If you go on a diet and lose weight, where does all that body fat actually go? It doesn't just disappear. Here's the fascinating story of exactly what happens to it, from the first moment your body taps into its fat stores to the very last step of the process.
Jump to:
- Why does fat loss happen?
- What is body fat?
- How does the body access its fat stores?
- What happens to glycerol?
- What happens to fatty acids?
- So where does the body fat actually go?
- Frequently asked questions
Why does fat loss happen?
Fat loss occurs when the body is in a calorie deficit. That means eating less energy from food than the body burns through activity and normal function.
Energy is measured in calories, or more precisely kilocalories. One kilocalorie is the amount of energy needed to heat one kilogram of water by one degree Celsius. Everything the body does, from staying warm and pumping blood to processing information and producing sweat, requires energy, and we get that energy from food.
If we take in more food energy than the body needs at any given moment, it stores the surplus for later. The main way it does this is by building up stores of body fat.
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What is body fat?
A 70kg person with around 15% body fat carries approximately 10kg of body fat, which represents roughly 90,000 calories of stored energy, theoretically enough to last 35 to 45 days or more. Fat accounts for between 92 and 98% of the energy stored in the body, with the remainder stored as glycogen, the form of carbohydrate held in the muscles and liver.
Body fat is stored in cells called adipocytes. The scientific term for body fat tissue is adipose tissue. Each adipocyte contains one large fat droplet made up of millions of molecules called triglycerides.
Triglycerides are the basic unit of fat. Each triglyceride consists of three fatty acids, which are long chains of carbon atoms, attached to a backbone molecule called glycerol.
How does the body access its fat stores?
When the body is low on energy, it detects this and sends a signal to fat cells to produce enzymes called lipases. Lipases break down triglycerides into their component parts: fatty acids and glycerol. These then pass out of the fat cells and into the bloodstream.
Fat doesn't dissolve well in water, so fatty acids are transported through the blood by carrier molecules called lipoproteins, which carry them to the parts of the body that need energy. Glycerol, by contrast, is water-soluble and can travel through the blood without a carrier.
What happens to glycerol?
Glycerol travels to the liver, where it is mostly used to produce glucose in a process called gluconeogenesis. As glycerol is converted to glucose, it loses some of the carbon it contains, which is released as carbon dioxide.
The glucose produced by gluconeogenesis can then be used by tissues that prefer to run on glucose, particularly the brain and red blood cells. Glucose is burned in a process called aerobic respiration: it passes through a series of steps in the cell, including glycolysis and the Krebs cycle inside the mitochondria. As it breaks down, it releases energy that is used to make ATP (adenosine triphosphate), the energy currency of the cell. Carbon dioxide and water are produced as waste products.
What happens to fatty acids?
Fatty acids released from fat cells can go to one of two places.
Route one: the liver
In the liver, fatty acids are converted into ketones. These are the substances produced in significant amounts on a low-carbohydrate diet. Ketones cannot be used for energy by the liver itself, but can be used by other organs, including the heart, the muscles and the brain. Like glucose, ketones are metabolised through the Krebs cycle in the mitochondria to produce ATP, with carbon dioxide produced as a byproduct.
Route two: direct energy use in tissues
Fatty acids can also be used directly for energy in tissues such as muscle. The heart is a particularly significant example, getting up to 90% of its energy from fatty acids, with the remainder coming from other fuels such as glucose and ketones.
Inside the mitochondria, fatty acids are broken down for energy in two main steps. The first is beta oxidation, in which the long fatty acid chain has pieces broken off two carbon atoms at a time. Fatty acids are chains of carbon atoms with hydrogen atoms attached, and enzymes in the mitochondria work along the chain, cutting off two carbons at a time. As this happens, some water is also released as a byproduct. The cut-off pieces then enter the Krebs cycle, which produces ATP and more carbon dioxide.
So where does the body fat actually go?
Body fat is stored as triglycerides, which are made up of carbon, hydrogen and oxygen atoms. Through the entire process of being broken down to produce energy, those atoms end up in just two places: carbon dioxide and water.
You breathe out the carbon dioxide through your lungs, along with some water vapour. The remaining water is lost through sweat or urine.
So when you're in a calorie deficit and your body dips into its fat stores for energy, you are quite literally breathing out, sweating out and peeing out what used to be your body fat.
Frequently asked questions
Where does body fat go when you lose weight?
Body fat is chemically converted into carbon dioxide and water through the process of metabolism. The carbon dioxide is breathed out through the lungs, and the water is either exhaled as vapour, sweated out or excreted in urine.
What are triglycerides?
Triglycerides are the basic unit of fat and the form in which body fat is stored in adipocytes (fat cells). Each triglyceride is made up of three fatty acids attached to a glycerol backbone.
What are lipases?
Lipases are enzymes that break down triglycerides into fatty acids and glycerol when the body needs to access its fat stores for energy.
What is gluconeogenesis?
Gluconeogenesis is the process by which the body produces glucose from non-carbohydrate sources, including glycerol (from fat breakdown) and components of protein. It primarily takes place in the liver.
What is beta oxidation?
Beta oxidation is the first step in how fatty acids are broken down for energy inside the mitochondria. Enzymes work along the fatty acid chain, cutting off two carbon atoms at a time, which then enter the Krebs cycle to produce ATP.
How much energy is stored in body fat?
A 70kg person with around 15% body fat carries approximately 10kg of body fat, representing roughly 90,000 calories of stored energy, theoretically enough to sustain the body for 35 to 45 days or more.
Take home message
Body fat doesn't vanish when you lose weight. It is chemically converted into carbon dioxide and water through the processes of metabolism. Most of it leaves the body through the lungs as carbon dioxide, with the rest excreted as water through sweat and urine. The trigger for all of this is a calorie deficit: eating less energy than the body uses, which forces it to draw on stored fat to meet its needs.
