Kidney Research and Clinical Practice 2019 Mar; 38(1): 90-99  https://doi.org/10.23876/j.krcp.18.0094
Time-varying effects of body mass index on mortality among hemodialysis patients: Results from a nationwide Korean registry
Sejoong Kim1,*, Jong Cheol Jeong2,*, Shin Young Ahn3, Kibbeum Doh4, Dong-Chan Jin5, Ki Young Na1
1Department of Internal Medicine, Seoul National University Bundang Hospital, Seongnam, Korea, 2Department of Nephrology, Ajou University School of Medicine, Suwon, Korea, 3Division of Nephrology, Department of Internal Medicine, Korea University Guro Hospital, Korea University Medical Center, Seoul, Korea, 4Medical Research Collaborating Center, Seoul National University Bundang Hospital, Seongnam, Korea, 5Department of Internal Medicine, College of Medicine, The Catholic University of Korea, Seoul, Korea
Correspondence to: Ki Young Na, Department of Internal Medicine, Seoul National University Bundang Hospital, 82 Gumi-ro 173beon-gil, Bundang-gu, Seongnam 13620, Korea.
E-mail: kyna@snubh.org, ORCID: https://orcid.org/0000-0002-8872-8236
*Sejoong Kim and Jong Cheol Jeong contributed equally to this study as the co-first authors.
Received: August 7, 2018; Revised: October 15, 2018; Accepted: October 27, 2018; Published online: March 31, 2019.
© The Korean Society of Nephrology. All rights reserved.

This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
Background: Unlike patterns observed in the general population, obesity is associated with better survival among hemodialysis patients, which could be explained by reverse causation or illness-related weight loss. However, the time-varying effect of body mass index (BMI) on hemodialysis survival has not been investigated. Therefore, this study investigated the time-varying effect of BMI on mortality after starting hemodialysis.
Methods: In the present study, we examined Korean Society of Nephrology data from 16,069 adult patients who started hemodialysis during or after the year 2000. Complete survival data were obtained from Statistics Korea. Survival analysis was performed using Cox regression and a non-proportional hazard fractional polynomial model.
Results: During the median follow-up of 8.6 years, 9,272 patients (57.7%) died. Compared to individuals with normal BMI (18.5-24.9 kg/m2), the underweight group (< 18.5 kg/m2) had a higer mortality hazard ratio (HR, 1.292; 95% confidence interval [CI], 1.203-1.387; P < 0.001) and the overweight group (25.0-29.9 kg/m2) had a lower mortality HR (0.904; 95% CI, 0.829-0.985; P = 0.022). The underweight group had increasing HRs during the first 3 to 7 years after starting hemodialysis, which varied according to age group. The young obese group (< 40 years old) had a U-shaped temporal trend in their mortality HRs, which reflected increased mortality after 7 years.
Conclusion: The obese hemodialysis group had better survival during the early post-dialysis period, although the beneficial effect of obesity disappeared 7 years after starting hemodialysis. The young obese group also had an increased mortality HR after 7 years.
Keywords: Body mass index, Obesity, Renal dialysis, Reverse causation, Time-varying hazard


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