Lean body mass performs the essential cellular functions of life. Lean body mass must not be reduced or life itself is reduced.
The body is composed of extracellular fluid, fat mass, and lean mass (muscles and organs). It is fat mass, not muscles and organ tissues, that a dieter wants to lose. Lean body mass consists of parenchymal organ tissues (liver, brain, kidney, spleen, pancreas, etc.) including muscles (which may be increased in obese patient). The cellular functions of this lean mass are essential to life and so reduction of lean body mass must be avoided.
Loss of lean muscle mass can be not only unhealthy, but also unsightly, causing cosmetic malproportion, loss of body mass in the wrong areas. Loss of lean muscle mass (upper limbs, lower limbs, buttocks, and chest) with preservation of abdominal fat gives the appearance of spindly limbs and a protruberant belly, not typically considered a desirable result. The esthetic effect of such lean body mass reduction is entirely negative, indeed a cachectic appearance. Only a minimal amount of fat mass is necessary for health and the associated healthy appearance.