How the Body Gets Rid of Fat
All body processes need energy to run well. When the body is expending more energy than it is taking in (e.g. when exercising), body cells count on internally stored energy sources, like compound carbohydrates and fats, for energy. The first resource the body turns to is glycogen, which is a compound carbohydrate created by the body. When that basis is nearly exhausted, the body begins lipolysis, the metabolism of fat for energy. In this process, fats, attained from fat cells, are broken down into glycerol and fatty acids, which can be used to make energy. The main by-products of metabolism are carbon dioxide and water; carbon dioxide is expelled through the respiratory system. Fats are also concealed by the sebaceous glands (in the skin).
 Actual Energy Obtained From Food
The energy humans get from food is restricted by the efficiency of digestion and the effectiveness of utilization. The efficiency of digestion is mainly dependent on the type of food being eaten. Weakly chewed seeds are badly digested. Refined sugars and fats are absorbed almost completely. In spite of the claims of certain popular diets, chewing and digesting does not use a considerable amount of the energy offered by any food (that anyone would want to eat). Even celery, known for being low in caloric value, includes enough sugars (as well as sucrose, glucose and galactose) to easily compensate for the cost of (energy invested in) chewing it.
The effectiveness of energy utilization by skeletal muscles is around 20 percent. That is, of the chemical energy used, 20 percent does work and 80 percent creates heat.
Thermoregulation
According to the principles of thermoregulation, humans are endotherms. We use energy to maintain our blood temperature at body temperature, which is about 37 °C (98.6 °F). This is attained by metabolism and blood circulation, by trembling to stay warm, and by sweating to stay cool.
Besides thermoregulation, humans use energy keeping the vital organs (especially the lungs, heart and brain) functioning. Apart from sleeping, our skeletal muscles are working, chiefly to maintain straight posture. The average work done just to stay alive is the basal metabolic rate, which (for humans) is about 1 watt per kilogram of body mass. Consequently, an average man of 75 kilograms who just rests (or only walks a few steps) burns about 75 watts (continuously), or about 6,500 kilojoules (1,500 Calories) per day.
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