The truth about metabolism - Harvard Health

It is the internal process by which your body expends energy and burns calories. It runs 24/7 to keep your body functioning by converting the food and nutrients you consume into energy.

This process works at different intensities in different people. The speed of your metabolism is determined mostly by your genes, regardless of your body size and composition, but how much you weigh mostly comes down to diet and activity levels.

Age affects metabolism, making it slow over the years, even if you start out with a fast metabolism. Differences in metabolism speed are evident in how easy or hard it is for people to gain or lose weight.

A slow metabolism burns fewer calories, thus storing more fat; that's why some people have difficulty losing weight by just cutting calories. A fast metabolism burns calories at a quicker rate, which explains why some people can eat a lot and not gain extra pounds.