Lora A. Sporny, adjunct associate professor of nutrition education at Columbia University, answers.
In order to understand the disappearance of body fat, we must enter the world of biochemistry. All fats, whether solid or liquid, exist in chemical form as triglycerides, which consist of a glycerol molecule and three fatty acid chains. Each triglyceride macromolecule's appearance is similar to the letter "E"--with the glycerol being the vertical line and the fatty acids as the three horizontal lines. Many of these triglycerides are stored as droplets of oil within the fat cells that make up the fat tissue located throughout the body. They represent a fuel source to support bodily activities, like gasoline held in a car's fuel tank.
People who are overweight or obese--as roughly 66 percent of American adults are--possess large fat cells brimming with triglyceride fuel. When trimming calories and/or increasing exercise during weight loss, the enzyme hormone-sensitive lipase, located within fat cells, responds to hormonal messages and disassembles triglycerides into their component glycerol and fatty acids. These components then slip out of the fat cells and into the bloodstream, where they are accessible to tissues throughout the body. The liver preferentially absorbs the glycerol and some of the fatty acids--the remainder of which is taken in by muscle.
Once inside liver or muscle cells, the triglyceride ingredients are further disassembled and modified, eventually resulting in large quantities of a compound called acetyl-CoA. Within the cells' mitochondria--the powerhouses of the cells--the acetyl-CoA combines with the compound oxaloacetate to form citric acid. This synthesis kicks off the citric acid cycle (or Krebs cycle), a set of chemical reactions that creates usable energy from fat, protein and carbohydrates. As these mitochondrial activities unfold, they generate carbon dioxide, water and heat, as well as adenosine triphosphate (ATP), an energy-carrying molecule that fuels cellular activities.
The carbon dioxide is then expelled from the lungs during exhalation. The water exits the body as urine and perspiration. The heat that is generated helps to maintain body temperature at a comfortable 98.6 degrees Fahrenheit. And the ATP powers cellular activities that require energy--from moving your muscles during exercise, to maintaining your heart's 100,000-plus beats each day, to digesting each mouthful of food that you swallow and processing nutrients into bodily tissues.