Subjects and study design

The Nishimura Health Survey is an ongoing cohort investigation of risk factors for chronic diseases including hypertension, metabolic syndrome, diabetes mellitus, and chronic kidney disease. The Nishimura Clinic (Kyoto, Japan) provides regular health check-up for employees of various companies. In Japan, annual routine health examination of employees is legally mandated, and the employers usually pay all or most of the health-check costs. We performed a retrospective cohort study to assess the relationship between total bilirubin and GGT levels at baseline and incident metabolic syndrome during a follow-up of 2.8 ± 1.2 years. Among 20,852 subjects who underwent health examinations from April 1, 2013, to March 31, 2018, a total of 12,334 subjects underwent two or more health examinations. We excluded 34 subjects with data not examined on at least one variable. From the remaining 12,300 subjects, we excluded 2046 subjects with alcohol intake of > 20 g/day and 1262 subjects with metabolic syndrome at baseline. Finally, 8992 subjects were selected as eligible for the study (Fig. 1). The subjects were divided into four study groups according to the median values of total bilirubin and GGT: (i) both total bilirubin and GGT low (total bilirubin and GGT less than the median value, both-low group), (ii) total bilirubin low and GGT high (total bilirubin less than the median value and GGT equal to or higher than the median value, GGT-high group), (iii) total bilirubin high and GGT low (total bilirubin equal to or higher than the median value and GGT less than the median value, total bilirubin-high group), and (iv) both total bilirubin and GGT high (total bilirubin and GGT equal to or higher than the median value, both-high group).

Flowchart of the inclusion and exclusion of subjects

In addition, all the subjects were divided into another four study groups according to the reference ranges of total bilirubin and GGT: (i) both total bilirubin and GGT levels within the reference range (total bilirubin and GGT levels equal to or lower than the upper reference range, both-reference group), (ii) total bilirubin level within the reference range and GGT level higher than the reference range (total bilirubin level equal to or lower than the upper reference range and GGT level higher than the upper reference range, GGT-higher than reference group), (iii) total bilirubin level above the reference range and GGT level within the reference range (total bilirubin level higher than the upper reference range and GGT level equal to or lower than the upper reference range, total bilirubin-higher than reference group), and (iv) both total bilirubin and GGT levels above the reference range (total bilirubin and GGT levels higher than the upper reference range, both-higher than reference group.

All procedures of the present study were approved by the local research ethics committee and were conducted in accordance with the Declaration of Helsinki. Informed consent was obtained from all subjects.