The number of fat cells in your body doesn't change throughout your life. When you gain weight, excess calories are stored in your fat cells, making each cell larger. When you lose weight, your body uses the fat stored in your fat cells for energy, and the fat cells become smaller as the energy in them is used.
Calories In
You need a certain minimum amount of energy for your body to keep living. Many people need about 1,200 calories a day just to keep breathing and functioning. In addition, you need calories to fuel your physical activities. The more active you are, the more calories you need to keep going. However, if you eat more calories than you need for daily life and activities, the excess energy is stored in your fat cells as triglycerides.
Calories Out
If you use more calories than you eat, your body draws on energy stored in the fat cells. The triglycerides are turned into glycerol and fatty acids that your liver, kidneys and muscles use for energy, through a complex chain of chemical reactions that produce carbon dioxide, water, heat and adenosine triphosphate, or ATP, which is the fuel that your cells use for every process in your body. Once that energy is used up, it disappears. Water is excreted as urine and perspiration; carbon dioxide is excreted by your lungs when you exhale and the heat that is produced from the use of energy keeps your body warm.
Ultimate Source of Energy
All our energy ultimately comes from the sun. Plants absorb sunlight and carbon dioxide, and along with the nutrients they absorb from the soil, plants create carbohydrates, which store the energy from the sun as sugars and starch. Oxygen is a by-product of this process, produced when plants use part of carbon dioxide to make sugar and starch.
Energy Transfer
Animals that eat plants consume the energy the plants have stored up. Some animals eat other animals as well as plants. Animal protein is turned into amino acids, which are also used for energy in your body. If you are overweight, you have a lot of potential energy from your food stored up in your fat cells, as if you have been stacking up a lot of firewood for the fireplace. When you start to burn the fuel, or firewood, it produces energy and heat. The energy is used for activities, the heat keeps your body warm and your fat cells shrink.
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