The Brain
Runs on Fuel.

A fifth of everything, for three pounds of tissue.

The brain is about 2% of your body weight and burns roughly 20% of your resting energy — a tenfold concentration of demand, running every second of every day, awake or asleep.[1] It has almost no storage of its own; it lives hand-to-mouth on whatever fuel the blood delivers. That relentless appetite is why the brain is so exquisitely sensitive to which fuel is arriving and whether it can use it.

The brain is not glucose-only.

You'll often hear that "the brain can only run on glucose." It's one of the most repeated half-truths in physiology. Glucose is the brain's default fuel, yes — but it is not the only one it can use. When glucose is scarce — during fasting, or on a very low-carbohydrate diet — the liver makes ketone bodies from fat, and the brain takes them up and burns them readily.

This isn't a fringe claim; it was shown cleanly in humans in the 1960s. Measure what a fasting brain actually consumes and you find ketones supplying a large share of its fuel — in deep fasting, the majority — with glucose demand falling to match.[2] The brain, it turns out, has a second fuel line built in, and switching to it is a normal, ancient adaptation, not an emergency.[3]

Brain insulin resistance is an energy crisis.

Insulin isn't only a signal in the body; it has jobs in the brain too, including helping neurons take up and use glucose efficiently. When the same resistance this whole site is about reaches the brain, those neurons start struggling to fuel themselves on their default fuel — a slow, regional energy shortfall in exactly the areas that depend most on steady glucose.[4]

This is the thread picked up in Type 3 Diabetes — the idea that part of what we see in Alzheimer's looks like the brain's own version of the fire. Here, the point is narrower and more mechanical: if the glucose line is jammed, a brain with a second fuel line has somewhere else to turn. That isn't a cure for anything — it's a plumbing fact with large implications.

One fact, pointing three ways.

Everything so far reduces to one sentence: the brain is an energy organ with a backup fuel line. From there, three roads open.

• Toward epilepsy — the firmest ground of all. A century of using a ketone-based diet to calm a storming brain, where the fuel switch itself is the treatment. That's the next chapter, The Storming Brain.
• Toward dementia — the brain-energy reading of Alzheimer's as a regional fuel crisis, carried in Type 3 Diabetes, held honestly about what's shown and what isn't.
• Toward mood and the mind — the frontier, where the same fuel logic is being tested in psychiatry. The most exciting and the most easily overclaimed, and the one that has to earn its place behind the other two.

A spare fuel line, held honestly.

It's easy to get carried away here, so the grading matters. That the brain can run on ketones, and that this is a normal adaptation, is settled. That the fuel switch treats drug-resistant epilepsy is well established. Beyond those, the picture gets more hopeful than proven — and the honest thing is to say exactly where the line sits.

• A therapeutic ketogenic diet is a medical intervention, not a casual tweak. For epilepsy and the like it's done with a clinical team — it needs monitoring and isn't right for everyone.
• Nothing here is a reason to change medication. Especially not seizure medication — never alter it on the strength of a web page. This is understanding, to bring to a doctor.

An energy organ, with a spare tank.

Strip the brain down and it is, before anything else, a ravenous little engine that cannot store its own fuel. Most of the time it runs on glucose and never thinks about it. But the backup tank — the ketone line, quietly built in — is the detail that matters most, because when the main line jams, having somewhere else to turn is the difference between a starving brain and a fuelled one.

The brain runs on fuel. The surprise, and the hope, is that it was always built to run on two.