Abstract
Objectives
This study aims to prospectively explore the association between sedentary time and the risk of all-cause mortality in adults based on a cohort from rural areas of China.
Methods
The study population included 20,194 adults at baseline (2007–2008) who participated in the Rural Chinese Cohort Study. Cox’s proportional hazard regression model was used to analyze the hazard ratios (HRs) and 95% confidence intervals (CIs) of sedentary time and all-cause mortality, and a restricted cubic spline was used to model the dose-response relation. We also carried out a series of sensitivity analyses to verify the robustness of our main results.
Results
The median follow-up duration was 6 years, with a total of 17,265 participants (response rate 85.5%) followed up, and 1,106 deaths observed. Data for 17,048 participants were analyzed, with the mean age of participants being 52.00. Compared with sedentary time <4 h/day group, the risk of all-cause mortality was significantly increased in the 8–11 h/day (HR=1.27, 95%CI:1.03–1.56) and ≥11 h/ day groups (HR=1.48, 95%CI:1.20–1.84). With increases in sedentary time, the risk of all-cause mortality increased gradually (Ptrend <0.001). For each 1 h/day increase in sedentary time, the risk of all-cause mortality increased by 3% (HR=1.03, 95%CI: 1.01–1.05). Sensitivity analyses showed our main results were consistent.
Conclusions
Prolonged sedentary time increases the risk of all-cause mortality in the adult rural Chinese population. Reducing sedentary time may have important implications for reducing mortality risk.
Key words: Sedentary time, all-cause mortality, dose-response relation, cohort study
Introduction
Sedentary behaviors are conventionally defined as any activities such as sitting or lying down, involving energy expenditure of 1.0–1.5 metabolic equivalents (METs). In 2012, the Lancet Physical Activity Series Working Group published an article pointing out that 41.5% (41.3–41.7) of adults in the world were sedentary for more than 4 hours each day, 55.2% (54.2–56.1) in the United States, 64.1% (63.5–64.7) in Europe, and 23.8% (23.1–24.5) in Southeast Asia (1). Bauman et al. investigated 49,493 adults aged 18–65 years from 20 countries, revealing that the average sedentary time was approximately 346.2 minutes/day (2). Studies using accelerometers to objectively measure sedentary time have shown that the average sedentary time for American adults was about 7.7 hours per day (3) and for Chinese adults about 8.5 hours per day (4). Sedentary behavior is highly prevalent in modern society with technological advances (5), and is increasingly being considered a potentially important risk factor for health outcomes in adults.
Epidemiological studies suggest that less sedentary time may protect against premature death, independently of moderate to vigorous physical activity (MVPA). There is also increasing evidence that prolonged sedentary time affects health. Some studies (6, 7, 8, 9, 10) have reported that prolonged sedentary time is associated with increased risk of hypertension, diabetes, obesity, cardiovascular diseases, cancer, and mortality. A systematic review and meta-analysis8 including 34 studies with 1,331,468 participants indicated that cohort studies on the association between sedentary time and all-cause mortality were mainly conducted in developed countries such as the United States, Britain, and Canada, with studies in the Asian region mainly focused on Japan. Currently, there are limited prospective cohort studies investigating the sedentary time-mortality association among rural adults in China.
The present study therefore aims to prospectively explore the association between sedentary time and all-cause mortality and to test a dose-response relationship in adults based on a large rural Chinese cohort study. We performed a series of sensitivity analyses to test the robustness of our main results.
Methods
The Rural Chinese Cohort Study enrolled 20,194 adults (≥18 years old) living in the rural area of Luoyang city in the middle of China during July and August 2007 and July and August 2008. A total of 17,265 individuals (response rate 85.5%) completed the follow-up examination during July and August 2013 and July and October 2014. Details of the study design, participants, methods, and measurements have been previously described (11). For this study, we excluded participants with missing data on sedentary time (n=181) at baseline. Ultimately, 17,084 participants were included in the final analysis. The study was approved by the Ethics Committee of Shenzhen University Health Science Center, with written informed consent obtained from all participants.
