Creating an Energy Deficit
Fat is a long-term energy store. Energy is vitally important for your body, so it is under tight regulation by hormones and other cellular signals. When energy (food) intake is lower than what is needed, hormones will signal the release of more fat (triglycerides) from adipose (fat) tissue. The triglycerides will travel to the tissue that is in need of the energy (usually the muscles, heart, liver) and burns them in addition to stored carbohydrate (glycogen) and stored amino acids (muscle proteins). If this continues, then fat stores (body fat) will decrease. At rest, when not starving, we get half of our energy from fat and half from carbohydrates. So, we are always burning fat, but will not have a net loss of body fat if it is replaced by the food we eat (fat or carbohydrate).
Resting Metabolic Rate
Your body is always using energy (Calories) to maintain and repair itself, even at rest. The amount of energy your body uses is directly related to how big your body is, or more accurately, how much muscle mass your body has. The more muscle mass you have, the more energy it consumes 24 hours a day. Men and younger adults will burn more calories than females and older adults for this reason. Your resting metabolic rate is the amount of energy your body burns in 24 hours, while at rest, just to maintain and repair (and grow) itself (does not include any physical activity or exercise). It can be estimated with a body composition analysis. For example, a 45 year-old, average-build female will have a resting metabolic rate of about 1300-1400 Calories per day.
Elevating Your Metabolism
Often, when someone is trying to lose fat they will focus on total calories burned during their exercise session. However, this is a small fraction of time when compared to the whole week (for example if you run 30 minutes, three times per week you have raised your metabolism 1.5 hours vs. 168 total hours in a week). The key to greater fat loss is elevating your metabolism 168 hours per week. How do you raise your metabolism all the time? The answer is with strength training and interval endurance exercise. Not only do you burn calories while you perform these exercises, but you will raise your resting metabolic rate for up to 48 hours afterwards. Resistance training will also build muscle, which also raises your metabolism. For example, if a person adds five pounds of muscle to their body, they will burn almost 20,000 extra Calories in a year.
Metabolic Resistance Training and Interval Endurance Exercise
Resistance training stimulates muscle to grow stronger and increase in size. When performed with a moderate to heavy resistance, extra energy is needed to repair and return the muscle fibers to their resting condition. The key is stimulating as much muscle as possible. That means using all of the muscles with multiple exercises or exercises that combine upper and lower body muslces in one session. Also, you need to use a significant number of fibers in each muscle (meaning that the resistance has to be adequate) and contract with a fast enough speed to use your larger, fast-twitch fibers (these are typically not used during lower intensity exercise).
Interval endurance exercise allows you to work at a higher level for short bouts (0:10 to 2:00) with rests in between. The higher intensity exercise uses the larger, fast-twitch fibers and stimulates your metabolism. Interval endurance exercise could also include your upper body, too. Interval endurance exercise may be too intense for some beginners and may need to start with a lower intensity exercise.
The combination of creating a calorie deficit, full-body resistance training (two days per week) and interval training (two days per week) will help you burn body fat, get stronger, build muscle, and raise your metabolism in the same amount of time you were already exercising.