CARBS 2 | March ‘25

Why Your Brain Is Thirsty
For Carbs

Despite getting trashed in the pop-nutrition discourse, carbs are still the champions of the energy substrate debate. Besides the body preferring them for high-intensity exercise, the brain also prefers them, and in their simplest, most common form—the glucose molecule. A typical adult can use up to 120 grams of glucose per day. Honey is the most concentrated source of glucose most people regularly eat, so when you’re sweetening your tea, you can almost literally think of it as brain food!

Here’s some other reasons simple carbohydrates (typically in the form of sugars) are important for brain function:

🧠 1. Glucose Is Readily Available and Fast-Acting

The brain is a high-energy organ, using about 20–25% of the body’s glucose despite being only ~2% of body mass. Because glucose is water-soluble, it travels easily in the blood and can transfer to tissues quickly. It also metabolizes very quickly (as discussed in the last newsletter), offering rapid ATP production, which is vital for neurons that constantly fire and need stable energy.

🔒 2. Fatty Acids Can't Cross the Blood-Brain Barrier (Efficiently)

Fat stores can power your low-intensity, daily activities and metabolism, but they don’t give the brain the boost it needs for all of that electrical activity, nor can they really fit through the blood-brain-barrier (BBB). Long-chain fatty acids (the primary fat-based energy source) are mostly blocked by the BBB, which protects the brain from toxins but also limits what fuels get in. Glucose, on the other hand, uses special glucose transporters (like GLUT1 and GLUT3) to cross.

⚡ 3. Neurons Have Minimal Energy Storage

Neurons can’t store much glycogen in their astrocytes, so they rely on a constant glucose (i.e. basic sugar) supply from the blood. They are highly specialized for signaling, not storage, and accumulating glycogen could disrupt their fine-tuned ion balances and structure.

🔌 4. Glucose Supports Both Energy and Biosynthesis

Glucose is also needed for neurotransmitter synthesis (e.g., glutamate, GABA), which literally have the “glu” of “glucose” in their name. It’s also needed for lipid synthesis (the fatty sheaths around of myelin around neurons and membranes around other cells) and antioxidant production (e.g., NADPH via the pentose phosphate pathway).

🧴 5. Ketones Are a Backup, Not a Primary Fuel

During fasting, starvation, or keto diets, the liver makes ketone bodies (like beta-hydroxybutyrate), which the brain can use for fuel. But even then, it still demands some glucose (usually 20–30%) — the switch is more like a plan B, not a first choice, because they don't support all of the brains necessary functions.

Don’t Shy Away From Sugars

The real culprit in high-sugar related disease is exaggerated caloric-surplus, so don’t starve your brain to (supposedly) save your body. Within healthy-caloric thresholds, a little extra sugar from heathy, whole-food carbohydrates won’t hurt.

(...and also, don’t be afraid to have that extra piece of dark chocolate 😉)