How and when does FAT leave the body??

It isn't fat at that point - it is converted to FFA , then leaves the fat cell & goes into the blood, and then it is sent to and absorbed into muscle cells and burned there in the mitochondria. it's all in the article.

^this

Mitochondria is the powerhouse of energy cells.

Just glad it does!(leave the body)

I have read that you keep the same number of fat cells but they shrink in size as you burn the fat.

Fat is stored energy that your body can use. It is not as efficient as glucose but when your muscles are active and running low on energy (aka glucose being converted to ATP) they can covert it. Cells that are responsible for providing energy have organelles called mitochondria. The mitochondria are responsible for the process that creates the ATP. So, muscles that require lots of energy and especially quick energy, will have cells with many mitochondria present in each cell to try to keep up with the energy needs of the muscle. See mathjulz explanation for the remainder .... It is pretty darn good. You have a very good memory, mathjulz. I personally have CRS.

Energy can be gotten anaerobically as well. When muscles are at rest, they can tap into lipids for energy and conserve ATP, but this process is not efficient enough to provide necessary energy for lots of activity.

So, the more muscle you have......not just bulky, sexy, hot, muscle, but all muscle, including cardiovascular and smooth muscle, the more energy you will burn. Cardio and weight bearing exercises burn energy, build muscles that continue to burn energy, and make your insides happy, too. Smooth muscles can be found internally and help to move blood along your vessels, for instance. ( They surround the vessels. That is just one example of smooth muscle. ) you have 3 muscle types, smooth, skeletal, and cardio.

That's all I got without an anatomy and physiology lesson and ridiculously unpronounceable words.

Easier visual to create in your mind. Take a bag of balloons. Inflate them they fill a car... Deflate them and they fit back into the bag... The adipose tissue doesn't go away (except thru liposuction or other extreme measures) it simply deflates the adipocytes by releasing the fatty acids that have been stored. These are used by other tissues in the body for reconstruction and energy. Your body if it has a caloric deficit triggers through hormones the release of these fatty acids from the adipocytes (individual cells within adipose tissue) and they travel in the bloodstream until used by the body, redeposited in adipose tissue, or targeted for waste removal when part of a defective cell structure...

Cell biology explanation:

Fat (lipids) are long chains of molecules. When the chains are broken in the right place, it liberates a lot of energy. The energy is used to turn ADP + phosphate into ATP, which is what cells use for energy (breaking it back into ADP + phosphate). So, you have this broken chain of molecules which are useless to the body. The blood pulls them from the cell (through the cell wall, the same way water, glucose chains, and everything else goes into and out of the cell). The kidneys (and sometimes liver, but mostly kidneys) filter the blood, pulling the broken chains out. This is part of what is in your urine. So, you pee out the remnants of the fat.

ETA: it's been almost 15 years since I took cell biology in college, so I'm sure I've forgotten a significant amount of the details, but I think I got the basics about right.

Well duh! That's what Fire_Rock said just before you!! :laugh:

I like the Dr. Who explanation.

Me too, lol!