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A significant gain in weight and body fat results from an energy imbalance over a prolonged period of time, which means that energy intake exceeds energy expenditure. In other words, weight gain and body fat accumulation results from a prolonged period of eating more calories than you burn. Thus, you didn't go to bed one night fit and trim and wake-up the next morning fat and over-weight.
Consequently, you can change your energy balance by either altering the amount of calories you consume or increasing your energy expenditure. Your total daily energy expenditure is the sum total of the following: your resting energy expenditure + the thermogenic effect of the foods you consume + the energy expenditure related to your daily activity. Consequently, if your total daily caloric intake exceeds your total daily energy expenditure, you gain weight in the form of body fat.
Under most circumstances the largest component of your total daily energy requirement is your resting energy expenditure (also referred to as REE). Your REE is composed of the metabolic requirements of your organs, and muscle mass. The energy requirements of your organs remain fairly constant under most circumstances therefore, the energy expenditure related to muscle metabolism is the only part of your REE that varies considerably.
Muscle is the active component of your body that burns the most calories at rest. It is estimated that a pound of muscle burns approximately 6 calories daily at rest. So, for each pound of muscle you gain you also increase your energy expenditure by 6 calories each day. While that may sound like a small number, it makes a big difference over time. For example, let's say you gain 5 pounds of lean muscle in the course of a year. That equates to an additional 30 calories in your daily energy expenditure. Over the course of a year that equates to nearly a 11,000 calorie increase in your REE.
The synthesis and breakdown of muscle protein is principally responsible for the energy expenditures of resting muscle. The energy to provide for this process of muscle protein turnover is derived mainly from the oxidation of fat. Yes, body fat is the preferred fuel of resting muscle, and it is also the preferred fuel for your muscles during low to moderate intensity activities.
Now, let's look at the above example of a 5 pound muscle gain in a different light. It takes a 3500 calorie deficient to lose 1 pound of body fat. So, the 11,000 calorie increase in your REE due to a 5 pound gain in muscle equates to a 3 pound loss of body fat over the course of a year. Now, do you see why it's important to build and maintain your muscle mass if you want to get lean? Thus, I remind you again, "Build the muscle and you will burn the fat".