What is fat gain?

Most people think about weight gain and fat loss in terms of calories, but calories are a unit, not a substance. You don’t eat calories, you eat molecules. This short article explains the physics and chemistry of weight, mass, and energy, why dietary fat is stored more efficiently than carbohydrate, and why a low-carbohydrate diet is not necessarily more effective for fat loss.

This article is actually a copied section from “Adaptive Capacity Framework”, available to members.

What is weight, and weight gain?

If you weigh 70kg, the scale is experiencing 687N. Gravity pulls you down with a force equal to your mass × 9.81 m/s² (your weight in newtons). Mass is the amount of “stuff” you are made from, but less substance and more property. It is a number that tells you how an object will behave when forces, gravity, or energy are involved. Curiously, if you take a proton apart, the three quarks inside have almost no mass, only about 1% of the proton’s total mass^[1]. The other 99% comes from the energy of the strong force, the gluon fields and the motion of the quarks, via E = mc².

So most of your mass is really trapped energy; however, that’s nuclear physics, not chemistry. Fat loss is a chemical process: atoms are rearranged into more stable molecules—carbon dioxide (CO₂) and water (H₂O), and the mass leaves your body through your breath, sweat, and urine. Fat leaves your body almost entirely as exhaled CO₂ and water. And it’s nitrogen from protein breakdown that leaves as urea in urine.

At the everyday scale, mass and energy behave like different things. Mass is atoms, physical matter you can weigh. Chemical energy is a property of how those atoms are arranged, and the mass changes that accompany it are far too small to register on a scale. At the nuclear scale, mass and energy are equivalent, but chemistry doesn’t reach those energies.

When you gain fat mass, it is the carbon, hydrogen, and oxygen atoms in the food consumed being digested, absorbed, and then rearranged and incorporated as fat (lipids) in fat cells (adipose tissue), and other cells like muscle and the liver. Those atoms can also be stored as glycogen—branched chains of glucose, in the liver and muscles. For protein, it is the amino acids carbon, hydrogen, oxygen, and nitrogen, being incorporated into muscle and other tissues.

A glucose molecule (C₆H₁₂O₆) has more energy than the carbon dioxide (CO₂) and water (H₂O) it eventually becomes, not because it contains calories like a container, but because its atoms are held together in a higher-energy configuration. When glucose reacts with oxygen (O₂), those atoms rearrange into more stable molecules: CO₂ and H₂O. The energy difference between the starting and ending arrangements is released, first captured in a usable chemical form (ATP), then ultimately given off as heat.

So calories don’t accumulate as a substance in the body, atoms do, and they are in an arrangement that can release energy when reordered. When we say food has calories, we mean its atoms can rearrange into more stable molecules, releasing the difference. A kilocalorie (kcal) is a unit, not a thing. We don’t eat calories, we eat molecules.

You might not know, but dietary fat and carbohydrate are not stored with the same efficiency. When consumed in excess, 90-95% of excess dietary fat energy is stored, compared to only 75-85% of excess carbohydrate energy^[2]. Dietary fat requires little conversion to body fat, as we break them down during digestion, and rebuild them again for storage. The “re-esterification” of fatty acids into triglycerides has a thermic cost of only 2-3%^[3]. Whereas converting carbohydrate to body fat (de novo lipogenesis, DNL) costs ~20-25% of the energy, and requires sustained carbohydrate overfeeding at a rate faster than it can be oxidised or stored as glycogen. Much of this DNL happens in the liver, where it outpaces the liver’s ability to export the fat it produces, and hepatic (liver) fat accumulates. Past a threshold, this is fatty liver disease: metabolic dysfunction–associated steatotic liver disease (MASLD).

So our body fat is primarily derived from the fat we eat, not from the carbohydrate or protein.

