Short reportSerum bilirubin as a predictor of incident metabolic syndrome: A 4-year retrospective longitudinal study of 6205 initially healthy Korean men
Introduction
The metabolic syndrome (MetS) is a complex disorder involving a combination of risk factors of metabolic origin that appear to promote the development of cardiovascular disease (CVD) [1]. Chronic inflammation, insulin resistance and oxidative stress are important factors in the initiation and progression of MetS [1].
Bilirubin has been regarded as a potentially toxic metabolite of haem catabolism [2], but recent data have suggested that it has numerous other beneficial effects, such as scavenging excess reactive oxygen species and anti-inflammatory actions [3]. In line with these beneficial effects, mildly elevated bilirubin has been negatively associated with oxidative stress-mediated diseases, including CVD [4] and MetS [5], [6], [7], although most studies were cross-sectional in design.
Recently, Oda et al. [8] reported that total bilirubin was not a risk factor for MetS in a longitudinal study. However, in contrast to that report, our recent longitudinal study of bilirubin and incident type 2 diabetes showed a negative association between bilirubin and waist circumference (WC), fasting plasma glucose (FPG) and hypertriglyceridaemia, which are individual components of MetS [9]. These findings raise the possibility that bilirubin could be a risk factor for MetS. Thus, the longitudinal effects of baseline serum bilirubin concentrations on the incidence of MetS were further investigated over a 4-year follow-up study.
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Methods
The study cohort comprised 8068 Korean men who had undergone comprehensive routine health examinations at the Health Screening and Promotion Center of the Asan Medical Center in Seoul, Republic of Korea, in 2007 and who had returned for follow-up examinations in 2011. Of these men, 1674 subjects with MetS were excluded at baseline. In addition, 189 subjects who were missing values for baseline bilirubin level were also excluded. After these exclusions, 6205 men with a mean age of 49.4 years
Results
Subjects were categorized into four quartiles (Q1–Q4) according to baseline total bilirubin concentrations, and their clinical and biochemical characteristics according to bilirubin quartile categories are shown in Table 1. As the bilirubin quartile increased, the prevalence of current smokers and frequent exercisers decreased. On checking the interaction between smoking status and bilirubin to test whether the effect of bilirubin on incident MetS differed according to smoking status, there was
Discussion
In the present longitudinal study, a significant negative relationship was demonstrated between baseline bilirubin levels and incident MetS, thereby suggesting that mildly elevated bilirubin may play a protective role against incident MetS. Furthermore, serum bilirubin was negatively associated with the risk of incident hypertriglyceridaemia.
Several recent studies have also reported a negative association between bilirubin and MetS [5], [6], [7]. However, most of these studies were
Disclosure of interest
The authors declare that they have no conflicts of interest concerning this article.
Funding: No funding source is reported.
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These authors contributed equally to this work.