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Department of Medicine
85 North Medical Drive East
Room 201
Salt Lake City, UT 84112
E-mail: srinivasan.beddhu{at}hsc.utah.edu
University of Utah School of Medicine
Salt Lake City, UT 84112
Dear Sir:
In a recent issue of the journal, Johansen et al (1) examined an important question—What is the association of body size with survival adjusted for muscle mass in incident dialysis patients? However, there are really 3 questions: 1) What is the independent association between muscle mass and mortality, 2) What is the independent association between BMI and mortality, and 3) How does mortality vary across different levels of BMI and muscle mass combined. Based on the answer to the question posed by Johansen et al, inferences on whether body composition influences the survival of incident dialysis patients with a high BMI could not be drawn.
We reexamined the data from our earlier study (2), which the authors graciously discussed. Details on study population, inclusion criteria, data collection, and statistical methods were described earlier (2). In 70028 incident hemodialysis patients in the United States, from 1 January 1995 to 31 December 1999, the associations of BMI categories described by Johansen et al with survival were examined in a multivariable parameteric proportional hazards survival model adjusted for urinary creatinine, demographics, comorbid conditions, serum albumin, and functional status. The results (Figure 1) are similar to those reported by Johansen et al.
FIGURE 1.. Association of body size with survival in incident hemodialysis patients. Reference BMI: 22 to <25.
To further examine the influence of body composition on survival in high-BMI patients, each of the BMI groups was divided into groups on the basis of muscle mass: low (urinary creatinine 25th percentile, ie, 0.55 g/d), normal, or high (urinary creatinine >0.55 g/d) subgroups. The hazard ratios from the multivariable parameteric proportional hazards survival model, adjusted for all of the above factors except urinary creatinine, are presented in Figure 2.
FIGURE 2.. Association of body composition with survival in incident hemodialysis patients. Reference group: 22 to <25, with urinary creatinine >0.55 g/d. , urinary creatinine > 0.55 g/d; , urinary creatinine 0.55 g/d.
At first glance, Figures 1 and 2 appear contradictory, but, in reality, they are not. Adjustment for urinary creatinine in the multivariable model (Figure 1) does not mean that the hazard of death is constant across the spectrum of urinary creatinine values in any given BMI group (Figure 2). Whether the association of BMI with survival is confounded by muscle mass is examined in Figure 1. Whether those with a large body size but low muscle mass have a survival advantage over "healthy" patients with a normal BMI and a normal or high muscle mass is examined in Figure 2.
In our study we summarized the findings in Figure 2 as "the survival advantage conferred by high BMI in dialysis patients is limited to patients with normal or high muscle mass." We understand the concerns of Johansen et al that this could be construed as independence. We rephrase our conclusions as follows. Patients with a high BMI and low muscle mass have a higher mortality than do "healthy" incident dialysis patients with a normal BMI and normal or high muscle mass. On the other hand, patients with a high BMI and normal or high muscle mass have a lower mortality than do "healthy" incident dialysis patients with a normal BMI and normal or high muscle mass. Thus, compared with "healthy" incident dialysis patients with a normal BMI and normal or high muscle mass, those with a high BMI have a lower mortality only if their muscle mass is normal or high.
In conclusion, the questions addressed in the 2 studies were related but had different emphases. We absolutely agree with Johansen et al that body size is an important determinant of survival in incident dialysis patients. However, we stand by our earlier conclusion that, in incident dialysis patients, body size and body composition influence survival. In incident dialysis patients, adiposity confers a survival advantage over undernutrition, but higher muscle mass is better than higher body fat. We agree that, given the current data, incident dialysis patients should not be encouraged to lose weight but should be encouraged to increase muscle mass rather than fat mass.
ACKNOWLEDGMENTS
None of the authors had a conflict of interest.
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