Our metabolic rate is the amount of energy required by a person at rest for maintaining homeostasis--to maintain such processes as breathing, circulation, cell growth, body temperature regulation, how you metabolize nutrients, regulate brain function, etc. Your metabolic rate determines how you maintain your weight and in people with a high metabolic rate, they may not easily put on weight or may experience a dramatic weight loss. A high metabolic rate can be a cause for alarm, or it can be normal depending on the situation.. For example, males have higher BMR's than females. Lactating females can have higher basal metabolic rates as can pregnant females. The age of an individual is a factor--young children have higher metabolic rates than elderly people do as their energy needs are greater at an age where the body is growing and developing. Sometimes, the basal metabolic rate becomes high due to fever--fever is the body's defense against pathogens. For every degree of rise in body temperature, the BMR increases 7 percent. Cancer patients can have a high BMR since cancer cells don't operate the same way normal cells do and their metabolic rates are higher. Burn patients can frequently have higher than normal basal metabolic rates. People with the condition known as hyperthyroidism can have a higher metabolic rate. They experience extreme weight loss despite a high caloric intake. The hormone thyroxine is used to regulate metabolism. In hyperthyroidism, too much thyroxine is produced, leading to accelerated metabolic rate, sweating, irregular heartbeat, nervousness among other symptoms.
Fast metabolism simply refers to a high rate of metabolism. Many people find that they are always hungry and no matter how much they eat, they do not put on weight. It is simply because their bodies have a fast metabolism and utilizes the food much faster than most of the people. Suspected reasons for fast metabolism include genetics, stress, exercises, an overactive thyroid gland or hyperthyroidism, smoking, etc.
The food is broken down into simpler molecules using the process of cellular respiration. Co.plex molecules in our food are broken down during the process of catabolism, during which energy is also released and is stored in the form of ATP or adenosine triphosphate molecules. The simpler molecules are used along with the generated energy, in the process of anabolism, to generate biomass, muscles and regular upkeep and operation of our body.
Hope this helps.