It's a result of the amount of energy we release into our bodies (catabolism) minus the amount of energy our bodies use up (anabolism). The excess energy is stored either as fat or glycogen in the muscles and liver, with fat being the most caloric dense of the two.

Although becoming overweight is a result of the body storing excess energy as fat, sometimes, hormonal problems or an underlying medical condition may affect metabolism.

• A larger body mass requires more calories.
• For people of the same weight, the one with a higher muscle to fat ratio will have higher caloric needs.
• The progress of age leads to reductions in muscle mass, testosterone/estrogen production, physical activity.
• An average man burns more calories than an average woman of the same age and weight, due to men having a higher muscle to fat ratios.

It refers to the biochemical processes that occur within living organisms to maintain life. These processes allow us to grow, reproduce, repair damage, and respond to our environment.

When people use the word "metabolism" they are often referring to catabolism and anabolism. People's body weight is a result of catabolism minus anabolism.

It is a common belief that slim people have a higher metabolism and overweight people have a slower metabolism. But, this is rarely the case.

Overweight is in most cases a result of a lifestyle in which one group generally consumes more calories than they need. There isn’t much we can do to significantly change our resting metabolic rate, but long-term strategies, such as increasing muscle mass, may eventually have an effect.