It is in every cell in your body. It refers to a series of chemical processes in each cell that turn the calories you eat into fuel to keep you alive.

The body's major organs — the brain, liver, kidneys, and heart — account for about half of the energy burned at rest, while fat, the digestive system, and especially the body's muscles account for the rest.

There are 3 main ways:

• the basal metabolism, the energy used for your body's basic functioning while at rest
• the energy used to break down food (also known as the thermic effect of food)
• the energy used in physical activity.

Most of the energy you burn is from your resting metabolism.

Metabolism can vary a lot between people, and researchers don't understand why.

2 people with the same size and body composition can have different metabolic rates. One can consume a huge meal and gain no weight, while the other has to carefully count calories to not gain weight.

Getting older slows down the metabolism.

The effect happens gradually, even if you have the same amount of fat and muscle tissue. So when you're 60, you burn fewer calories at rest than when you're 20.

There's a lot of hype around" speeding up your metabolism for weight loss, but that's just a myth.

While there are certain foods (coffee, chili, and other spices) that may speed the basal metabolic rate up just a little, the change is so negligible and short-lived, it would never have an impact on your waistline. Building more muscles, however, can be more helpful.

Drastic dieting can slow down your metabolism. This may be the body's way of vigorously defending a certain weight range, called the set point.

Researchers don't fully understand why this metabolic slowdown happens though.