All participants responded to the same questionnaires and underwent anthropometric and laboratory measurement at baseline and in follow-up examinations. Information regarding demographic characteristics, behavioral measures, and personal and family history of disease of all participants was obtained through face-to-face interviews. Smoking was defined as currently smoking and/or smoking at least 100 cigarettes during the lifetime (11). Alcohol drinking was defined as having consumed alcohol (12) or more times in the last year (11). Physical activity levels were classified as low, moderate, or high using the scoring protocol of the International Physical Activity Questionnaire (12). Height and weight were measured twice using standard clinical procedures, with the average used for analysis. Body mass index (BMI) was calculated as the ratio of mean weight (kg) to the square of mean height (m2). Blood pressure was measured three times on the right arm using an electronic sphygmomanometer (HEM-770AFuzzy, Omron, Japan), with the average used for analysis. Overnight fasting blood samples were obtained from each participant for assessing fasting plasma glucose (FPG), total cholesterol (TC), triglycerides (TG), and high-density lipoprotein cholesterol (HDL-C) levels using an HITACHI automatic clinical analyzer (Model 7060, Tokyo), according to standard clinical procedures.
Daily total sedentary time was measured using the International Physical Activity Questionnaire which has been shown to have acceptable reliability and validity (12). Sedentary time was assessed with the question: “During the last 7 days, how much time did you usually spend sitting during work and leisure time on a) a weekday? and on b) a weekend day?” Average total sedentary time (h/day) was calculated as the sum of weekday sitting minutes*5 and weekend day sitting minutes*2, then divided by 7. Sedentary time was divided into 4 categories, <4, 4–8, 8–11 and ≥11 h/day, in line with previous studies (13, 14, 15), with <4 h/day as the reference category.
Data about mortality was collected by trained staff using a specially-designed questionnaire in face-to-face interviews with relatives, local village physicians, or other healthcare providers. We later verified the corresponding information for mortality with the local Center for Disease Control and Prevention. All-cause mortality was defined according to codes A00-Z99 of the International Classification of Diseases, 10th Revision.
Baseline characteristics of study participants are summarized in the categories of sedentary time. Non-normally distributed continuous variables are presented as median (interquartile range), and categorical variables as number (%). Kruskal-Wallis H and chi-square tests were conducted to evaluate differences in continuous skewed data and categorical data, respectively. Differences in survival probabilities of all-cause mortality across categories of sedentary time were examined using Kaplan-Meier survival curves. Cox’s proportional-hazard regression model was used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for all-cause mortality risk across different sedentary time categories with three models: model 1, unadjusted; model 2, adjusted for age, sex, marital status, educational level, income, smoking, alcohol drinking, and physical activity at baseline; and model 3, further adjusted for BMI, systolic blood pressure (SBP), FPG, TC, TG, and HDL-C at baseline. To assess a dose-response relationship between sedentary time and all-cause mortality, Cox’s proportional-hazard model with restricted cubic splines in which sedentary time was treated as a continuous scale was fitted, with reference set at 4 h/day. Further, to evaluate the stability of the main study results, we conducted a series of sensitivity analyses, excluding: (1) deaths within 1 year of follow-up; (2) participants with cancer at baseline; (3) participants with cardiovascular diseases (myocardial infarction, stroke, or heart failure) at baseline; and (4) deaths within 1 year of follow-up and participants with cancer and cardiovascular diseases at baseline. Finally, we performed subgroup analyses for gender and age.
All the analyses were conducted using SAS 9.4 (SAS Inst., Cary, NC, USA) and Stata V.12 (Stata Corp, College Station, TX, USA), with two-sided P<0.05 considered statistically significant.
Results
Data for 17,048 participants (6,741 men and 10,343 women) were analyzed, the mean (interquartile range, IQR) age of participants being 52.00 (42.00–61.00). During the 6.01 (5.08–6.53) years of follow-up, a total of 1,106 deaths were observed. The baseline characteristics of the study population stratified by sedentary time groups are presented in Table 1. Compared with the sedentary time <4 h/day group, those who reported high levels of sedentary time were more likely to be older, women, less physically active, and have higher BMI, SBP, diastolic blood pressure, FPG, TC, and TG (all P <0.001).
Table 1.
