Also, metabolism can be "boosted" by increasing energy usage through exercise.  A simplified definition of metabolism is the process in which energy sources (primarily food and stored body fat) are broken down to supply energy to the body. The higher the energy requirement, the faster the metabolic process occurs.  Therefore, someone with a "slow metabolism" just simply isn't expending much energy, or in other words, they are lazy. One of fat people's favorite excuses, hypothyroidism, is a real disorder that causes a less effective metabolism due to insufficent hormones involved in the process, but according to the American Thyroid Association, this only accounts for 5-20 pounds of weight gain and does not cause obesity.