Although fat has more energy (calories) per gram than proteins and carbohydrates, your body has a more difficult time pulling the energy out of fatty foods. Imagine a chain of long balloons — the kind people twist into shapes that resemble dachshunds, flowers, and other amusing things. When you drop one of these balloons into water, it floats. That's exactly what happens when you swallow fat-rich foods. The fat floats on top of the watery food-and-liquid mixture in your stomach, which limits the effect that lipases — fat-busting digestive enzymes in the mix below — can have on it. Because fat is digested more slowly than proteins and carbohydrates, you feel fuller (a condition called satiety) longer after eating high-fat food.
When the fat moves down your digestive tract into your small intestine, an intestinal hormone called cholestokinin beeps your gallbladder, signaling for the release of bile. Bile is an emulsifier, a substance that enables fat to mix with water so that lipases can start breaking the fat into glycerol and fatty acids. These smaller fragments may be stored in special cells (fat cells) in adipose tissue, or they may be absorbed into cells in the intestinal wall, where one of the following happens:
I They're combined with oxygen (or burned) to produce heat/energy, water, and the waste product carbon dioxide.
I They're used to make lipoproteins that haul fats, including cholesterol, through your bloodstream.
Glucose, the molecule you get by digesting carbohydrates, is the body's basic source of energy. Burning glucose is easier and more efficient than burning fat, so your body always goes for carbohydrates first. But if you've used up all your available glucose — maybe you're stranded in a cabin in the Arctic, you haven't eaten for a week, a blizzard's howling outside, and the corner deli 500 miles down the road doesn't deliver — then it's time to start in on your body fat.
The first step is for an enzyme in your fat cells to break up stored triglycerides (the form of fat in adipose tissue). The enzyme action releases glycerol and fatty acids, which travel through your blood to body cells, where they combine with oxygen to produce heat/energy, plus water — lots of water — and the waste product carbon dioxide. As anyone who has used a high-protein/high-fat/low-carb weight-loss diet such as the Atkins regimen can tell you, in addition to all that water, burning fat without glucose produces a second waste product called ketones. In extreme cases, high concentrations of ketones (a condition known as ketosis) alter the acid/alkaline balance (or pH) of your blood and may trip you into a coma. Left untreated, ketosis can lead to death. Medically, this condition is most common among people with diabetes. For people on a low-carb diet, the more likely sign of ketosis is stinky urine or breath that smells like acetone (nail polish remover).
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