"Metabolism is the term for the chemical processes that occur inside the body to keep us alive and our organs working properly," says registered dietitian Sioned Quirke.
She goes on: "These chemical processes require energy. The minimum amount of energy your body needs to carry out these chemical processes is called the basal metabolic rate (BMR)."
Your BMR is the rate at which you burn calories and therefore affects how quickly you put on weight or lose it.
What factors affect metabolism?
People burn calories at different rates depending on a number of factors.
"There's a genetic component," says Dr Jane Naufahu, lecturer in human nutrition and performance at the University of Westminster.
"There are variables in term of gender, age and lean body mass."
You may have been as thin as a pick in your teens but with age our metabolism slows so if we eat the same amount as we did in our youth we'll put on weight.
In fact as our BMR drops by 5% each decade after the age of 40.
"Our BMR's tend to decrease with age as we tend to lose muscle mass," says Sioned.
Men usually have a faster metabolism (BMR) than women do. They tend to burn more calories at rest than women. That's why on diets men and women have different calorie allowances. It's down to the fact that men generally have more muscle and muscle burns more calories than fat.
"They tend to have more lean mass and a bigger body size so they need more energy to power it," says Dr Naufahu.
Weight and muscle
Similarly heavier people tend to have a higher metabolic rate than lighter people as their bodies are bigger and need more energy or calories to keep it running.
It's also the case that the more body muscle you have the faster your metabolism. It's estimated a pound (lb) of muscle burns around 6 calories a day whereas a pound (lb) of fat only burns 2 calories.
Unfair as it seems some people simply burn calories quicker than others. The hereditary aspect of metabolism is not fully understood.
It may be to do with your body composition.
"You are born with a certain type of body type, you may be naturally lean with low body mass or you may be more portly," says Dr Naufahu.
"To a certain extent we do inherit our metabolic rate from our parents," according to Miguel Toribio-Mateas, chairman of the British Association for Applied Nutrition and Nutritional Therapy.
Occasionally people have a defect in their thyroid glands which slows down their metabolism.
"Reduced BMR could be modified by changes in endocrine function including levels of cortisol, growth hormone and testosterone," says dietitian Perryn Carroll.
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