What is the Energetic Equivalent of Body Tissues? Implications for Weight Loss:
The energetic equivalent of a given amount of body mass either gained or lost depends on its composition. Lean tissue such as muscle is about 80% water with the remaining 20% being largely protein with smaller amounts of fat and carbohydrate. In contrast adipose tissue is only about 15% water, the remainder (85%) being storage lipid with a very small amount of protein contributing to the cell structure and intracellular enzymes. Given these differences in the composition of body tissues, the amount of energy represented by a kilogram of body weight will differ accordingly.
For example, the body energy lost when a Kg of muscle is lost would be roughly 1120 kcal (200g x5.6 kcal/g for protein). Only about 800 kcal of this energy would be net energy available to supply the boy’s fuel needs; the remaining 320 kcal would be required to metabolize the nitrogen derived from the breakdown of the amino acids in the protein to the excretory product, urea. Loss of a kilogram of adipose tissue (85% lipid) represents a body energy loss of about 7905 kcal (850 g x 9.3 kcal/g). In contrast to protein, essentially all of the energy represented by storage fat can contribute to net fuel energy. Thus, a kilogram of body fat represents nearly 10 times (7905 kcal/kg fat ÷ 800 kcal/kg lean) the energy for fuel represented by a kilogram of lean tissue. These figures provide some perspective on weight loss claims appearing in the popular press. Take the example of a claim touting a 2.5 Kg weight loss in a week by simply following a new diet (i.e. no exercise involved). If the weight loss were really adipose tissue, as hoped by the dieter, the loss would represent 41,995 kcal of body energy. For the average sedentary individual only about 2500 kcal is needed each day to maintain body weight, or 17,500 kcal per week. It is therefore impossible to lose 5 lbs (2.5 Kg) of fat in a week by dieting alone – in order to lose 5 lbs of fat, the individual would have to eat nothing for 16.5 days!
Healthy weight loss is about 0.5 to 2 pounds per week, depending on total body weight.