When you think of the process of losing weight, chances are good that you imagine progressively lower numbers on a scale and a more svelte figure, not the biochemistry involved in reducing body mass. But when you lose weight, it's ultimately because you shed the by-products of fuel metabolism -- carbon dioxide, or CO2, and water, or H2O. A number of pathways end in this shedding of molecules, most of which leave your body in the breaths you exhale.
Glycogen Metabolism
Glycogen is the body's storage form of glucose, a six-carbon sugar that serves as the predominant fuel for your brain, muscles and other tissues. It is stored in granules in the liver and in muscles. During times of increased need, glycogen is broken down in a series of reactions into glucose, which then undergoes glycolysis -- the conversion of glucose into energy in the form of adenosine triphosphate. This takes place in the mitochondria of cells, and the resulting waste products are CO2 and H2O, which leave your body through your lungs, skin and gastrointestinal tract.
Lipid Metabolism
What is usually termed "fat burning" is formally known as lipid metabolism. A fat, or lipid, molecule consists of a glycerol molecule bound by the three fatty acids, and is also called a triglyceride. In the initial stages of fat metabolism, the three fatty acids are cleaved from their glycerol backbone. The glycerol enters the glycolytic pathway and is converted to adenosine triphosphate or ATP, the substance cells ultimately draw on for energy. CO2 and H20 are produced as waste products, while the fatty acids are transformed into acetyl coenzyme A and ultimately to more energy. The CO2 leaves the system in the same manner as that derived from glycogen metabolism.
Protein Metabolism
Protein breakdown is the least efficient way of obtaining energy from body tissues, as it is not stored for this purpose but instead is incorporated into structural tissues such as muscle and organs. Nevertheless, amino acids derived from the breakdown of protein can be converted to glucose, which then enters the glycolytic pathway just as glucose from carbohydrates and glycerol from triglycerides do. In addition to CO2 and water, protein breakdown yields nitrogen and, in some cases, sulfur.
Carbon Dioxide Excess
As the vast majority of CO2 resulting from metabolic pathways is lost through the lungs via respiration, respiratory disorders result not only in low blood-oxygen levels, but a buildup of CO2 in the system. When this occurs, the CO2 combines with H2O to form carbonic acid, and the acidity of the blood rises, causing a condition known as respiratory acidosis. When the kidneys fail and are unable to produce enough bicarbonate to buffer the acid in the bloodstream, acidity again rises; this is termed metabolic acidosis. In both cases, the acidosis can result in shock, organ failure and other complications.
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