MECHANISM ENGINE
What Are Fasting States (Fed vs Fasted)?
I thought I knew what fasting was before I actually started. Skip breakfast, eat lunch and dinner, job done. Turns out I’d missed the entire point. It’s not about skipping meals. It’s about which state your body is actually running in.
The Switch
There are, broadly, two states your metabolism runs in. Fed, where insulin’s up and the body’s using and storing incoming glucose. And fasted, where insulin drops and the body shifts to drawing on stored fuel instead, glycogen first, then fat. The Switch is what happens at the point the body moves from one state to the other, usually somewhere in the eight to twelve hour mark after your last meal for most people, though it varies. You can’t be in both states at once. Every snack, every grazing habit, keeps flicking the switch back to fed before the fasted state’s had a proper chance to do anything.
The fed state begins with eating and lasts several hours while insulin remains elevated to manage incoming glucose. The fasted state begins once insulin drops back to baseline, typically some hours after the last meal, at which point the body increasingly draws on glycogen and then fat stores for fuel. Time-restricted eating and intermittent fasting work by deliberately extending the fasted window, giving the body longer, uninterrupted stretches in that state.
Research on time-restricted eating consistently shows metabolic differences between fed and fasted states, with longer fasted windows associated with improved insulin sensitivity in several studies, though individual response varies and NHS guidance recommends checking with a GP before fasting on medication.
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Related mechanisms
Educational only, not medical advice. Speak to your GP before changing your fasting pattern, diet, or medication.
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.