What are ketones, when does the body start producing them, and what can they fuel?
The Short Answer
Ketones are molecules produced by the liver when fat is broken down for energy. Production begins when glycogen stores are depleted — typically 12-24 hours into a fast — and ramps up significantly by 24-48 hours. Ketones can fuel most tissues including, crucially, the brain. This metabolic flexibility allowed humans to survive periods without food, and it’s the physiological basis for the mental clarity many people experience during extended fasting.
Something to Sit With
Your body has this remarkable backup fuel system — the ability to run on stored fat and power the brain through ketones.
When you fast, you’re not depriving your body of fuel. You’re switching to a different fuel.
Does understanding this change how fasting feels?
Learn More
What Ketones Are.
When the body burns fat, fat cells release fatty acids into the bloodstream. Most tissues can burn fatty acids directly. But the brain cannot — fatty acids don’t cross the blood-brain barrier effectively.
Without an alternative fuel, the brain would be entirely dependent on glucose. During any period without food, the body would need to break down muscle to produce glucose for the brain — metabolically expensive and unsustainable.
Ketones solve this problem. The liver converts fatty acids into ketone bodies:[1]
- Beta-hydroxybutyrate (BHB): The most abundant, often measured in blood tests
- Acetoacetate: Can convert to BHB or acetone
- Acetone: A byproduct, expelled through breath (the “keto breath” some people experience)
These smaller molecules can cross the blood-brain barrier and fuel the brain directly. This allows the body to preserve glucose for the few tissues that absolutely require it (red blood cells, parts of the kidney) while running everything else — including the brain — on fat-derived fuel.
When Ketone Production Begins.
The timing depends on individual factors (starting glycogen levels, activity, metabolic flexibility), but the general pattern:
Hours 0-12: Body running primarily on glucose from recent food and beginning glycogen use. Minimal ketone production.
Hours 12-18: Glycogen being depleted. Liver starts producing small amounts of ketones. Blood ketone levels begin to rise.
Hours 18-24: Significant ketone production begins. Blood levels reach 0.2-0.5 mmol/L — light nutritional ketosis.
Hours 24-48: Ketone production increases substantially. Blood levels may reach 0.5-1.5 mmol/L. The body is now running primarily on fat and ketones.
48+ hours: Deep ketosis. Blood ketone levels may reach 2-5 mmol/L during extended fasts. The brain is getting a significant portion of its fuel from ketones.
What Ketones Can Fuel.
The brain. This is the crucial capability. During extended fasting, ketones can provide up to 75% of the brain’s energy needs.[1] This preserved cognitive function during food scarcity was evolutionarily essential.
Heart muscle. The heart actually prefers ketones over glucose in many circumstances. It runs efficiently on beta-hydroxybutyrate.
Skeletal muscle. Muscles can use ketones, though they can also burn fatty acids directly.
Most other tissues. Ketones are a versatile fuel accepted by most of the body.
What ketones cannot fuel: Red blood cells lack mitochondria and can only use glucose. A few other specialized tissues are glucose-dependent. The body maintains glucose production for these through gluconeogenesis.
The Fasting Experience.
Understanding ketone production explains several fasting phenomena:
The transition period (12-24 hours). Some people feel hungry, irritable, or low-energy. The body is switching fuel systems. Ketone production hasn’t yet ramped up to fully replace glucose.
Mental clarity (after 24+ hours). Many people report improved focus and mental clarity once ketone production is established.[2] The brain seems to run well on ketones — possibly better than on fluctuating glucose in some ways.
Reduced hunger (after 24+ hours). Ketones appear to have appetite-suppressing effects. Once fully in ketosis, many people find hunger diminishes despite not eating.
Stable energy. Unlike glucose, which spikes and crashes, ketone availability is steady during a fast. Energy levels stabilize.
Why This Matters for Shorter Fasts.
Even if you’re doing 16:8 intermittent fasting (not reaching deep ketosis), understanding the ketone system matters:
- You’re beginning the transition toward fat-burning
- You’re training metabolic flexibility — the ability to switch between fuels
- You’re creating conditions for accessing stored fat
- The longer your fast, the more you tap into this system
Further reading:
- Cahill GF Jr, “Fuel metabolism in starvation”[3] — Annual Review of Nutrition, 2006
- Veech RL, “The therapeutic implications of ketone bodies”[4] — Prostaglandins, Leukotrienes and Essential Fatty Acids, 2004
- Anton SD et al., “Flipping the metabolic switch: understanding and applying the health benefits of fasting”[5] — Obesity, 2018
Written by AI, directed and approved by me. How this is made →