Weight. Weight is a measure of the force of gravity acting as the total mass of an object. As such it reflects not only the overall size of the body but also the density of the combination of body tissues, including bone, muscle and body organs. Fat is lighter than water and therefore adipose tissue is lighter than muscle and organ tissue (which are mainly water) and both are lighter than bone. Increases in weight might therefore mean an increase in fat mass, muscle mass and/or fluid (remember, glycogen is stored with three times its weight in fluid). Over the long term, it’s true that changes in weight generally reflect changes in body fat, but in the short term the use of scales is not recommended as a measure of success of a fat loss program. Weight scales also vary significantly, from a sensitive bar balance or high quality electronic scale to the less sensitive but more often used bathroom-type scales.
The validity of weight as a measure of body fatness then is only fair, especially in certain types of individuals such as mesomorphic (muscular) males and elite athletes. Reliability of the measure on the other hand is quite high. Sensitivity is also reasonably high (i.e. around 0.8) detecting small changes in body mass. But, of course, this is not sensitive to fat as distinct from changes in other body tissue. Weight, therefore, is limited as a measure of fatness, except where combined with some other measures.
Myth-information. Weight loss through heat treatments such as saunas and steam baths represents fluid losses through sweat. These techniques have no permanent effect on fat loss.