Metabolic changes during fasting/starvation

Early Fasting (0 to 24 Hours: The Glycogen Window)

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• The Hormonal Shift: Insulin drops. Glucagon and Epinephrine rapidly rise.

• The Primary Fuel Source: Hepatic Glycogenolysis (breaking down liver glycogen).

• The Mechanism: Glucagon knocks on the liver's door and activates the enzyme Glycogen Phosphorylase. The liver chops up its stored glycogen into free glucose and dumps it into the bloodstream to feed the brain and Red Blood Cells (RBCs).

• The Limitation: The liver only stores about 100 grams of glycogen. By the 18 to 24-hour mark, the liver's glycogen vault is completely empty.

High-Yield Board Pearl: Muscle is Selfish

‘blank’ tissue also stores a massive amount of glycogen, but it cannot share it with the rest of the body or the brain. ‘blank’ cells lack the enzyme Glucose-6-Phosphatase. Therefore, ‘blank’ glycogen is strictly used for muscle contraction.

The Fasting State (1 to 3 Days: The Gluconeogenesis Era)

The glycogen is gone. If the body doesn't find a new way to make glucose, the brain will die. The body must start cannibalizing itself to print new glucose.

The Fasting State (1 to 3 Days: The Gluconeogenesis Era)

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• The Primary Fuel Source: Hepatic Gluconeogenesis.

• The Raw Materials (Where do we get the parts to make glucose?): muscle, fat

• The Energy Shift: The rest of the body (heart, muscles, kidneys) stops using glucose entirely to save it for the brain. They switch to burning Free Fatty Acids via Beta-Oxidation.

From Muscle

Muscle protein is violently broken down (proteolysis) into free amino acids. Alanine and Glutamine are shipped to the liver to be converted into glucose.

From Fat

• Adipose tissue undergoes massive lipolysis (breaking down triglycerides). This releases free fatty acids and a Glycerol backbone. The liver uses the glycerol to make glucose.

High-Yield Board Pearl: The Brain's Limitation

Why can't the brain just burn fat like the rest of the body? Free fatty acids travel through the blood bound to a massive protein called Albumin. This Albumin-Fatty Acid complex is physically too large to cross the Blood-Brain Barrier. The brain must have glucose (or ketones).

Prolonged Starvation (3 Days to Weeks: The Ketone Era)

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• The Primary Fuel Shift: Ketogenesis (Making Ketone Bodies).

• The Mechanism: The liver is burning so much fat that it creates a massive backlog of Acetyl-CoA. It takes this excess Acetyl-CoA and condenses it into Ketone Bodies (Acetoacetate and Beta-hydroxybutyrate).

• The Brain's Adaptation: The brain adapts to start using these ketone bodies! During prolonged starvation, ketones can provide up to 70% of the brain's energy requirements.

• The Result: Because the brain is now happily running off ketones, the liver can drastically slow down Gluconeogenesis. This halts the rapid breakdown of muscle tissue, extending the person's survival from weeks to months.

High-Yield Board Pearl: The Liver's Sacrifice

The liver is the only organ that manufactures ketone bodies, but the liver cannot use them for its own energy. It lacks a crucial enzyme called Thiophorase. The liver acts purely as the altruistic factory, making the fuel and shipping it out to save the brain.

Diabetic Ketoacidosis (DKA)

This is essentially "starvation in the midst of plenty." In Type 1 Diabetes, the patient has zero insulin. Even if their blood sugar is 600 mg/dL, the cells cannot see it. The body literally thinks it is starving to death. It triggers massive lipolysis and ketogenesis, flooding the blood with acidic ketone bodies and dropping the blood pH to lethal levels

The Ketogenic Diet

Used pharmacologically to treat refractory pediatric epilepsy. By eating zero carbohydrates and massive amounts of fat, the patient artificially forces their body into the Phase 3 starvation state. The continuous flood of ketone bodies to the brain raises the seizure threshold and calms electrical storms.

Alcoholic Ketoacidosis

Chronic alcoholics often drink their calories instead of eating, depleting their glycogen stores. The metabolism of alcohol heavily depletes the liver's NAD+ supply, completely shutting down gluconeogenesis. The body panics and starts churning out ketones, leading to a dangerous acidic state

Fed State

Timeframe: 0 - 4 Hours

Main Blood Glucose Source: Dietary Carbohydrates

Major Fuel for the Brain: Glucose

Major Fuel for Muscle/Heart: Glucose

Early Fasting

Timeframe: 4 - 24 Hours

Main Blood Glucose Source: Hepatic Glycogenolysis

Major Fuel for the Brain: Glucose

Major Fuel for Muscle/Heart: Free Fatty Acids

Fasting

Timeframe: 1 - 3 Days

Main Blood Glucose Source: Hepatic Gluconeogenesis

Major Fuel for the Brain: Glucose

Major Fuel for Muscle/Heart: Free Fatty Acids

Starvation

Timeframe: > 3 Days

Main Blood Glucose Source: Hepatic Gluconeogenesis

Major Fuel for the Brain: Ketone Bodies (and Glucose)

Major Fuel for Muscle/Heart: Free Fatty Acids