A person's body weight is basically a simple case of supply and demand. If the supply (food) exceeds the demand (i.e., exercise and energy needed for basic body functions), weight goes up and vice versa. If you think about it, weight gain is really just the body's way of efficiently conserving resources: it's simply tucking away extra energy for a rainy day. In many areas of the world where food is abundant, though, the rainy days rarely come -- thus the current obesity epidemic in a growing number of countries.
The way the human body stores energy is often compared to a car and its engine. But instead of gasoline, we store extra energy in the form of triglycerides in our fat cells. As more energy gets stored, the more those fat cells expand, along with our waistlines. While most people possess between 10 and 30 billion fat cells, obese people may have as many as 100 billion. By overeating for a prolonged length of time, they prompt their bodies to produce more cells to store all the excess energy [source: Park].
When a person goes on a diet, the number of fat cells stays the same -- it's simply their size that decreases. The fat doesn't just disappear, though. It gets converted into energy. When people burn more energy than they consume, either by restricting calorie intake or amping up exercise, it triggers the enzyme lipase to break down the stored triglycerides into their component parts of glycerol and fatty acids. These smaller components then travel through the bloodstream where they're picked up by either the liver or muscle cells. Once inside these cells, the glycerol and fatty acids are further broken down by a series of chemical reactions. Along with the energy-carrying molecule ATP, which powers cellular activities, these reactions produce carbon dioxide, heat and water. So what was once a cushiony padding is now being exhaled, perspired and excreted as urine, maintaining body temperature and fueling activity.(©iStockphoto/Thinkstock)
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