When you lose fat, you may wonder where it goes and how it leaves the body. The answer is more complex than you might expect and depends on several factors.
The basics of fat storage and breakdown
Fat is stored in the body in cells called adipocytes. These fat cells can increase and decrease in size as fat is stored or removed from them. When you gain weight and body fat, those fat cells expand to accommodate the extra fat being stored. When you lose weight and body fat, those same fat cells shrink as the fat is pulled out to be used for energy.
The number of fat cells you have is set during childhood and adolescence. However, the size of those cells can change throughout your life. When people lose a substantial amount of weight, the fat cell count remains the same, but the cells become much smaller than before weight loss.
Where does the fat go when cells shrink?
When fat is broken down, it gets released into the blood as free fatty acids that travel through the bloodstream and are absorbed by other tissues to be used for energy. The glycerol component of fat enters the glycolysis process and can be converted to glucose when needed by the body for energy.
So in simple terms, when fat cells shrink during weight and fat loss, the fat contained in the cells gets released and circulates in the blood until it finds another cell to enter for either energy usage, fat storage or other metabolic roles. The fat isn’t converted into energy or literally “burned” right there on the spot. It has to circulate throughout the system until needed.
Fat breakdown and usage during weight loss
When you lose body fat it ideally gets used for energy rather than getting stored back as fat. The way this works depends on several factors:
Diet composition
What kinds of foods you eat impacts what happens to fat when it gets released from storage in fat cells. A diet higher in carbs sends it toward fat storage. A diet higher in fat and protein sends it toward being burned for energy.
Energy balance
If you eat less energy (calories) than you expend, you burn through all the available glycogen (carb) stores. Then you start burning fat and protein for energy. In this state, the fat that gets released from fat cells circulates in the blood and gets burned by cells that need energy, so it doesn’t get re-stored as fat.
Exercise
When you exercise, your muscles use more energy than when you’re sedentary. They burn through glycogen first for fuel. After that, muscles start burning circulating fat from fat cells for energy. Exercise also impacts hormones in the body that regulate fat breakdown and storage.
Location of fat storage
Where fat gets stored and broken down from impacts what happens to it when released. Subcutaneous fat is stored right under the skin. This is the first type of fat to get broken down when significant weight loss occurs. Visceral fat is stored around organs deeper in the abdomen. This is more metabolically harmful and is harder to burn off.
Conversion of fat to energy and byproducts
While most stored body fat gets released, circulated, and then burned for energy during weight loss, a small portion of fat does get converted and excreted as waste products:
Water
Fat cells that shrink release water since fat cells can hold large amounts of water. Some research suggests around a 30% loss of the weight lost when dieting is from water loss.
Carbon dioxide
When fatty acids are burned for energy, the major byproduct is water and carbon dioxide, which you exhale from your lungs.
Ketones
In extreme dieting cases, your body burns fats for nearly all its energy needs. Small fragments of incomplete fat breakdown called ketones can exit the body through urine, breath, and sweat.
Can fat turn into muscle?
It’s a common misconception that fat can turn directly into muscle when you strength train while losing weight and fat. This isn’t physiologically possible.
Fat is stored energy whereas muscle is functional body tissue made up of bundles of protein fibers. Fat gets released from cells and circulates in the blood until it’s absorbed and burned up by other cells. It doesn’t somehow morph into muscle fiber.
However, if you strength train during weight loss, the size of the muscle fibers can increase even as fat cells shrink which gives the impression of “turning fat into muscle.” But they are in reality separate processes happening simultaneously.
The takeaway on where fat goes
When fat is burned for energy it enters the bloodstream as free fatty acids. They circulate and are absorbed by cells throughout the body that burn them for energy. A small portion is converted to carbon dioxide, water, and ketones. When fat isn’t burned immediately for energy, it can get reabsorbed back into fat cells. But during weight loss your body tries to use fat for energy as much as possible.