Reply to S Beddhu et al

Division of Nephrology
Department of Medicine
University of California, San Francisco
San Francisco VA Medical Center
San Francisco, CA 94121
E-mail: johanse{at}itsa.ucsf.edu

Dear Sir:

We appreciate the comments of Beddhu et al regarding our recent publication that examined the relation between body size and outcomes among incident hemodialysis patients (1). In particular, we agree with the idea that body composition, and perhaps muscle mass in particular, is important to consider for patients receiving dialysis. However, it is important that, in our discussion of the "best" way to adjust for muscle mass in these patients, we not lose sight of the larger issues at hand. First, although analyses using large data sets are often constrained to the use of body mass index or similar weight-for-height indexes as the primary indicator of body size, they are fundamentally not the best measures of body composition. The best way to address the contribution of muscle mass to survival among incident hemodialysis patients would be to measure muscle mass itself. Although this is not possible in large cohorts that can be established with the use of data from the US Renal Data System, body composition can be measured directly in smaller cohorts and the results used to determine which components are most important to patient survival.

Second, survival is only part of the story when it comes to associations between body composition and outcomes in patients receiving hemodialysis. Body fat mass and muscle mass could each be related in important ways to quality of life in these patients. For example, a larger muscle area is related to greater strength and improved physical performance (2). Conversely, it is possible that greater fat mass is associated with greater difficulty with physical activity and activities of daily living. These associations need further study before anyone can assign muscle or fat as "more important" in this patient population.

ACKNOWLEDGMENTS

Neither of the authors had a conflict of interest.

REFERENCES

1. Johansen KL, Young B, Kaysen GA, Chertow GM. Association of body size with outcomes among patients beginning dialysis. Am J Clin Nutr 2004;80:324–32.
2. Johansen K, Shubert T, Doyle J, Soher B, Sakkas G, Kent-Braun J. Muscle atrophy in patients receiving hemodialysis: effects on muscle strength, muscle quality, and physical function. Kidney Int 2003;63:291–7.