Baseline characteristics of study participants by sedentary time
| Baseline characteristics | Sedentary time (h/day) | P values | ||||
|---|---|---|---|---|---|---|
| All (n=17084) | <4 (n=4426) | 4-8 (n=7366) | 8-11 (n=3362) | ≥11 (n=1930) | ||
| Age (years) | 52 (42–61) | 47.00 (39.00–56.00) | 50.00 (41.00–59.00) | 56.00 (45.00–65.00) | 61.00 (51.00–70.00) | <0.001 |
| Sex (%) | <0.001 | |||||
| Men | 6741 (39.46) | 1998 (45.14) | 2813 (38.19) | 1180 (35.10) | 750 (38.86) | |
| Women | 10343 (60.54) | 2428 (54.86) | 4553 (61.81) | 2182 (64.90) | 1180 (61.14) | |
| Smoking (%) | 4618 (27.03) | 1444 (32.63) | 1939 (26.32) | 777 (23.11) | 458 (23.73) | <0.001 |
| Alcohol drinking (%) | 1915 (11.21) | 661 (14.93) | 828 (11.24) | 287 (8.54) | 139 (7.20) | <0.001 |
| Physical activity (%) | <0.001 | |||||
| Low | 5650 (33.07) | 754 (17.04) | 2034 (27.61) | 1496 (44.50) | 1366 (70.78) | |
| Moderate | 3580 (20.96) | 1119 (25.28) | 1672 (22.70) | 605 (18.00) | 184 (9.53) | |
| High | 7854 (45.97) | 2553 (57.68) | 3660 (49.69) | 1261 (37.51) | 380 (19.69) | |
| BMI (kg/m2) | 24.03 (21.63–26.60) | 23.75 (21.54–26.18) | 24.06 (21.70–26.65) | 24.17 (21.69–26.74) | 24.23 (21.55–26.96) | <0.001 |
| SBP (mmHg) | 123.00 (111.67–137.67) | 120.33 (110.33–133.33) | 122.00 (111.00–136.00) | 126.00 (113.00–141.67) | 130.00 (115.67–147.00) | <0.001 |
| DBP (mmHg) | 77.67 (70.67–85.67) | 76.67 (70.33–84.67) | 77.33 (70.67–85.33) | 78.33 (71.00–87.00) | 78.67 (71.33–87.33) | <0.001 |
| FPG (mmol/L) | 5.36 (5.00–5.81) | 5.31 (4.96–5.72) | 5.34 (5.00–5.78) | 5.40 (5.04–5.88) | 5.46 (5.06–5.96) | <0.001 |
| TC (mmol/L) | 4.41 (3.84–5.05) | 4.32 (3.76–4.96) | 4.37 (3.81–5.00) | 4.49 (3.93–5.13) | 4.59 (4.00–5.22) | <0.001 |
| TG (mmol/L) | 1.37 (0.96–1.98) | 1.32 (0.93–1.94) | 1.37 (0.97–1.99) | 1.42 (1.00–2.04) | 1.39 (1.01–1.99) | <0.001 |
| HDL-C (mmol/L) | 1.14 (0.98–1.32) | 1.14 (0.98–1.32) | 1.14 (0.99–1.32) | 1.14 (0.99–1.32) | 1.14 (0.98–1.32) | 0.978 |
| Death (%) | 1106 (6.47) | 153 (3.46) | 360 (4.89) | 299 (8.89) | 294 (15.23) | <0.001 |
Data are shown as median (IQR) or number (%); BMI, body mass index; SBP, systolic blood pressure; DBP, diastolic blood pressure; FPG, fasting plasma glucose; TC, total cholesterol; TG, triglycerides; HDL-C, high-density lipoprotein cholesterol.
Death rates (per 1000 person-years) increased with increasing sedentary time. Sedentary time of <4, 4–8, 8–11, and ≥11 h/day yielded death rates of 5.78, 8.22, 15.37 and 27.12, respectively (Table 2). A log-rank test for Kaplan-Meier survival analysis showed that the cumulative all-cause mortality across the four sedentary time categories was different (logrank χ2=398.080, P<0.001) (Figure 1). The risk of all-cause mortality was lowest in the sedentary time <4 h/day group, and highest in the sedentary time ≥11 h/day group.
Table 2.
