How Your Body Stores and Burns Fat

Every weight-loss plan, product and trend comes back to one idea: calories in minus calories out equals results. That's the idea behind CIMCOER. To use it well, it helps to know what your body actually does with the energy you eat: where it goes, when it gets stored as fat, and how that fat eventually gets used.

What a calorie is

A calorie (technically a kilocalorie, or kcal) is a unit of energy. Food contains energy in three main forms, called macronutrients, plus alcohol:

Carbohydrate: about 4 calories per gram
Protein: about 4 calories per gram
Fat: about 9 calories per gram
Alcohol: about 7 calories per gram

Your body runs on this energy all day, every day: for your heartbeat, breathing, brain and muscles. It also needs to keep energy in reserve for when you aren't eating, and its main reserve is fat.

Where calories go: calories out

The energy you use each day, your total daily energy expenditure, has three main parts:

Rough split of a typical person's daily calories out. Exact proportions vary from person to person.

Resting metabolism (about 60–70%) is the energy to keep you alive at rest. It depends mostly on your body size and how much lean mass you have.
The thermic effect of food (about 10%) is the energy spent digesting and processing what you eat. Protein costs the most to process.
Activity (the rest) covers both exercise and everyday movement such as walking, standing, chores and fidgeting. It varies the most between people and from day to day.

How your body stores energy

After a meal, digestion breaks food down into glucose (from carbohydrate), fatty acids (from fat) and amino acids (from protein), which enter your bloodstream. From there, your body does one of three things with them:

What happens to the energy from a meal.

Burns it right away to cover what you're using at that moment.
Stores it as glycogen. Glucose can be packed into glycogen in your liver (roughly 100 g) and muscles (roughly 400 g, depending on your size and training). This is a short-term tank and fills up quickly.
Stores it as fat. Energy that isn't needed now and doesn't fit in glycogen is stored as triglycerides in fat cells (adipocytes). Fat is very energy-dense and your body can keep adding to it, which makes it the long-term reserve.

Most of the fat you store comes straight from fat you eat. After a meal, dietary fat travels in particles called chylomicrons, and an enzyme in the walls of your blood vessels (lipoprotein lipase) helps unload it into fat cells. Turning carbohydrate into fat, called de novo lipogenesis, does happen, but in most people it's a small pathway unless they eat a large carbohydrate surplus. Excess carbohydrate mostly causes fat storage indirectly: your body burns the carbohydrate for fuel and stores the fat you ate instead.

Insulin, released after you eat, pushes your body toward storage and slows the release of fat. That's normal and healthy. Your body switches between storing after meals and releasing between meals and overnight. Whether your fat stores grow or shrink over weeks depends on the net total: how much was stored compared with how much was released.

How your body gets fat back out

When you take in less energy than you use, your body makes up the difference from its reserves. Hormonal signals such as lower insulin and higher adrenaline activate enzymes inside fat cells (adipose triglyceride lipase and hormone-sensitive lipase). These split each triglyceride into three fatty acids and one glycerol molecule. This process is called lipolysis.

The fatty acids travel through your blood to tissues like muscle and liver. There they enter the mitochondria and are burned for energy (beta-oxidation). The glycerol goes to the liver, where it can be turned into glucose.

In an energy deficit, stored fat is released, burned for energy, and leaves your body mostly as carbon dioxide.

So where does the fat actually go?

Most people guess it turns into energy or heat, or leaves as sweat. In fact, burning fat breaks it down into carbon dioxide and water. Researchers worked through the chemistry and found that when you lose 10 kg of fat, about 8.4 kg leaves your body as carbon dioxide that you breathe out. The other 1.6 kg becomes water, which leaves in urine, sweat and breath.¹ The energy is used to power your body. The mass is mostly exhaled.

How the math plays out in real life

One kilogram of body fat stores roughly 7,700 calories (about 3,500 per pound). That's why a daily deficit of about 500 calories is often quoted as losing about 0.5 kg (1 lb) a week. Treat it as a starting estimate, not a promise:

Early losses are partly water. Glycogen is stored with roughly three times its weight in water. When a deficit lowers glycogen in the first week or two, the scale can drop faster than fat loss alone explains.
Calories out shrinks as you lose weight. A smaller body needs less energy, and your body also adapts by moving and spending a bit less. The same intake produces slower loss over time, so weight loss curves flatten rather than following a straight line.²
Day-to-day weight is noisy. Salt, carbohydrate, hydration and digestion can move the scale by a kilogram or more overnight. Watch the weekly trend.

None of this breaks the equation. It just means both sides move over time, which is why tracking actual intake and watching the trend beats any fixed formula.

Takeaways

Your body is always storing and releasing fat. The balance over weeks decides the result.
Fat is lost when you consistently use more energy than you eat, and it leaves mostly as the carbon dioxide you exhale.
Knowing your real intake is the most useful step you can take. Look up any food to see what it adds.
How you create the deficit matters for whether it lasts. That's the subject of diet vs. dieting.