⏱️ Glycogen Depletion to Fat Oxidation Metabolic Switch Timeline

Glucose & Liver Glycogen — The First Four Hours

Right after a meal, glucose and stored glycogen run the body.

• ~100 g: Liver glycogen store (≈400 kcal reserve)
• ~90 mg/dL: Blood glucose (tightly regulated)
• ~120 g/day: Brain glucose use (obligate consumer)
• ~15%: Fat oxidation share (minor contributor)

Insulin keeps glycogen synthase active

High insulin favors glycogen storage over breakdown.

Liver buffers blood glucose

Hepatocytes release glucose to hold levels steady.

Liver glycogenolysis alone can hold blood glucose steady for about four hours.

Muscle glycogen stays local

Muscle stores fuel contraction, not blood glucose directly.

Liver Glycogen Drains Through Hour 4–12

Glycogenolysis accelerates as the fast stretches past breakfast.

• ~50–70%: Glycogen remaining (by hour 8)
• 2–3×: Glucagon rise (vs fed baseline)
• ~25%: Fat oxidation share (gradually climbing)
• begins: Gluconeogenesis (liver supplements supply)

Glucagon takes over

Falling insulin lets glucagon drive glycogen breakdown.

Gluconeogenesis starts early

Liver begins building new glucose from lactate and amino acids.

Adipose lipolysis picks up

Fat cells start releasing more free fatty acids.

By hour twelve, roughly half of liver glycogen is already spent.

Hour 12–18 — Glycogen Nearly Exhausted

Reserves run low while fat oxidation rises to fill the gap.

• ~15–25%: Glycogen remaining (near depletion)
• ~40–50%: Fat oxidation share (rapidly rising)
• 2× baseline: Free fatty acids (plasma concentration)
• trace: Ketone bodies (just beginning to rise)

Glycogen buffer thins out

Liver stores approach their functional lower limit.

Fatty acid delivery surges

Adipose tissue floods blood with free fatty acids.

Crossover approaches

Fat oxidation is about to overtake glycogen as dominant fuel.

The crossover point shifts later when baseline glycogen stores start higher.

Hour 18–24 — Fat Oxidation Takes the Lead

Fatty acid oxidation now covers most of resting energy needs.

• ~65–75%: Fat oxidation share (now dominant)
• <10%: Glycogen remaining (residual reserve)
• rising: Ketone bodies (liver ketogenesis active)
• ~70 mg/dL: Blood glucose (gluconeogenesis-supported)

Beta-oxidation ramps up

Mitochondria process fatty acids for the bulk of ATP.

Liver starts making ketones

Excess acetyl-CoA is converted into ketone bodies.

Glucose sparing protects the brain

Fat use elsewhere preserves scarce glucose for neurons.

Fat oxidation crossing fifty percent marks the true metabolic switch.

Hour 24+ — Sustained Fat Oxidation & Ketosis

Fat oxidation and ketone production settle into a stable rhythm.

• ~80–90%: Fat oxidation share (stable plateau)
• 1–2 mM: Ketone bodies (well-established ketosis)
• ~4–8%: Glycogen remaining (residual floor)
• ~65–70 mg/dL: Blood glucose (held by gluconeogenesis)

Ketones fuel the brain

Ketone bodies now supply a growing share of brain energy.

Protein sparing improves

Ketosis reduces reliance on muscle protein breakdown.

A stable fasted equilibrium

Fuel mix stays steady until refeeding or prolonged fasting.

This steady state can persist for days once fully established.