Metabolism is the sum total of all chemical reactions that take place in your body. It is based on three basic components: 1) Your resting metabolic rate or RMR, 2) physical activity, and 3) thermogenesis. (Metabolic rate is typically expressed in calories and reflects the energy released during the course of metabolism. )
? 1) Your RMR (also called basal metabolic rate or BMR) is the minimum number of calories required to maintain your body’s basic functions, such as the heart beat, tissue repair, and breathing. Your RMR constitutes about 60 to 75% of your total calories. Your muscle mass is the single most important predictor of your RMR. The more muscle, the higher your RMR. Combined with your age and gender, muscle accounts for 83% of RMR. Another 11% is explained by genetics, and the last few percentage points results from a person’s thyroid and nervous system activity.
? 2) The second biggest calorie contributor to metabolism is physical activity, which accounts for 15 to 30% of total calories in a moderately-active woman. This value varies enormously (intense exercise for long-durations, such as an ultra-marathon, can burn as much as 15 times the RMR).
? 3) The third and final contributor to metabolism is diet-induced thermogenesis (DIT), or the amount of calories burned over and above RMR when the body is assimilating food after a meal. DIT accounts for about 10 to 20% of your energy intake.