[Obesity in dialysis and reverse epidemiology: true or false?].

About 50% of patients who undergo dialysis are overweight or obese. Rather than being a disadvantage, the extra weight is associated with improved survival in this patient group. However, the relationship between weight and outcome is complex among dialysis patients. In the general population obesity constitutes a clear cardiovascular risk factor. By contrast, in obese dialysis patients the nutritional status may be better, and obesity thus provides, at least in the short term, some protection against malnutrition and the associated morbidity. On the other hand, some studies suggest that mortality in the long term is directly correlated with excess weight and obesity, which indicates that fat represents a risk factor also in uremia. In the elderly, particularly those affected by end-stage renal disease, endocrine and metabolic effects on the nitrogen balance cause the loss of muscle mass despite an excess of adipose tissue, which is a condition known as sarcopenic obesity. While a good nutritional state is found in some obese dialysis patients, which probably accounts for the improved survival of the obese group as a whole, there is a sizable proportion of sarcopenic obese, which is probably increasing. Sarcopenic obesity is not only characterized by the reduction of muscle mass but also by the accumulation of fat surrounding the abdominal viscera (visceral fat syndrome), which may be associated with a greater degree of metabolic and atherosclerotic disease. Several studies have shown that malnutrition associated with obesity, including sarcopenic obesity, is the risk factor most closely correlated with morbidity and mortality both in dialysis patients and the general population. The timely identification of this condition has therefore become necessary in the dialysis population now dominated by the elderly and very elderly. Body mass index is inadequate as a measure of sarcopenic obesity since it cannot define muscle mass nor indicate the localization of the fat in the visceral compartment. Other indices must be developed and validated in well performed clinical trials to identify fat localization and the presence of sarcopenia.