Cox’s proportional hazards model of association between sedentary time and all-cause mortality
| Characteristics | Sedentary time (h/day) | Ptrend | Per h/day increase | |||
|---|---|---|---|---|---|---|
| <4 (n=4426) | 4–8 (n=7366) | 8–11 (n=3362) | ≥11 (n=1930) | |||
| Deaths (n) | 153 | 360 | 299 | 294 | ||
| Person-years | 26476.81 | 43769.36 | 19454.06 | 10841.23 | ||
| Death rate (per 1000 person-years) | 5.78 | 8.22 | 15.37 | 27.12 | ||
| Model 1 HR (95% CI) | 1.00 | 1.43 (1.18–1.72) | 2.70 (2.22–3.28) | 4.79 (3.94–5.82) | <0.001 | 1.16 (1.14–1.17) |
| Model 2 HR (95% CI) | 1.00 | 1.13 (0.94–1.37) | 1.31 (1.07–1.60) | 1.50 (1.21–1.86) | <0.001 | 1.03 (1.01–1.05) |
| Model 3 HR (95% CI) | 1.00 | 1.12 (0.91–1.34) | 1.27 (1.03–1.56) | 1.48 (1.20–1.84) | <0.001 | 1.03 (1.01–1.05) |
HR, hazard ratio; CI, confidence interval; Model 1: Unadjusted; Model 2: Adjusted for age, sex, marital status, educational level, income, smoking, alcohol drinking, and physical activity at baseline; Model 3: Adjusted for variables in model 2 plus body mass index, systolic blood pressure, fasting plasma glucose, total cholesterol, triglycerides, and high-density lipoprotein cholesterol at baseline.
Figure 1.
Kaplan-Meier survival curves for different sedentary times and all-cause mortality
Cox’s proportional hazards regression models revealed a significantly increased risk of all-cause mortality for adults with high levels of sedentary time after adjusting for potential confounders (Table 2). Compared with the sedentary time of <4 h/day group, the adjusted HRs (95% CIs) in Model 3 were 1.10 (0.91–1.34), 1.27 (1.03–1.56) and 1.48 (1.20–1.84) for the sedentary times of 4–8, 8–11 and ≥11 h/day groups, respectively. The risk of all-cause mortality increased with increases in sedentary time (Ptrend <0.001). For each 1 h/day increase in sedentary time, the risk of all-cause mortality increased by 3% (HR=1.03 , 95%CI: 1.01–1.05) after adjusting for potential confounders (Model 3).
The results of Cox’s proportional-hazard models with restricted cubic splines showed there was a significant nonlinear dose-response relation between sedentary time and all-cause mortality after adjusting for age, sex, marital status, educational level, income, smoking, alcohol drinking, physical activity, BMI, SBP, FPG, TC, TG, and HDL-C at baseline (Pnonlinear <0.05) (Figure 2). When sedentary time reached >6 h/day, the risk of all-cause mortality gradually increased as sedentary time increased.
Figure 2.
Dose-response association between sedentary time and risk of all-cause mortality. Adjusted for age, sex, marital status, educational level, income, smoking, alcohol drinking, physical activity, body mass index, systolic blood pressure, fasting plasma glucose, total cholesterol, triglycerides, and high-density lipoprotein cholesterol at baseline
Sensitivity analyses excluding: (1) deaths within 1 year of follow-up; (2) participants with cancer at baseline; (3) participants with cardiovascular diseases (myocardial infarction, stroke, heart failure) at baseline; and (4) deaths within 1 year of follow-up and participants with cancer and cardiovascular diseases at baseline, yielded results consistent with the main findings; that is, compared with the sedentary time of <4 h/day group, the risk of all-cause mortality increased among the sedentary time 8–11 and ≥11 h/day groups (Table 3). The nonlinear dose-response association between sedentary time and all-cause mortality remained stable (Pnonlinear <0.05). When sedentary time reached >6 h/day, the risk of all-cause mortality gradually increased as sedentary time increased (Figure 3). In the subgroup analyses, associations between categories of sedentary time with all-cause mortality were consistent across age, but inconsistent between genders. (Table 4) The results showed that the association between sedentary time and all-cause mortality was significant in women, but not in men.
Table 3.
