Body composition and outcomes in chronic hemodialysis patients

In contrast to epidemiological data from the general population, maintenance hemodialysis (MHD) patients with a naturally small body size experience an increased mortality rate compared to their larger fellow patients. Since body mass index is a poor surrogate of body composition, attempts were made to delineate muscle, fat and visceral organ mass in MHD patients. Several lines of evidence indicate that (a) increased fat and muscle mass exerts protective effects, (b) some markers of inflammation may be increased with fat mass, and (c) a high visceral mass per body weight is associated with a reduced survival time. The reasons for the positive effects of fat and muscle mass on survival are not clear. A novel hypothesis predicts lower uremic toxin concentrations in larger subjects. This is based on the observation that both in healthy subjects and in dialysis patients, visceral organ mass is inversely related to body mass. Since visceral organs are the most prominent source of uremic toxins, large patients may have a lower toxin production rate per unit of body mass. Moreover, large patients have a greater volume of distribution (total body water, fat mass) resulting in lower toxin concentrations. Future studies should aim to tackle the Janus-like duality of obesity by a system biology approach.