Lean Body Mass refers to the mass of the body that is free of adipose tissue. Thus, lean body mass is the sum of the weight of bones, skin, muscles, organs and everything else except fat. If the weight stays stable then lean body mass will decrease in direct proportion to the increase in fat. Bioelectric impedance analysis is one of the methods used for measuring lean body mass. A calculation of the lean body mass helps in understanding the amount of muscle lost or gained during a period.