Sensitivity analysis of sedentary time and all-cause mortality risk
| Characteristics | Sedentary time (hours/day) | Ptrend | Per h/day increase | |||
|---|---|---|---|---|---|---|
| <4 | 4–8 | 8–11 | ≥11 | |||
| Excluding deaths within 1 year of follow-up (n=81) | ||||||
| Deaths (n) | 141 | 330 | 281 | 273 | ||
| Person-years | 26471.30 | 43752.20 | 19443.90 | 10830.19 | ||
| Death rate (per 1000 person-years) | 5.33 | 7.54 | 14.45 | 25.21 | ||
| Model 1 HR (95%CI) | 1.00 | 1.42 (1.17–1.73) | 2.76 (2.26–3.38) | 4.85 (3.96–5.95) | <0.001 | 1.16 (1.14–1.18) |
| Model 2 HR (95%CI) | 1.00 | 1.12 (0.92–1.36) | 1.31 (1.06–1.62) | 1.49 (1.19–1.86) | <0.001 | 1.03 (1.02–1.05) |
| Model 3 HR (95%CI) | 1.00 | 1.10 (0.90–1.34) | 1.29 (1.04–1.59) | 1.48 (1.19–1.85) | <0.001 | 1.03 (1.02–1.05) |
| Excluding participants with cancer at baseline (n=30) | ||||||
| Deaths (n) | 151 | 358 | 297 | 291 | ||
| Person-years | 26450.34 | 43716.59 | 19396.98 | 10828.43 | ||
| Death rate (per 1000 person-years) | 5.71 | 8.19 | 15.31 | 26.87 | ||
| Model 1 HR (95%CI) | 1.00 | 1.44 (1.19–1.74) | 2.72 (2.24–3.31) | 4.80 (3.95–5.85) | <0.001 | 1.16 (1.14–1.17) |
| Model 2 HR (95%CI) | 1.00 | 1.14 (0.94–1.38) | 1.31 (1.07–1.61) | 1.50 (1.21–1.86) | <0.001 | 1.03 (1.01–1.05) |
| Model 3 HR (95%CI) | 1.00 | 1.12 (0.93–1.36) | 1.29 (1.05–1.58) | 1.49 (1.20–1.84) | <0.001 | 1.03 (1.01–1.05) |
| Excluding participants with cardiovascular diseases at baseline (n=907) | ||||||
| Deaths (n) | 137 | 322 | 253 | 226 | ||
| Person-years | 25687.54 | 42138.84 | 18055.73 | 9683.94 | ||
| Death rate (per 1000 person-years) | 5.33 | 7.64 | 14.01 | 23.34 | ||
| Model 1 HR (95%CI) | 1.00 | 1.44 (1.18–1.75) | 2.67 (2.17–3.28) | 4.45 (3.60–5.51) | <0.001 | 1.15 (1.13–1.17) |
| Model 2 HR (95%CI) | 1.00 | 1.16 (0.95–1.42) | 1.33 (1.07–1.66) | 1.44 (1.14–1.82) | <0.001 | 1.03 (1.01–1.05) |
| Model 3 HR (95%CI) | 1.00 | 1.14 (0.93–1.40) | 1.31 (1.05–1.63) | 1.44 (1.14–1.81) | <0.001 | 1.03 (1.01–1.05) |
| Excluding deaths within 1 year of follow-up and participants with cancer and cardiovascular diseases at baseline (n=1004) | ||||||
| Deaths (n) | 124 | 293 | 236 | 208 | ||
| Person-years | 25663.73 | 42070.45 | 17990.77 | 9665.96 | ||
| Death rate (per 1000 person-years) | 4.83 | 6.97 | 13.12 | 21.52 | ||
| Model 1 HR (95%CI) | 1.00 | 1.45 (1.17–1.78) | 2.77 (2.23–3.44) | 4.56 (3.65–5.69) | <0.001 | 1.15 (1.13–1.17) |
| Model 2 HR (95%CI) | 1.00 | 1.15 (0.93–1.42) | 1.35 (1.08–1.70) | 1.43 (1.12–1.82) | 0.002 | 1.03 (1.01–1.05) |
| Model 3 HR (95%CI) | 1.00 | 1.13 (0.92–1.40) | 1.32 (1.05–1.66) | 1.42 (1.12–1.82) | 0.002 | 1.03 (1.01–1.05) |
HR, hazard ratio; CI, confidence interval; Model 1: Unadjusted; Model 2: Adjusted for age, sex, marital status, educational level, income, smoking, alcohol drinking, and physical activity at baseline; Model 3: Adjusted for variables in model 2 plus body mass index, systolic blood pressure, fasting plasma glucose, total cholesterol, triglycerides, and high-density lipoprotein cholesterol.
