), identifying and removing these exposures can be as important as any dietary intervention.

Practical priorities: filter your water, minimize pesticide exposure by choosing organic food when possible, evaluate your medication list for anticholinergic burden, and address any mold or indoor air quality issues in your home.

Know Your Numbers

You cannot address what you cannot measure. The standard annual blood panel is not sufficient for assessing metabolic and brain health. This is just one thing that sets functional medicine apart from conventional. Some biomarkers I recommend when I am working with a client presenting with cognitive decline:

• Fasting glucose and fasting insulin (to calculate HOMA-IR, a measure of insulin resistance)

• HbA1c (3-month average blood sugar)

• hsCRP (high-sensitivity C-reactive protein — a marker of systemic inflammation)

• Vitamin D, B12, folate, homocysteine

• A full omega-3 index (OmegaCheck)

• A Neural Zoomer panel (from Vibrant America) — a specialized test that looks at brain-specific autoimmune markers and neurological risk factors

What the Numbers Mean: Your Metabolic Brain Health Reference Ranges

These are the ranges I use in my practice when assessing metabolic and brain health risk. You’ll notice they are tighter than conventional lab reference ranges — because conventional ranges are built around what’s “normal” in a largely insulin-resistant population, not what’s optimal for a healthy brain.

Biomarker	Optimal	Concerning	Red Flag
Fasting Insulin	2–6 μIU/mL	10–19 μIU/mL	≥20 μIU/mL
Fasting Blood Glucose	75–85 mg/dL	86–100 mg/dL	>100 mg/dL
Fasting Triglycerides	<100 mg/dL	100–150 mg/dL	>150 mg/dL
HOMA-IR	<1.0	1.0–1.8	≥1.8

A note on fasting insulin specifically: this is the single most important marker that most conventional labs don’t run. You may need to request it explicitly. A level in the double digits — even at 10 or 12 — suggests early-stage insulin resistance. By the time insulin hits 20, we are looking at significant metabolic dysfunction, regardless of what fasting glucose shows.

HOMA-IR is calculated from fasting glucose and fasting insulin (glucose × insulin ÷ 405). It’s a simple equation your practitioner can run for you, and it is one of the most sensitive early indicators of insulin resistance available outside of a glucose tolerance test.

The markers below move beyond metabolic function into the nutritional and inflammatory foundations of brain health. All are worth knowing, and most are not included in standard annual labs.

Biomarker	Optimal	Concerning	Red Flag
hsCRP	<0.5 mg/L	0.5–1.0 mg/L	>1.0 mg/L
Vitamin D (25-OH)	>50 ng/mL (∼75 sweet spot)	30–50 ng/mL	<30 ng/mL
Vitamin B12	800–1200 pg/mL	500–799 pg/mL	<500 pg/mL
Folate	>24 ng/mL	12–24 ng/mL	<12 ng/mL
Homocysteine	5–7 μmol/L	8–12 μmol/L	>12 μmol/L
OmegaCheck (Omega-3 Index)	>8% (sweet spot)	6–8%	<6%

I also strongly recommend a polypharmacy review for anyone on multiple medications. Many commonly prescribed drugs — statins, proton pump inhibitors, anticholinergics, benzodiazepines, and others — have documented effects on cognition and nutrient absorption that are rarely discussed.

The Single Most Important Thing You Can Do

If you take nothing else from this article, take this: stabilize your blood sugar.

Every mechanism described in this article, brain insulin deficiency, cerebral glucose hypometabolism, blood-brain barrier breakdown, amyloid accumulation, tau tangles, hippocampal atrophy, suppressed autophagy, and disrupted sleep, is upstream of, or worsened by, chronic blood sugar instability. Blood sugar is the thread that runs through all of it.

You do not need a diagnosis to start. You do not need a prescription. You need to understand what your fasting insulin, fasting glucose, triglycerides, and HOMA-IR are telling you — and you need to take those numbers seriously before they become a clinical emergency.

Stable blood sugar is not just a metabolic goal. It is a brain protection strategy. And it is one of the few things in this entire picture that is almost entirely within your control.

My Stance as a Functional Nutrition Practitioner: Restorative Health in Action

I am not willing to accept that Alzheimer’s is simply a genetic destiny or an inevitable tax on aging. The research, the work of Dr. Dale Bredesen, Dr. Georgia Ede, Dr. David Perlmutter, and many others, tells a different story. A story in which the choices we make daily, about what we eat, how we move, what we supplement, and how we manage stress and toxin exposure, are among the most powerful determinants of our cognitive future.

The window for intervention is long. But it is not infinite. And it begins now.

If any part of this resonates with you,  if you have a family history of Alzheimer’s, if you’ve been told you’re prediabetic, if you’re noticing changes in your memory or focus,  please don’t wait for a diagnosis to act. The best time to start was years ago. The second-best time is today.

If you’d like to explore what a comprehensive and personalized brain health and metabolic assessment looks like, I’d love to connect. You can find me at in2GREAT.

Key researchers and resources referenced in this article: Dale Bredesen, MD (The End of Alzheimer’s), Georgia Ede, MD (Change Your Diet, Change Your Mind), Suzanne de la Monte, MD (Type 3 Diabetes research, Brown University), David Perlmutter, MD (Grain Brain). The information in this article is for educational purposes and does not constitute medical advice. Please consult a qualified healthcare provider for a personalized assessment and treatment.