MECHANISM ENGINE

What Is Ketosis?

Day three of an extended fast is when this properly kicks in for me, and it’s the strangest part of the whole process. The hunger that was loud on day one goes almost completely quiet. That’s not willpower. That’s ketosis doing something specific.

The Quiet

There’s a point, usually the second or third day of an extended fast, where the noise just stops. Not gradually, more like someone’s turned a dial down. That’s what I call The Quiet, and it lines up almost exactly with the body properly shifting into ketosis, running meaningfully on fat and ketones rather than glucose. It’s not that hunger disappears forever. It’s that once the body’s confident it has a reliable, steady fuel source, the urgency around finding food, the constant background nag, eases right off.

Ketosis is a metabolic state where the liver converts fatty acids into ketone bodies, which can be used as fuel by the brain and other tissues once glucose and glycogen availability drops. It typically begins somewhere between 12 and 24 hours into a fast for most people, though timing varies with prior diet and glycogen stores, and deepens over the following days of continued fasting or a very low carbohydrate intake.

Ketone body metabolism as an alternative fuel source is well established in the research literature; NHS materials note that extended fasting should be approached cautiously and is not recommended without medical guidance for anyone on medication or with an existing health condition.

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Related mechanisms

Educational only, not medical advice. Speak to your GP before changing your fasting pattern, diet, or medication.

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What this is based on

NHS and NICE guidance, cross-referenced against Neil’s own NHS blood test record going back to April 2012. See the full timeline and current numbers.