Taken from Variables for Weight Lifting and Exercise © 2003 Anthony Sell - All Rights Reserved
The key to a healthy metabolism lies largely in the amount of muscle mass that an individual has. Other considerations in changing the rate of metabolism are the level of activity prior to training, the normal amount of food eaten (also prior to training), and the amount of rest normally attained.
An athlete that is accustomed to eating a diet that has a maintenance level caloric intake on a busy schedule with little or no weight training will have a more significant change in metabolism once they start weight training, than someone who has done light weightlifting and is merely increasing intensity or changing focus.
Metabolism, by definition, is the body's ability to change stored and consumed energy into useful fuel. A person with a higher metabolic rate will not only spend more energy per day, but will also be able to utilize more of their energy stores and consumed nutrients. This in turn will allow for a more cut and defined appearance.