What is fast metabolism and what does it have to do with a person gaining or losing weight? Basically metabolism means the collective chemical processes that your body uses in order to convert the food that you eat into energy that the body can use to function. By eating, you ingest a certain number of calories and those are then broken down. The basal metabolic rate (BMR) is a term that is often used when discussing issues concerning weight loss or weight gain and refers to the rate at which your body breaks down food. This rate is measured at the time that the body is not engaging in physical activity (is at rest) and therefore measures the amount of energy that your body is using in order to sustain the basic life functions (things such as breathing and your internal organs such as the lungs, heart and others). A fast metabolism refers to your body breaking down food at a fast rate.
The thyroid is the gland that is the key component in all of this. It is very small and is located in your throat. It is an endocrine gland, meaning that it secretes its hormones (chemicals) directly into the blood. The hormones that the thyroid gland releases are the ones responsible for the regulation of metabolism. These hormones stimulate catabolic and anabolic reactions. The catabolic reaction is the one that breaks down the food so that it can be digested and the nutrients coming from the food can go to the different parts of the body where they are needed. The anabolic reaction uses these small broken down pieces to construct components of cells. So the way your thyroid works and how fast it works affects the rate of your metabolism. Fast metabolism means that all these processes are done at a faster rate than normal.
If you want to know how many calories to consume, for example if you want to gain weight, then knowing your BMR can be very helpful. There are different methods of determining this (for example there are different BMR calculators). After you determine your BMR, then you can also determine how many calories you need to eat in order to reach your goals. People with a fast metabolism usually find it hard to gain weight, while the ones with a slow metabolism usually gain weight very easily. However, just like anything in life, all these things can be overcome with a proper plan and even if you have a fast metabolism you can still grow to be big. You just have to work hard at it.
To determine your BMR, then you can use the calculator here: