There’s 2 questions implied here:
- Where does fat go?
- Where does weight go?
Believe it or not, these are two entirely different questions. Fat loss is not always weight loss and weight loss is not always fat loss.
The quick answer is:
- Fat shrinks through direct conversion to energy and through energy redistribution within your body.
- Weight is shed through a number of places, from sweating, to urination, to defecation, to exhalation of carbon dioxide, to heat radiation.
We have 2 main types of dynamic soft tissue in our body: adipose tissue and muscle tissue. Adipose tissue is your body’s energy store, it’s your gas tank, basically. Your muscle tissue is your engine and everything that keeps you moving.
Muscle is much denser than fat. If you gain muscle and lose fat, you’ll look skinnier and could end up weighing more in the process.
I’ll start with where fat goes and circle back to where weight goes.
So where does fat go?
When you’re fasted and at rest, your body gets 60%-70% of it’s energy from fat, the rest comes from glycogen stores (more on that later).
Fat is stored in, or fat cells, which make up adipose tissue, or fat tissue. An adult has about 8 million adipocytes and they don’t ever go away (though they are replaced gradually over the course of years as the cells are maintained). When you eat, some of the food you eat is stored in them. As ou eat the cells grow. When you gain fat, they’re swollen, and sometimes will split into two (like shares on the stock market). When you lose fat, they actually shrink. You never really kill these cells except in some extreme weight loss procedures (cryolipolysis, liposuction, etc).
When you need to store fat a process called takes place. This process turns sugars and proteins and free fatty acids into trigylcerides for storage inside the fat cells. The process can take place in the liver or inside your fat cells. Insulin is the transport mechanism that deposits things into the fat cells.
When your body needs energy, particular hormones (, , , , , and ) will attach to the adipocytes and initiate a process called . Your fat cell will turn the triglycerides into free fatty acids and glycerol and release them into your blood stream. They get picked up by LDLs and VLDLs and are taken to cells that need them. Fat is then processed into acetyl sugar through a process called . The sugar is then used in the to create a ton of ATP for energy.
That is one of the ways that your fat cells are shrunk, for direct energy use.
The other way is because of glycogen store depletion. Glycogen is another name for stored glucose. Your body depends on glucose. It’s easier to turn into ATP than fat. It can be converted into about 4 ATP, it’s also smaller than ATP, and stable in water (unlike ATP) making it an ideal primary energy store for your body, plus it can create ATP without using oxygen unlike fat and amino acids.
Your brain, liver, kidneys, and muscle tissue all hold glycogen, without glycogen they won’t function.
When your glycogen stores are low and your body doesn’t have any sugar in your system from eating, it will use fat and amino acids to replenish your glycogen stores through a process called. This is the primary method that your fat shrinks from exercise.
When you start doing INTENSE exercise, your muscles won’t have access to oxygen, so they have to rely on their ATP stores (and the phosphocreatine recycling) which only last a few seconds. After that, your body will use anaerobic glycolysis to create more energy. This is less efficient than aerobic glycolysis, giving 8xs fewer ATP per molecule, but it’s the best your body can do under the circumstances.
At the same time your aerobic energy systems kick into gear, aerobic glycolysis and lipolysis. Your body has enough glycogen to run for 2-4 hours. While your glycogen system is in use, your fat stores will start into gear. All of your aerobic systems take in oxygen and release CO2, water, and a few other metabolites. The CO2 is taken in the blood stream to your lungs to be exhaled. The water and other metabolites are taken by your blood to your liver and kidneys to be filtered out. Some of the water is kept by your body for it’s own uses.
So how does your body lose weight?
Through all of this it looks like you only redistribute your weight, doesn’t it? Your body expels actual weight through several mechanisms, some of which are related directly to metabolism:
- Expulsion of CO2 generated from aerobic energy creation by breathing (this is not really news, we’ve known about this since the 1930s). You cannot sit around breathing and expect to lose weight, unless you don’t eat anything, which is a really foolish way to go about things. An average person will exhale about 2.3 pounds of carbon dioxide daily. 
- Expulsion of water vapor generated from aerobic energy creation by breathing. We breathe out about a pound of water over the course of the day.
- Sweating. We sweat constantly, throughout the day, just not always noticeably, in temperate environment you’ll only lose about a pound, but under more trying circumstances you could lose as much as 20 pounds. 
- Skin and hair cell sloughing. Skin and hair cells die and are replaced constantly. Every day you lose millions of skin cells. This isn’t terribly significant on a day to day basis, but you could end up shedding hundreds of pounds of skin cells over the course of your life time. 
- Urination, it’s one of our body’s key waste processes removing metabolites from our system.
- Pooping, removes food waste and effectively reduces our weight.
- Heat radiation. When ATP reacts, a lot of energy is lost from heat. The heat is radiated or convected from the body through a number of mechanisms. Your body burns through it’s own body weight in ATP every day. It’s a pretty amazing thing.
All of these different processes contribute to your losing weight from day to day.
Which leads me back to my first point, your weight isn’t important, your fitness is. Your weight will fluctuate by pounds from day to day, don’t obsess over the weight, just aim to get fit and eat right. Your fat will shrink, your muscles will become strong. You’ll look great and feel great.