Figure 3.
Sensitivity analysis of the dose-response association between sedentary time and risk of all-cause mortality
Table 4.
Association between sedentary time and all-cause mortality (HRs and 95% CI) by different subgroups
| Subgroups | Sedentary time (h/day) | Ptrend | Per h/day increase | |||
|---|---|---|---|---|---|---|
| <4 | 4–8 | 8–11 | ≥11 | |||
| Gender | ||||||
| Men | ||||||
| Deaths (n) | 98 | 209 | 160 | 151 | ||
| Person-years | 12012.47 | 16630.9 | 6732.2 | 4191.34 | ||
| Death rate (per 1000 person-years) | 8.16 | 12.57 | 23.77 | 36.03 | ||
| Model 1 HR (95% CI) | 1.00 | 1.55 (1.22–1.97) | 2.97 (2.31–3.81) | 4.48 (3.48–5.78) | <0.001 | 1.14 (1.12–1.17) |
| Model 2 HR (95% CI) | 1.00 | 1.14 (0.90–1.45) | 1.28 (0.99–1.67) | 1.32 (1.00–1.75) | <0.035 | 1.02 (1.00–1.04) |
| Model 3 HR (95% CI) | 1.00 | 1.12 (0.88–1.43) | 1.26 (0.97–1.64) | 1.30 (0.99–1.72) | <0.043 | 1.02 (1.00–1.04) |
| Women | ||||||
| Deaths (n) | 55 | 151 | 139 | 143 | ||
| Person-years | 14464.34 | 27138.46 | 12721.86 | 6649.89 | ||
| Death rate (per 1000 person-years) | 3.80 | 5.56 | 10.93 | 21.50 | ||
| Model 1 HR (95% CI) | 1.00 | 1.46 (1.07–1.99) | 2.90 (2.12–3.96) | 5.79 (4.25–7.91) | <0.001 | 1.18 (1.15–1.21) |
| Model 2 HR (95% CI) | 1.00 | 1.13 (0.83–1.55) | 1.36 (0.98–1.88) | 1.79 (1.27–2.50) | <0.001 | 1.05 (1.02–1.08) |
| Model 3 HR (95% CI) | 1.00 | 1.11 (0.81–1.52) | 1.33 (0.96–1.85) | 1.76 (1.26–2.47) | <0.001 | 1.05 (1.02–1.08) |
| Age | ||||||
| <60 years old | ||||||
| Deaths (n) | 75 | 135 | 81 | 50 | ||
| Person-years | 22464.89 | 34316.02 | 12020.01 | 5027.34 | ||
| Death rate (per 1000 person-years) | 3.34 | 3.93 | 6.74 | 9.95 | ||
| Model 1 HR (95% CI) | 1.00 | 1.18 (0.89–1.57) | 2.05 (1.50–2.81) | 3.02 (2.11–4.32) | <0.001 | 1.11 (1.08–1.15) |
| Model 2 HR (95% CI) | 1.00 | 1.09 (0.82–1.45) | 1.57 (1.14–2.17) | 1.90 (1.31–2.77) | <0.001 | 1.06 (1.03–1.10) |
| Model 3 HR (95% CI) | 1.00 | 1.07 (0.80–1.42) | 1.53 (1.11–2.11) | 1.81 (1.24–2.64) | <0.001 | 1.06 (1.02–1.09) |
| ≥60 years old | ||||||
| Deaths (n) | 78 | 225 | 218 | 244 | ||
| Person-years | 4011.92 | 9453.34 | 7434.05 | 5813.89 | ||
| Death rate (per 1000 person-years) | 19.44 | 23.80 | 29.33 | 41.97 | ||
| Model 1 HR (95% CI) | 1.00 | 1.22 (0.95–1.58) | 1.49 (1.15–1.93) | 2.15 (1.66–2.77) | <0.001 | 1.07 (1.05–1.09) |
| Model 2 HR (95% CI) | 1.00 | 1.13 (0.87–1.46) | 1.17 (0.90–1.53) | 1.38 (1.05–1.81) | 0.012 | 1.02 (1.00–1.04) |
| Model 3 HR (95% CI) | 1.00 | 1.12 (0.86–1.45) | 1.16 (0.89–1.51) | 1.38 (1.05–1.81) | 0.010 | 1.02 (1.00–1.04) |
HR, hazard ratio; CI, confidence interval; Model 1: Unadjusted; Model 2: Adjusted for age, sex, marital status, educational level, income, smoking, alcohol drinking, and physical activity at baseline; Model 3: adjusted for variables in model 2 plus body mass index, systolic blood pressure, fasting plasma glucose, total cholesterol, triglycerides, and high-density lipoprotein cholesterol.
