Why card · first principles

Why does some of a surplus become muscle and the rest fat?

A store must have a form; the body has three, and only one of them is built at a rate.

why/the-partitionDomain · chemistry of storageNo verdict, no grade · first principles are settled
At rest

The model you brought

"Eat extra and train, and the extra goes to the muscle you trained. The body puts the calories where the work is."

Whyborrowed · logistics

The why

Energy can only be kept in bonds, and the body has three kinds of bond to keep it in: glycogen, protein and fat. Glycogen is a day's worth and full within days of eating past it. Protein is built by machinery that runs at a rate, into tissue that has a job, so it can take only so much a day however much is offered. Fat has no ceiling. Whatever the ceiling on building leaves over goes to the store with no ceiling. Nothing sends it there; it is simply the only door still open.

The forms are not equal in cost. A kilogram of fat holds about 39.5 megajoules; a kilogram of new lean tissue, mostly water around its protein, about 7.6. So the same surplus makes about five times the weight in lean tissue that it would in fat, if the machinery can build it, and the spare goes to the dense store. And the carbohydrate you eat is not, mostly, what becomes the fat: in a human it is burned first and spares the fat you ate, which is stored as it is; only when carbohydrate alone exceeds what you spend does the liver make fat from it in earnest.

Whysettled · first principle
energy surplusglycogena day’s worth · already fullprotein · musclebuilt at a rate · into tissue with a jobfatno ceiling · takes the remainderthe valve · opened by training,not by the pour
Three stores under one pour. On the left a small jar, glycogen, already full: a day's worth, and the pour runs off it. In the middle a vessel with a narrow neck and a valve, protein: the pour passes only as fast as the valve lets it, and the valve is opened by training and by the amino acids on hand, never by the size of the pour. On the right a wide tank with no lid, fat: whatever the other two cannot take falls in. Nothing steers the pour; the shapes do. To draw: jar full to the brim with overflow, the necked vessel with its valve half open, the open tank, and one stream from above splitting three ways by width.

What it unlocks

  • Why a bigger surplus is a fatter one: the valve does not open wider because more is poured.
  • Why the beginner and the lifter get different results from the same plate: the beginner's valve is barely open and training opens it wide; the lifter's is near its limit already.
  • Why "lean mass" on a scan is not muscle: the water held around protein and glycogen weighs, and moves, on its own.
  • Why the surplus and the deficit are not mirror images: emptying the fat store is free of any valve, but refilling the protein store never is. Chapter two.

The principle in one line

A store has a form, and the form sets its capacity, its cost and its rate. Glycogen: about 15 g per kilogram of you at the very most, about 500 g of headroom in practice, roughly a day's energy, and full within days of eating past it; then, and only then, the liver makes fat from carbohydrate in quantity. Protein: built by ribosomes at a rate that training raises and a meal's amino acids saturate, into muscle, organ and enzyme that have to earn their keep, so the store cannot be force-filled. Fat: triglyceride in adipocytes, the densest store by five to one and the only one with no ceiling, filled mostly by the fat you ate while the carbohydrate you ate is burned in its place. The pour does not choose. The three shapes do.

Changed

—No revisions. First principles move rarely; when the reader's picture is wrong, it is the picture that changes, and that is recorded here.