Fat is the way that the body stores energy during times of plenty so that it will have it during times of famine. When you loose weight, it is because you are using more energy than you are taking in in the food you are eating. So, your body makes up for this energy deficit by burning fat.

It turns into carbon dioxide and water vapor and you exhale it--same as whale blubber, or seal and walrus fat, do in an Eskimo lamp.

The fat doesn't go anywhere. Your fat cells shrink but you have the same number of fat cells that you always had. Edit: I got the answer from my Anatomy & Physiology textbook. We have a set amount of fat cells. In a healthy body they grow or shrink based on how many calories we eat and expend. The same is true about muscle cells they grow or shrink but we always have the same amount.

First, fat is converted to glycogen, which is a larger and more complicated form of glucose(a simple sugar molecule). Like glucose, it is then broken down into energy which the body can use. The glycogen is converted to energy, water and carbon dioxide. Energy and water are used up for various body activities, while carbon dioxide is exhaled out of the body during breathing. Therefore the fat does not disappear, it is 'burnt' up to release energy.

In a word, you exhale it. Fat molecules are completely burned to carbon dioxode and water vapor, both of which are exhaled. There is no ash.

I'll add a second answer, though my first one was accurate enough; you actually don't increase the amount of CO2 and water vapor you exhale just because you're on a diet. Rather, it takes lots and lots of overeating to keep you at an obese weight. To oversimplify, if a man should weigh 150 pounds but weighs 300 and uses about as many calories dragging around his 300 pounds as he used to use when he was out at the track or gym every day, he doesn't exhale any more CO2 and water vapor at 300 pounds than at 150--but he has to eat enough for two to stay so big, and when he stops doing that, his weight will automatically go back to where it was. This is oversimplification but in principle correct.

cleveland

i think it comes out in your sweat (rate my answer usful please)

Fats and carbohydrates are based around the molecular formula CH2O, or multiple thereof (such as C2H4O2). When you expend energy using oxygen (O2) from the air (aerobic respiration) then the O2 combines with the C, giving C02 and H2O, and releases energy. The energy you require, the more carbohydrate you burn up. I won't go into anaerobic respiration, except to say that that is why you feel sore after exercising too much.

Fat cells remain- they just change size. You never really "lose fat" unless you have liposiction. Even with lipo theres the chance of weight gain- it just goes to other areas where the fat cells have not been sucked out. I have a friend who had total body lipo- and she gained weight. It all went to her breasts.

Wikipedia.com say's; Vitamins A, D, E, and K are fat-soluble, meaning they can only be digested, absorbed, and transported in conjunction with fats. Fats are sources of essential fatty acids, an important dietary requirement. Fats play a vital role in maintaining healthy skin and hair, insulating body organs against shock, maintaining body temperature, and promoting healthy cell function. They are also known as lipids. They also serve as energy stores for the body. Fats are broken down in the body to release glycerol and free fatty acids. The glycerol can be converted to glucose by the liver and thus used as a source of energy. The fatty acids are a good source of energy for many tissues, especially heart and skeletal muscle. The fat content of a food can be analyzed by extraction. The exact method varies on what type of fat you are analyzing, for example, polyunsaturated and monounsaturated fats are tested quite differently. Fat also serves as a useful buffer towards a host of diseases. When a particular substance, whether chemical or biotic -- reaches unsafe levels in the bloodstream, the body can effectively dilute -- or at least maintain equilibrium of -- the offending substances by storing it in new fat tissue. This helps to protect vital organs, until such time as the offending substances can be metabolized and/or removed from the body by such means as excretion, urination, accidental or intentional bloodletting, sebum excretion, and hair growth.

My uneducated theory is that you sweat it out.