Participants
This cross-sectional study used the data from 200 participants who were recruited for an interventional study via local newspaper advertisements between 2014 and 2015. In total, there were 180 eligible participants, after excluding those without objectively evaluated sedentary behavior and physical activity or those who had insufficient accelerometer data. After excluding participants who had missing values of required variables such as blood samples (n = 6), the final analyses were conducted 174 middle-aged and older Japanese adults (Fig. 1). This study was approved by the Ethics Committee in University of Tsukuba (Tai 019-19) and conformed to the principles outlined in the Declaration of Helsinki, and all participants provided written informed consent.
Sedentary behavior and physical activity
The time spent in sedentary behavior, light-intensity physical activity (LPA), and MVPA were assessed using a uniaxial accelerometer (Lifecorder, Suzuken Co., Ltd., Nagoya, Japan) that samples vertical acceleration signals in the range from 0.06 to 1.94 G at 32 Hz. The accuracy and detailed algorithm of this accelerometer has been described elsewhere [14]. The epoch length was 1 minute, the time used by the accelerometer for measuring activity. Participants were instructed to wear constantly the accelerometer on the level of participant’s waist during waking and sleeping hour for 7 consecutive days, except while bathing and swimming. A day with at least 10 hours of wear time was considered valid. This accelerometer records the scores of physical activity intensity consisted of a scale from 0 – 9 (level 0: rest; level 0.5: micro activity; level 1 – 9: movement) according to the acceleration signal patterns [15]. In the present study, these scores were reclassified into four activity levels based on the previous investigation [16] as follows: sedentary or sleep (≤ 1.5 Mets: level 0 – 0.5), light (1.6 – 2.9 Mets: level 1 – 3), moderate (3.0 – 6.0 Mets: level 4 – 6), and vigorous (> 6.0 Mets: level 7 – 9), and were reported as the time spent in each activity level. The time spent in sedentary behavior was calculated as the time of sedentary or sleep (< 1.5 Mets: level 0 – 0.5) minus the sleep time assessed by the validated questionnaire (The Japanese version of the Pittsburgh Sleep Quality Index). Also, the moderate and vigorous physical activity time were combined to form the time spent in MVPA.
Renal function
Estimated glomerular filtration rate (eGFR) was calculated by the Japanese eGFR equations based on standardized serum creatinine or cystatin C as follows: eGFRcr (mL/min/1.73 m2) = 194 × serum creatinine−1.094 × Age−0.287 × 0.739 (if female), eGFRcys (mL/min/1.73 m2) = {104 × serum cystatin C−1.019 × 0.996 Age × 0.929 (if female)} – 8 [17, 18]. To improve estimated accuracy, the average values of eGFRcr and eGFRcys were used as the index of renal function.
Covariates
Brachial systolic and diastolic blood pressure and heart rate were simultaneously measured using semi-automatic vascular testing device with electrocardiogram and oscillometric extremities cuffs (form PWV/ABI, Colin Medical technology, Japan). Fasting blood samples were collected in the morning following a more than 12-hour overnight fasting to measure serum or plasma concentrations of high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol, triglyceride, glucose, creatinine, and cystatin C.
Statistical analysis
All statistical analyses were performed using SPSS Statistics 25.0 (IBM Corp., Tokyo, Japan). Data were presented as the means ± SD (for normal distribution), median with interquartile range (for skewed distribution), or frequency counts (for categorical data), as appropriate. Univariate linear associations of sedentary behavior, LPA, and MVPA with eGFR were examined using Spearman’s rank correlation coefficients (rs). Joint associations of sedentary behavior, LPA, and MVPA (four groups stratified according to each median value: higher sedentary behavior/higher physical activity, higher sedentary behavior/lower physical activity, lower sedentary behavior/higher physical activity, lower sedentary behavior/lower physical activity) with eGFR were examined using two-way analysis of co-variance (ANCOVA) to adjust for total waking time and covariates including age, sex, body mass index, systolic blood pressure, heart rate, HDL cholesterol, LDL cholesterol, triglycerides, fasting blood glucose, antihypertensive medicine, lipid-lowering medicine, hypoglycemic medicine, and current smoking. Independent associations of the exposure variables (sedentary behavior, LPA, and MVPA) with the outcome variable (eGFR) were assessed using three multiple linear regression models including a single factor model, partition model, and isotemporal substitution model. For enhanced interpretability of the results, the exposure variables (sedentary behavior, LPA, and MVPA) were scaled to 30 min/day units, respectively [13]. Briefly, the single factor models evaluated separately the associations between each exposure variable and eGFR, with adjustment for total waking time and covariates (Model 1 – 3). The partition model evaluated simultaneously the associations between all exposure variables and eGFR, with adjustment for covariates, but without adjusting for total waking time (Model 4). The isotemporal substitution models estimated the substitution associations between replacing one exposure variable with an equal amount of time in another exposure variable (e.g., replacement of 30 min/day of sedentary behavior with 30 min/day of MVPA) and GFRaverage (Model 5 – 7). This estimation can be accomplished by omitting replacing target exposure variable from the model and entering total waking time and covariates. A more detailed description of these regression models is presented elsewhere [9].