By Dr. Atli Arnarson
Your metabolism is the chemical engine that keeps you alive.
The speed at which it runs varies by individual. Those with a slow metabolism tend to have more leftover fuel (calories), which gets stored as fat.
On the other hand, those with a fast metabolism burn more calories and are less likely to accumulate a lot of fat.
This is a review of why some people have a fast metabolism and how you can speed up your metabolism to burn more calories.
What Is Metabolism?
Metabolism is a term that collectively refers to all the chemical processes in your body. The faster your metabolism, the more calories your body needs.
This is the reason some people can eat a lot without gaining weight, while others seem to need less to accumulate fat.
The "speed of metabolism" is commonly known as metabolic rate. It's the number of calories you burn in a given amount of time, also known as calorie expenditure.
Metabolic rate can be divided into several categories:
- Basal metabolic rate (BMR): Your metabolic rate when you are asleep or at deep rest. It is the minimum metabolic rate needed to keep your body warm, lungs breathing, heart pumping and brain ticking.
- Resting metabolic rate (RMR): The minimum metabolic rate required to keep you alive and functioning while at rest. On average, it accounts for up to 50–75 percent of total calorie expenditure (1).
- Thermic effect of food (TEF): The number of calories burned when your body is digesting and processing food. The rise in metabolic rate after meals usually represents about 10 percent of total energy expenditure (2).
- Thermic effect of exercise (TEE): The number of calories burned during exercise.
- Non-exercise activity thermogenesis (NEAT): The number of calories burned during activities other than exercise. This includes fidgeting, changing posture, standing and walking around (3).
Summary: Metabolic rate is also known as calorie expenditure. It is the number of calories used by the body in a given amount of time.
What Factors Affect Metabolic Rate?
Numerous factors affect your metabolic rate. To name a few, these include:
- Age: The older you get, the slower your metabolic rate becomes. This is one of the reasons people tend to gain weight as they age (4).
- Muscle mass: The greater your muscle mass, the more calories you burn (5).
- Body size: The bigger you are, the more calories you burn (6).
- Environmental temperature: When your body is exposed to cold, it needs to burn more calories to prevent your body temperature from falling (7).
- Physical activity: All body movements require calories. The more active you are, the more calories you'll burn. Your metabolism will speed up accordingly (8).
- Hormone disorders: Cushing's syndrome and hypothyroidism slow down metabolic rate and increase the risk of weight gain (9).
Summary: Multiple factors affect metabolic rate or the number of calories burned. These include age, muscle mass, body size and physical activity.
Are Some People Born With a Fast Metabolism?
Metabolic rates vary between people, even when they are newborns.
In other words, some people are born with a faster metabolism than others.
One thing is clear — not everyone is created equal when it comes to metabolic rate.
Most of this variation is due to people's age, as well as their environment and behavior. However, the role of genetics in these individual differences needs to be studied further.
Summary: Metabolic rates vary by individual, even among infants. However, it is unclear how much of this variation is due to genetics.
Metabolic adaptation, also known as adaptive thermogenesis or "starvation mode," may also play an important role in the development of obesity.
Starvation mode is the body's response to a calorie deficit. When your body doesn't get enough food, it tries to compensate by reducing its metabolic rate and the number of calories it burns.
Summary: Metabolic adaptation or starvation mode is when metabolic rate slows down during a calorie-reduced diet or a fast. It varies between people and tends to be more pronounced among obese individuals.