Losing weight should mean losing fat.
Unfortunately, the organism also loses proteins. But what happens with the fat inside our body? All animal fats, whether solid or liquid, are stored 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.
The acid chains can be saturated (in most animal fats) or unsaturated (in most plant fats, mostly known as oils), meaning they miss some hydrogen atoms from the acid chain. Unsaturated fats are easier to digest and burn by the organisms.
Most 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 store for shortage times.
People who are overweight or obese (roughly 66 % of American adults are) have huge fat cells fulfilled with triglyceride fuel. Fat cells number is practically constant during your entire life, that's why every person has his own particular pattern of fattening.
When diet or calories burning do not cover daily energy spending, the body releases catabolic hormones which activate the enzyme called lipase, located within fat cells (lipids = fats). Lipase breaks down triglycerides into glycerol and fatty acids. These components get out of the fat cells 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 the muscles. The brain can not use fats for its energetic needs, just glucose!
Once inside the liver or muscle cells, the fatty ingredients are further disassembled and modified, eventually resulting in large quantities of a compound called acetyl-CoA. Within the cells' mitochondria, where all the burnings in the organism take place, the acetyl-CoA combines with 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 any organic substance (fat, protein and carbohydrates).
The result of the Krebs cycle (burning) is 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. ATP powers all cellular activities that require energy--from moving your muscles during exercise, to maintaining your hearts 100,000-plus beats each day, to digesting each mouthful of food that you swallow and processing nutrients into bodily tissues, to gland secretions, brain and nerves activity and so on...
But one question remains: if the brain can not use fats, how does it get fed during starvation or diet periods? Well, the organism possesses a metabolic way, which consumes some energy, to transform glycerol and fatty acids into glucose which the brain can then use ...In fact, it is a reversed phenomenon of what happens when we get fat: during the fattening process, the glucose we get from food (especially bakery products and sugar) passes to fat. In fact, most of the fat we possess does not come from the fat in our food but from sugars.