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HOW COME? How fat is burned off

When people exercise to lose weight, what physically happens to the fat they "burn off?" asks reader Julie

If Monday's scale says you've gained a few from Thanksgiving stuffing and pie -- or Black Friday frappuccinos and hot wings -- fear not. Most of that gain is probably retained water. And any bits of extra fat will probably vanish after a few days of eating less. But how does something so solid simply disappear?

Whether we lose fat by eating less, exercising more, or both, most fat is burned up inside our muscles.

Muscles burn a mix of sugar and fat for fuel. About 2 to 3 grams of glucose is available from our bloodstream. And the liver holds about 100 grams of glycogen, glucose's storage form. But most of the body's glycogen, about 400 grams, is stored right in the muscles. Altogether, up to 2,000 calories' worth of sugary fuel is on tap for the muscles at any given time.

But more than 100,000 calories of energy can be supplied by just 30 pounds of body fat. Fat cells expand or shrink as fat is added or released. (And the body can and does manufacture new fat cells when existing cells are full to the brim.) The fat in fat cells is made up of a molecule called glycerol, with three fatty acid molecules attached.

To burn fat for energy, fatty acids must be freed from glycerol. Hormones such as epinephrine (adrenaline), glucagons (made by the pancreas when blood sugar drops) and human growth hormone get the process going. Helpfully, the levels of these hormones increase during vigorous exercise, such as running.

The hormones prompt cells to release enzymes called lipases. Three kinds of lipase swing into action, dismantling the glycerol/fatty acid bundles. Then, the newly separated glycerol and fatty acids travel through the blood to the liver. There, some are used to make extra glucose for energy.

Lipase enzymes also break down fat stored in the muscles themselves. Both this fat and the fat recruited from the body's fat cells can be burned for energy. How? Muscles contain tiny energy factories called the mitochondria. The mitochondria break down the freed-up fatty acids, in a complex, four-step process, leaving a chemical called acetyl-CoA.

Bits of acetyl-CoA molecules are oxidized to form carbon dioxide. Other chemicals generated in this process combine to produce water, and a molecule called adenosine triphosphate (ATP). The energy powerhouse of our cells, ATP is like the battery in a flashlight.

The leftover water is recycled by the body or is excreted, through urine, sweat, and exhaling. The carbon dioxide is carried by the blood to the lungs, where it's also breathed out into the air. And over time, we can button our jeans again.

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