How does the body use and store energy?
A certain amount of calories are needed to supply the energy required for everyday activities and metabolism (the chemical process by which the body converts food into energy and various functions, including food digestion, circulation, and temperature regulation). When more calories are consumed than are needed, these extra calories are stored primarily as fat--whether the calories come from fat, carbohydrates, or proteins.
During digestion, enzymes in the small intestine break down carbohydrates into simple sugars like glucose, proteins into amino acids, and triglycerides (dietary fat) into fatty acids and glycerol. Simple sugars and amino acids are rapidly absorbed from the small intestine into the bloodstream. The liver converts other sugars, like fructose and lactose, into glucose, which is used as a source of energy. Amino acids can be used as an energy source but serve mainly as building blocks for body proteins. Fatty acids combine with bile salts to form tiny droplets that promote their entry into cells in the intestinal wall, where they are again formed into triglycerides. The triglycerides are packaged into transport lipoproteins, which carry the triglycerides to the adipose tissue (fat) for storage.
Any excess carbohydrates and protein not immediately used for energy are converted to glycogen and triglycerides in the liver. These triglycerides are transported from the liver on another lipoprotein for storage in individual adipose tissue cells, located just beneath the skin or around the intestines.
To store more fat, the body either creates more fat cells (a process called hyperplasia, which generally occurs only in childhood-onset obesity, during pregnancy, or with rapid weight gain in adults) or enlarges existing fat cells (hypertrophy, the primary way that adults increase their adipose tissue). If faced with a shortage of calories--as when a person diets--the body uses the fat stored in these cells as a source of energy. Unfortunately, once fat cells are formed they can shrink, but they are not eliminated.
