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Metabolism is the body’s process of using food for energy and growth. It involves two major processes: catabolism and anabolism. Catabolism is the process in which foods and other substances are broken down into simpler substances, which may release or store energy. Anabolism is the process by which simple substances are built up into more complex substances. This process uses energy.

Calories are the body’s energy source, and the body’s metabolism regulates how it uses these calories. Excess energy is generally stored in the body as fat. Therefore, when the body takes in more energy (calories) than it uses, weight is gained. If more energy is used than is taken in, the body uses its fat stores for energy, and weight is lost.

There are three primary methods in which the body uses calories, regardless of whether they are taken from stored sources or directly from food. The basal metabolic rate is the energy used when the body is at rest. The thermic effect of food (TEF) is the energy used to process (e.g., digest, absorb, transport, store) consumed food. Energy is also used during physical activity (e.g., walking, running, any other movement). The sum of all three of these is known as the total energy expenditure (TEE) of the body. A person’s metabolic rate does not remain the same at all times. Certain events (e.g., pregnancy, aging) and activities (e.g., exercise, changes in eating habits) can cause metabolism to decrease or increase.

Some energy from catabolism is stored as compounds such as adenosine triphosphate (ATP). These compounds readily release energy and can easily be stored as fatty acids in the adipose tissues. The breakdown of food into these compounds occurs as a series of metabolic pathways (chemical reactions that lead to catabolism or anabolism).

The three main energy-producing nutrients (carbohydrates, fats, proteins) are broken down in different ways to produce energy the body can use. First, they create a metabolite called acetyl CoA. Acetyl CoA can be used to create fats or ATP, two forms of stored energy. If the body has plenty of ATP, acetyl CoA is built up to make fats. If the body’s ATP levels are low, acetyl CoA enters a metabolic pathway – either the electron transport chain or Krebs cycle – to produce more ATP.