Body mass index, muscle and fat in chronic kidney disease: questions about survival.

The human body can be roughly divided into two major compartments, fat mass and lean body mass. Adipose tissue is now considered to be a highly active tissue and, in addition to storing calories as triglycerides, it also secretes a large variety of compounds, including cytokines, chemokines and hormone-like factors such as leptin, adiponectin and resistin. On the other hand, muscle plays a central role in whole-body protein metabolism by serving as the principal provider for amino acids to maintain protein synthesis in vital tissues and organs and by providing hepatic gluconeogenic precursors. Although not a good indicator of body composition, the Quetelet index, also called body mass index (BMI), is often used for practical reasons. It is well known that high BMI predicts mortality and cardiovascular disease (CVD) in the general population. However, observational reports in the dialysis population have suggested that obesity is associated with improved survival, a phenomenon that is not well understood and subject to controversies. This review describes the characteristics of BMI in the general population and in chronic kidney disease (CKD) patients, as well as the respective role of muscle, whole body fat and fat distribution towards mortality, with particular emphasis on patients with CKD.