Carbohydrate consumption acts as a metabolic switch: the rise in blood glucose raises the hormone insulin which signals carbohydrate oxidation and storage (glycogen), and fat tissue (adipose) stops releasing fat and starts storing it. This continues until the dietary glucose in the blood is used and stored resulting in the return of blood glucose back to baseline. This metabolically switches back to fat release and use for energy. So when we consume carbohydrate, we increase carbohydrate use for energy over fat.

But no, a low carbohydrate diet is not more efficient for lowering body fat. This is because you still need to consume enough energy, which is going to be more dietary fat.

• Typical (2520kcal)
  • 150g protein (600kcal)
  • 300g carbohydrate (1200kcal)
  • 80g fat (720kcal)
• Low-carbohydrate (2520kcal)
  • 150g protein (600kcal)
  • 50g carbohydrate (200kcal)
  • 190g fat (1720kcal)

The reason people report more easily losing fat on a low-carbohydrate diet, is that trying to eat ~190g of fat per day is often missed. They unknowingly restrict energy. Though this can be effective, it’s not a great dietary pattern for athletes. It also restricts food choice (carbohydrates).

Consume more of any food than you need in a window of time, and you will store the excess energy primarily as fat. However dietary fat is stored most readily and efficiently (90-95%), which is why, if you had to overeat, go for whole-food protein and carbohydrate, not fat.

Here is a hypothetical situation…

• If you consume 50g of glucose, you will immediately gain 50g of mass (you weigh 50g more)
• 50g of glucose is ~200 kcal of energy
• Walking briskly for an hour requires ~200 kcal
• In this very controlled scenario (all other things not changing) after walking an hour you would have lost 50g of mass (you weigh 50g less). The glucose atoms exit your body as exhaled CO₂ and water. Metabolising the glucose doesn’t destroy mass; it rearranges it. Whereas the 50g gain is instantaneous, the 50g loss is gradual, exhaled molecule by molecule over the hour of walking.
• If you did truly nothing (all other things not changing) both the glucose mass (~50g) and its energy (~200 kcal) would be stored as muscle or liver glycogen, very rarely body fat. You cannot extract the energy and keep the atoms—those are mutually exclusive fates.

So when we lose fat, where does it go?

84% of it leaves your body as exhaled CO₂, and 16% as H₂O (urine, sweat, and breath)^[4]; the energy radiates off as heat.

But in a more pragmatic sense, the reason we gain body fat more easily than we lose it is one of energy density versus time (and effort). A few hundred kcal worth of food can be eaten in seconds, but using that energy takes far longer. Storing energy is rapid and relatively passive; expending it is slow and takes effort.

Why does fat have more energy than carbohydrate?

Fat provides roughly 9kcal/g versus carbohydrate’s 4kcal/g. This isn’t because fat has more atoms per gram—it’s because fat atoms are in a more reduced (unoxidised) state.

Carbohydrates are already partially oxidised. Glucose is C₆H₁₂O₆, one oxygen for every carbon. The carbons are already half-way to becoming CO₂. Fat is largely unoxidised: palmitic acid is C₁₆H₃₂O₂, sixteen carbons but only two oxygens. Its carbons are bonded to other carbons and hydrogens, far from their stable endpoint.

When we metabolise (rearrange) both, their atoms end up in the same stable oxidised endpoints (CO₂ and H₂O). But fat atoms start further from that endpoint, so more energy is released during the rearrangement. Per gram, more of fat’s mass is in energy-rich (reduced) bonds, and less is tied up in bonds that are already close to their final (oxidised) state.

Glucose is used before fat during exercise, not because it provides more energy per gram (it doesn’t, fat has over twice the energy density), but because it is rapidly accessible. Hence higher intensity exercise (higher rate of energy transfer requirement) predominantly uses glucose. Glucose is closer to the oxidised endpoint, takes fewer metabolic steps to break down, and its first stage (glycolysis) can run without oxygen. The full oxidation to CO₂ and H₂O still requires oxygen, and per litre of oxygen consumed glucose actually yields slightly more energy than complete fat oxidation.

Footnotes