Supplement · Integration of Metabolism
Integration of Metabolism: How the Body Switches Fuels
The pathways in this course don't run in isolation — they're switched on and off in a timed sequence as you move from fed to fasting to starving, coordinated by two hormones and traded between organs. This capstone is where glycolysis, gluconeogenesis, β-oxidation, ketogenesis, and the urea cycle become one connected system.
Every pathway in this course has been running in your body the whole time you read it — but not all at once. Glycolysis, gluconeogenesis, β-oxidation, ketogenesis, glycogen synthesis and breakdown: the cell does not run them simultaneously, because many are opposites that would cancel into a futile cycle. Instead it switches them on and off in a timed sequence as you move from fed to fasting to starving, coordinated by two hormones and traded between organs. This is the chapter where the separate maps become one system.
The body defends blood glucose, in a fixed order
Watch the hours after a meal and you see a relay, each runner handing blood glucose to the next.
Fed (0–4 hours). Glucose floods in from the gut, insulin rises, and the message to every tissue is store: the liver lays down glycogen and makes fat, muscle takes up glucose, adipose stores triacylglycerol. Glucose is the fuel of the hour.
Early fasting (4–16 hours). No more dietary glucose. Glucagon rises, and the liver maintains blood sugar by breaking down its glycogen. That store is finite — largely gone within about a day.
Fasting (1–2 days). With glycogen spent, the liver makes glucose from scratch by gluconeogenesis, drawing on lactate, the amino acid alanine, and glycerol. Meanwhile adipose tissue releases free fatty acids, which become the major fuel for muscle and liver. (The handoff between glycogenolysis and gluconeogenesis is a gradient, not a clean switch — by an overnight fast the two already contribute roughly equally.)
Starvation (days and beyond). Now the liver converts the flood of fatty-acid-derived acetyl-CoA into ketone bodies and exports them. This is the adaptation that saves your life, for a reason worth stating carefully.
The body never lets blood glucose drift far — it just keeps changing who pays for it. For the first few hours the meal covers the bill; then liver glycogen, good for maybe a day; then gluconeogenesis, built from lactate, the glycerol off your fat, and amino acids pulled from muscle. Run that too long and you would burn through your own muscle. The escape hatch is ketogenesis: the liver ships ketone bodies the brain learns to burn, cutting its glucose demand from roughly 120 to 40 grams a day. That single adaptation is the reason a healthy person survives weeks without food instead of days.
Why ketones matter: protecting the brain
The brain is the problem child of starvation. It normally burns about 120 grams of glucose a day, and it cannot meaningfully oxidize long-chain fatty acids — they ride through the blood bound to albumin and are largely kept out of brain tissue. If the only option were glucose, the body would have to make all 120 grams by gluconeogenesis, and the main raw material would be your own muscle protein. You would consume yourself within weeks.
Ketone bodies are the escape. They are small and water-soluble, they cross into the brain, and over several days the brain adapts to burn them — cutting its glucose requirement to roughly 40 grams a day. That protein-sparing effect is the difference between surviving a long fast and not. Note the elegant division of labor: the liver makes ketone bodies but cannot use them (it lacks the enzyme to reactivate them), so everything it produces is shipped out to the brain, heart, and muscle. Ketogenesis is an export business.
Organs trade fuel
No organ is metabolically self-sufficient, so the body runs a fuel economy. A sprinting muscle ferments glucose to lactate and ships it to the liver, which rebuilds glucose and sends it back — the Cori cycle the Coris first mapped. Muscle also exports carbon and nitrogen as alanine in the glucose-alanine cycle, handing the liver both a gluconeogenic substrate and the nitrogen it will dump as urea. Adipose exports fatty acids; the liver exports glucose and ketones. Toggle the map and watch the traffic reverse between the fed and fasted states.
Two states, two logics. Fed, insulin is high and the body distributes and stores: the gut and liver hand glucose to every tissue, and adipose and liver bank the surplus as fat. Fasted, the priority flips to mobilize and protect the brain: adipose releases fatty acids, muscle returns lactate and alanine to the liver, and the liver — the only organ that can — makes fresh glucose and ketone bodies to keep the brain running. No tissue is selfish here; the blood is just a ledger, and the hormones decide which way the entries point.
The two switches, and the one funnel
All of this coordination needs a controller, and there are two, working on different timescales. Across the whole body, the insulin-to-glucagon ratio is the master switch: insulin is the fed, anabolic signal; glucagon the fasting, catabolic one. They are powerful because they act reciprocally — glucagon’s cAMP/PKA cascade phosphorylates glycogen synthase off and phosphorylase on in the same stroke, so one signal swings an entire pathway. Inside each cell, AMPK is the local fuel gauge: when the AMP-to-ATP ratio climbs, it shuts off building and turns on burning.
What makes any of this controllable is the fact Hans Krebs put his finger on: “all foodstuffs are burnt through a common terminal pathway.” Carbohydrate, fat, and protein, however different, are all stripped to acetyl-CoA and fed through the citric acid cycle. There is one funnel to regulate, not a hundred — and that is why the body can flip cleanly between storing and spending.
The failure mode proves the rule. In untreated type 1 diabetes there is plenty of glucose in the blood, but with no insulin the cell cannot read the “fed” signal, so it runs the starvation program anyway: unrestrained lipolysis, runaway ketogenesis, and diabetic ketoacidosis — the body starving in the middle of plenty. Get the integration right and the whole course coheres: metabolism is not a list of pathways but a single, switched, organ-spanning system for keeping you alive between meals.
How we measure it
Inter-organ fuel cycling
Trading partly-spent fuel between tissues so the whole body stays supplied: the Cori cycle (lactate→glucose), the glucose-alanine cycle (carbon + nitrogen to the liver), and ketone-body export from liver to brain and muscle.
Hormonal reciprocal control
Insulin (fed/anabolic) and glucagon (fasting/catabolic) push the same enzyme pairs in opposite directions — glycogen synthase vs phosphorylase, ACC, PFK-2/FBPase-2 — mostly through phosphorylation, so one signal flips many switches at once.
Fuel sensing
AMPK reads the cell's AMP:ATP ratio and, when fuel runs low, shuts off building and turns on burning — the cell-level counterpart to the whole-body hormonal switch.