Metabolic States of the Body

Core Concepts

The absorptive (fed) state: build and store

For roughly two to four hours after a meal, absorbed glucose, amino acids, and triglycerides flood the blood. Rising glucose triggers insulin release, and insulin pushes every tissue into storage mode: liver and muscle store glucose as glycogen (glycogenesis); adipose tissue stores fatty acids as triglycerides while insulin inhibits lipolysis; muscle takes up amino acids and builds protein; most cells burn glucose for ATP. Glucose is the preferred fuel, and the body largely spares its fat and protein stores.

The postabsorptive (fasting) state: release and synthesize

As absorption ends, blood glucose falls, insulin drops, and glucagon rises. The fasting state runs on three processes:

• Glycogenolysis — the liver breaks down its glycogen and releases glucose, keeping blood glucose normal for roughly the first four to twelve hours. Muscle glycogen is also broken down, but muscle cannot release glucose into the blood (it lacks glucose-6-phosphatase), so it fuels the muscle itself.
• Gluconeogenesis — the liver (and to a lesser extent the kidneys) builds new glucose from lactate, glycerol, and glucogenic amino acids.
• Lipolysis — adipose tissue releases fatty acids and glycerol; most tissues — heart, skeletal muscle, liver — now burn fatty acids for ATP.

The key principle is glucose sparing: the body stops burning glucose wherever it can, reserving it for the brain and red blood cells.

The brain's fuel problem and the shift to ketones

The brain normally runs almost entirely on glucose, using a large share of it — commonly cited as on the order of 100–120 grams per day. Liver glycogen holds only a fraction of that, so a fast longer than about a day forces heavy gluconeogenesis from protein, wasting muscle. The adaptation is ketogenesis: the liver, flooded with fatty acids, diverts acetyl-CoA into ketone bodies — acetoacetate, beta-hydroxybutyrate, and acetone — which the heart, skeletal muscle, and (after several days of adaptation) the brain use as fuel. "Going into ketosis" means the fuel mix shifts so the brain can run on fat-derived ketones while protein is spared. Acetone is volatile and leaves in the breath, which is why heavy ketosis gives the breath a sweet, fruity odor.

Prolonged fasting and starvation

Over days to weeks, the same strategy deepens: fat is the main fuel, ketones increasingly feed the brain, and the metabolic rate drifts downward. Protein breakdown slows once ketone adaptation is complete — the body is "protein sparing" — until, when fat is nearly exhausted, protein becomes the dominant fuel and vital organs fail. Classic exam timeline: fed state → glycogen depletion → gluconeogenesis peaks → ketosis ramps up → protein sparing → terminal protein loss.

Hormonal control beyond insulin and glucagon

Glucagon, epinephrine, cortisol, and growth hormone are counter-regulatory hormones: they raise blood glucose by promoting glycogenolysis, gluconeogenesis, and lipolysis — which is why chronic stress or illness tilts the body toward a fasting-like, catabolic pattern even while eating.

When the system breaks: diabetes mellitus

In type 1 diabetes, beta cells fail and insulin is essentially absent, so the body behaves as if permanently fasting: glucose cannot enter cells, blood glucose soars, and unopposed lipolysis and ketogenesis produce ketones faster than tissues can use them — diabetic ketoacidosis (DKA), a metabolic acidosis with high glucose, ketones, and dehydration. In type 2 diabetes, insulin is present but tissues resist it: the fed-state signal is blunted and glucose production stays inappropriately high. These are educational mechanisms; diagnosis and treatment are clinical decisions made by providers.

Common Confusions