Metabolism describes all of the chemical processes that occur in the body that results in energy being made available. There are two main processes that make energy available to the body.
Catabolism is the breakdown of foods or tissues into available energy.
Anabolism uses these smaller molecules for cellular processes such as cell repair and muscle building. This is also the process for the storage of excess energy (e.g. the role of insulin to move glucose from the blood into cells).
Macronutrients are our primary source of energy. These have to be broken down into smaller molecules to become available for use by the body:
Carbohydrates ➡️ sugars (mostly glucose)
Proteins ➡️ amino acids
Fats ➡️ fatty acids
Your body needs energy to complete all vital functions such as breathing, moving, digesting, thinking and healing. When it has excess energy, it will store it in fat cells. When the body has inadequate energy available, it will break down available stores such as fats and muscle to complete vital functions.
Your metabolic rate is how effectively your body is able to use available energy to complete these vital functions. Factors that influence metabolic rate include hormonal balance, gender, body composition and genetics, just to name a few.