Body mass index and mortality in kidney transplant recipients: a systematic review and meta-analysis

Abstract

Background: A higher body mass index (BMI) seems to be linked to survival advantage in maintenance hemodialysis patients. However, it is uncertain if this 'obesity survival paradox' is also observed in kidney transplant recipients. Hence, we systematically reviewed the literature on the impact of pre-transplantation BMI on all-cause mortality in this population.

Methods: We searched MEDLINE, EMBASE, Web of Science, CINAHL, and Cochrane CENTRAL for relevant studies up to July 2013. Two investigators independently selected the studies using predefined criteria, abstracted the data from the included studies, and independently assessed each study's quality using the Newcastle-Ottawa Quality Assessment Scale. In addition to the qualitative synthesis, we quantitatively pooled the results of the studies with clinical, methodological, and statistical homogeneity.

Results: We screened 7,123 records, from which we included 11 studies (with a total of 305,392 participants) in this systematic review and 4 studies in the meta-analyses. In the only study that included children, obesity was linked to higher mortality in children of 6-12 years old. For adults, our meta-analyses indicated that compared to normal BMI, underweight [Hazard Ratio (HR): 1.09; 95% Confidence Interval (CI): 1.02-1.20], overweight (HR: 1.07; 95% CI: 1.04-1.12), and obese (HR: 1.20; 95% CI: 1.14-1.23) levels of BMI were associated with higher mortality.

Conclusion: The presence of the obesity survival paradox is unlikely in kidney transplant recipients since both extremes of pre-transplantation BMI are linked to higher mortality in this population.

Figure 1

Figure 2. The results from re-analyzing a report of ‘Scientific Registry of Transplant Recipients’ data

Re-analysis of the largest reported dataset by Hatamizadeh et al.¹¹ revealed the adjusted hazard ratios (HRs) of ‘all-cause mortality’ (A), ‘graft failure’ (B), and ‘combined mortality or graft failure’ (C) for underweight, overweight, and obese classes I, II, and III, compared to the normal BMI. The above categories were defined according to WHO BMI classification. ²⁰ The vertical axes are in logarithmic scale. The hazard ratios and their 95% confidence intervals (CIs) are tabulated (D).

Figure 3. Meta-analyses of hazard ratios of all-cause mortality

A, B, and C illustrate the meta-analysis of the hazard ratios (HRs) of all-cause mortality in ‘underweight’, ‘overweight’, and ‘obese’ BMI classes, respectively, compared to normal BMI. The three BMI cut-points between the ‘underweight’, ‘normal BMI, ‘overweight’, and ‘obese’ classes were 18.5 ^11,15 or 20 ^9,12, 25, and 30, respectively. The horizontal axes are in logarithmic scale. D puts the results of A, B, and C together, showing the pooled hazard ratios of all-cause mortality (in logarithmic scale). * Results are derived from re-analyzing Hatamizadeh et al. ¹¹ data.

Figure 4. Meta-analyses of hazard ratios of graft failure

A, B, and C illustrate the meta-analysis of the hazard ratios (HRs) of graft failure in ‘underweight’, ‘overweight’, and ‘obese’ BMI classes, respectively, compared to normal BMI. The cut-points between the ‘underweight’, ‘normal BMI, ‘overweight’, and ‘obese’ classes were the same as Figure 3. Graft failure was defined as re-initiation of dialysis or re-transplantation (patients who died before graft failure were censored). The horizontal axes are in logarithmic scale. D puts the results of A, B, and C together, showing the pooled hazard ratios of graft failure (in logarithmic scale). * Results are derived from re-analyzing Hatamizadeh et al. ¹¹ data.

Figure 5. Meta-analyses of hazard ratios of combined mortality or graft failure

A, B, and C illustrate the meta-analysis of the hazard ratios (HRs) of combined mortality or graft failure in ‘underweight’, ‘overweight’, and ‘obese’ BMI classes, respectively, compared to normal BMI. The cut-points between the ‘underweight’, ‘normal BMI, ‘overweight’, and ‘obese’ classes were the same as Figure 3. The horizontal axes are in logarithmic scale. D puts the results of A, B, and C together, showing the pooled hazard ratios of combined mortality or graft failure (in logarithmic scale). * Results are derived from re-analyzing Hatamizadeh et al. ¹¹ data.