Food Energy: Wiki
Energy intake to the body that is not used up is mostly stored as fat in the fat tissue. Consider the following theoretical calculation.
* Fat contains about 3,500 kilocalories per pound (32 kJ/g).
o If you eat 3,500 kcal more than your body needs, you will put on 0.3 pounds (0.14 kg) of fat (assuming 30% digestion efficiency, or less depending on food conversion inefficiency not included in the "body needs" category).
o If you burn 3,500 kcal more than you eat, you lose about 1 pound (0.45 kg) of fat, assuming that only fat is burnt (this is close to 100% since even the waste heat counts toward the 3,500 kcal). However, energy sources can come from catabolism of protein (muscles), and fat may be preferentially saved. The use of different body materials as available must be considered.
Because the body must expend energy to create fat, the amount of energy a person has to expend to lose weight is much smaller than the amount they have to consume in order to gain weight. However, the body may increase its basal metabolic rate to preferentially "burn off" new calories or decrease it to conserve energy to balance out increased activity. Thus, actual weight changes vary between individuals. Also, the computations above assume that all the weight gained and lost is in the form of fat. In reality, this is a mixture of protein, carbohydrates, etc. (in muscle tissue, organs, etc.).