A 16-hour fast — 0 seconds

Follow a representative sixteen-hour window between meals, from the last mouthful to the point where the liver is making most of the glucose in circulation.

Representative model

Change event

Absorption finishes

For the first few hours the gut is still delivering glucose from the meal, so the early part of a fasting window is not yet a fasted state.

The insulin and glucagon balance shifts

As meal-derived glucose stops arriving, pancreatic insulin secretion falls; glucagon secretion rises more gradually. The falling insulin signal is the larger and better measured of the two changes over a window this short.

Fat-derived fuels take a larger share

Fatty acids released from fat tissue supply more of the body's energy, and the liver converts some of them into ketone bodies. At this stage of a fast, measured ketone concentrations are still low; the large rise belongs to much longer fasts.

Stored liver glucose is released

The liver breaks down its glycogen store and releases glucose into the bloodstream. That store is finite, so its contribution shrinks as the hours pass.

The liver makes glucose of its own

The liver also builds new glucose from precursors such as glycerol, lactate and amino acids. Its share of the glucose in circulation grows as the glycogen store is drawn down.

Interactive 3D is loading into this same frame contract.

Last meal ends · 00:00:00
Absorption at its peak · 01:00:00
Postabsorptive state · 04:00:00
Liver supply shifts · 08:00:00
Twelve-hour mark · 12:00:00
Sixteen-hour mark · 16:00:00

Happening now

The last meal has just finished. Digestion and absorption are still under way, so the displayed glucose range is rising rather than falling.

Explore the mechanism Absorption finishes

For the first few hours the gut is still delivering glucose from the meal, so the early part of a fasting window is not yet a fasted state.

Evidence for this step

After a meal, glucose absorbed from the gut typically stops arriving within a few hours, and the body moves into a postabsorptive state in which the glucose in circulation is supplied by the liver rather than by food. moderate · View sources

Model status

Representative modelEducational only

Educational physiology only; not medical advice, diagnosis or treatment guidance.

This event shows a representative pattern, not a personal prediction.
Describing what a representative body does between meals is not a suggestion that anyone fast, or fast for any particular length of time.
The hour at which each change becomes noticeable varies with the last meal, sleep, activity, body composition and metabolic context.
Displayed ranges are deliberately wide teaching ranges, drawn around the group averages reported in the cited studies rather than around any individual, and they have not yet received formal domain approval for publication.
The studies behind this sequence measured small groups of young adults, and one of them enrolled men only. Where women and men were compared, the size of the fat-release response differed between them. Older adults and people with a different body composition are under-represented in the evidence shown here.
The dominant-fuel readout can only say glucose-dominant, returning-to-mixed or mixed. It cannot yet express that the mix keeps leaning further toward fat through the second half of this window, so the mechanism text carries that detail instead.
Ketone bodies and fatty acids are described in words only. This event carries no numeric ketone value: the metric registry has no reviewed ketone entry, and the one directly measured fourteen-hour figure behind the wording here comes from a single small study of eight healthy adults.

Open the liver in system focus

A 16-hour fast — 0 seconds

Blood glucose

Insulin response

Glucose uptake

Dominant circulating fuel

Fullness