Beyond Sugar: How the Brain Uses Fat for Energy

For years, public nutrition and medical education have framed glucose as the brain’s primary or default fuel, often implying that sugar is the main driver of cognition, focus, and mental clarity.¹

The truth is more nuanced.

The brain is metabolically flexible. It can use multiple fuels, and fat-derived energy plays a far bigger role than most people realize.² At the same time, chronically high blood sugar, most often driven by frequent intake of refined carbohydrates and sugars, doesn’t just affect energy. It can also interfere with how the body handles other nutrients, including vitamin C, which matters for immune and skin health.³

This isn’t about avoiding carbohydrates or sugar entirely, the body still uses glucose. It’s about recognizing that glucose isn’t the brain’s only fuel, and that relying on it alone can create instability. Understanding how the brain meets its energy needs using different fuels helps explain why steadier energy matters.

The Brain Isn’t Powered by Glucose Alone

Glucose aka sugar, is a common fuel source for the brain, but it’s not the only one.

Research shows the brain can also rely heavily on fat-derived fuels called ketones, especially when glucose availability or utilization is limited.³ In fact, neurons themselves are capable of tapping into fat stores for energy, something that challenges the idea that fat is merely “backup fuel” for the brain.¹

This metabolic flexibility is built into human biology.
It’s not extreme.
It’s not new.

So What Are Ketones, Exactly?

Ketones are energy molecules made from fat, primarily in the liver, when the body shifts some of its energy production away from glucose.⁴

The three main ketones are:

• Beta-hydroxybutyrate (BHB) — the primary circulating ketone and the one most used by the brain
  
  

• Acetoacetate (AcAc) — an intermediate ketone used by tissues
  
  

• Acetone — a minor byproduct that’s mostly exhaled
  
  

When ketones are available, the brain can use them directly to make ATP, the molecule that powers cellular activity. In other words: yes, ketones provide real, usable energy.³

Ketones vs. Glucose: Different Roles, Different Strengths

Glucose:

• Is fast and readily available
  
  

• Works well when intake, digestion, and insulin signaling are stable
  
  

• Can fluctuate more with meal timing and blood sugar swings
  
  

Ketones:

• Provide a steady, low-volatility energy source
  
  

• Are less dependent on rapid insulin signaling
  
  

• Help cover energy needs when glucose supply or utilization is inconsistent³
  
  

This is why ketones are often described in research as a way to support brain energy during gaps, not replace glucose entirely.³

How Does the Body Use Ketones for Fuel?

The brain has a constant, high energy demand, but it has limited ability to store fuel. That’s why it evolved multiple energy pathways.

In simplified terms:

• Fat is mobilized (from diet and/or body stores)
  
  

• The liver converts fatty acids into ketones
  
  

• Ketones circulate in the bloodstream
  
  

• Brain cells convert ketones into energy inside the mitochondria⁴
  
  

This pathway exists so the brain can stay fueled even when glucose is less available, not so people can deprive themselves of food.

Are Ketones “Better” Fuel Than Glucose?

“Better” depends on context.

What the research does show is that:

• Ketones are the only significant alternative fuel for the brain when glucose metabolism is impaired or insufficient³
  
  

• In certain metabolic states, ketones can supply a large share of the brain’s energy needs⁴
  
  

That doesn’t mean glucose is bad. It means the brain is adaptable.

A steady goal isn’t to eliminate glucose, it’s to support metabolic flexibility and consistent energy availability.

How Much of the Brain’s Energy Can Come From Fat?

You may see claims that the brain can run “almost entirely” on ketones.

The most consistently cited figure in peer-reviewed research is closer to ~60% of the brain’s energy needs during prolonged low-glucose states, not a fixed 80%.³⁴

What matters more than the number is the principle:

Fat-derived fuels can meaningfully support brain energy, far more than most people have been taught.

How Ketones May Support Cognitive Function

Ketones don’t just act as fuel.

Research suggests ketones may support brain function by:

• Helping bridge energy gaps when glucose delivery or utilization is inconsistent³
  
  

• Supporting cellular energy efficiency⁴
  
  

• Influencing signaling pathways involved in resilience and stress response (an area still under active research)³
  
  

This is not a cognitive “hack.” It’s basic metabolic support.

How Blood Sugar Affects Vitamin C Uptake

High blood sugar doesn’t only affect energy, it can also affect nutrient uptake.

Vitamin C (ascorbic acid) is essential for:

• Immune function
  
  

• Collagen production (skin, connective tissue, wound repair)
  
  

Here’s the key detail: glucose and vitamin C can compete during absorption and cellular uptake.

The oxidized form of vitamin C (dehydroascorbic acid) uses some of the same transport pathways as glucose. In higher-glucose environments, vitamin C uptake into cells may be reduced.²⁵

This isn’t about deficiency, it’s about how nutrients are handled once they’re in the system.

Why This Matters for Immune and Skin Health

Vitamin C plays a direct role in immune cell function, antioxidant protection, and collagen synthesis.²⁵

If glucose consistently crowds out vitamin C at the cellular level, those systems may not be supported as effectively, even when intake looks “adequate” on paper.

This is not about avoiding fruit or carbohydrates. It’s about understanding that chronic blood sugar instability can influence how well nutrients are actually used, and that foundational nutrients help create the environment in which other systems function optimally.⁶

A Steady Takeaway

Ketones remind us that the brain is designed with more than one way to meet its energy needs. They are a normal part of human metabolism that allow the brain to stay fueled when conditions aren’t perfectly stable.¹³

Vitamin D3 fits into this picture in a quieter, foundational way. It doesn’t act as a fuel source, but it helps shape the environment the brain and immune system operate within.⁶

Together, ketones and vitamin D3 point to the same principle: the brain functions best when its basic requirements are met consistently, without extremes.

Sources

NIH (NIDDK): Blood Glucose and Brain Energy https://www.niddk.nih.gov/health-information/diabetes/overview/preventing-problems/low-blood-glucose-hypoglycemia

Harvard T.H. Chan – Carbohydrates and Brain Fuel https://www.hsph.harvard.edu/nutritionsource/carbohydrates/

Neurons Can Tap Into Fat for Fuel- National Institutes of Health (NIH Research Matters)
https://www.nih.gov/news-events/nih-research-matters/neurons-can-tap-into-fat-fuel

Ketone Bodies as a Fuel for the Brain During Glucose Hypometabolism
Cunnane SC, Courchesne-Loyer A, et al. — Frontiers in Nutrition
https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2021.783659/full

Ketone Bodies: A Review of Physiology, Pathophysiology and Application of Monitoring to Diabetes - Puchalska P, Crawford PA — Journal of Diabetes Science and Technology / PMC
https://pmc.ncbi.nlm.nih.gov/articles/PMC7699472/

Is Fat the Best Brain Fuel? - Food for the Brain Foundation
https://foodforthebrain.org/is-fat-the-best-brain-fuel/

Vitamin C Transporters and Their Function - Savini I, Rossi A, Pierro C, Avigliano L, Catani MV — Molecular Aspects of Medicine / PMC
https://pmc.ncbi.nlm.nih.gov/articles/PMC3610982/

Glucose–Ascorbate Antagonism: Impact on Cellular Vitamin C Uptake - Wilson JX — Annals of the New York Academy of Sciences
https://pubmed.ncbi.nlm.nih.gov/9550452/

Blog Image- https://unsplash.com/photos/mother-and-daughter-preparing-salad-in-the-kitchen-P15HvVE_6iQ?utm_source=unsplash&utm_medium=referral&utm_content=creditShareLink