Adjusted for age, sex, marital status, educational level, income, smoking, alcohol drinking, physical activity, body mass index, systolic blood pressure, fasting plasma glucose, total cholesterol, triglycerides, and high-density lipoprotein cholesterol at baseline.
Discussion
Our analysis of 17,048 participants from a 6-year prospective cohort study revealed that prolonged sedentary time was associated with an increased risk of all-cause mortality in rural Chinese adults, and that there was a significant nonlinear dose-response relation between sedentary time and all-cause mortality. The series of sensitivity analyses conducted after excluding participants who died in the first year of follow-up and those who presented with cancer and CVD at baseline showed that the main results of this study were consistent.
Our results are similar to those of most previous epidemiological studies. A cohort study based on a large sample of American adults followed up for 8.5 years (n=240,819) found that long-term sedentary behavior (≥ 9 h/ day) can increase the risk of all-cause mortality by 19%16. A systematic review and meta-analysis of 47 studies involving the relationship between sedentary time and mortality risk in adults showed that long-term sedentary behavior increased the risk of all-cause mortality (HR:1.24, 95% CI:1.09–1.41), and was independent of physical activity level10. Another recently published meta-analysis including 1,331,468 subjects in 34 prospective cohort studies also showed that sedentary behavior was associated with the risk of all-cause mortality, and that there was a nonlinear dose-response relationship8. Subgroup analysis by sex showed that the association between sedentary time and all-cause mortality was significant in women, but not in men. A large multi-ethnic cohort of 134,596 adults (61,395 men and 73,201 women) from five racial/ethnic groups showed that long-term sedentary behavior (≥ 10 h/day) significantly increased the risk of all-cause mortality in women (HR=1.11, 95% CI: 1.04–1.19) after adjusting for potential confounding factors, but not in men (HR=1.04, 95% CI: 0.98–1.11) (17). The reason for the gender difference in the association of sedentary time and all-cause mortality may be explained by different dietary behavior, the MET level of sedentary activities, or the frequency of sedentary rest (17). The reason may also be that prolonged sedentary time among the female population is more common than among the male population in this study. In the future, more studies are needed to analyze the relationship between sedentary time and all-cause mortality in different gender groups to verify the results of this study. Our study not only further validated the above research results, but also provided epidemiological evidence for the study field of sedentary time and all-cause mortality in rural Chinese adults.
Various mechanisms may explain the relationship between sedentary time and mortality. First, sitting for a long time will weaken the capacity of muscles to contract, and then affect the metabolism and blood vessel health of the body (18). Studies have found that prolonged sedentary time reduces contraction activity of skeletal muscles, thus reducing the activity of lipoprotein lipase in muscles and damaging the metabolic function (19, 20). Lipase is a key enzyme in the regulation of lipid metabolism. Low lipoprotein lipase activity is associated with increased glucose and lipid levels which increase levels of triglycerides while decreasing levels of high-density lipoprotein and insulin sensitivity, all risk factors for mortality (21, 22, 23). Second, prolonged sedentary time can reduce energy expenditure in the body, leading to weight gain, high blood pressure, and abnormal blood sugar levels, all of which can affect the occurrence of disease and mortality (24). Intervention trials have shown that breaking prolonged sedentary time by standing or walking can indeed reduce the concentration of postprandial glucose, insulin, and lipids (25, 26, 27).
Studying the association of sedentary time and all-cause mortality reveals important implications for public health. With the development of a social economy and the changes in people’s lifestyles, prolonged sedentary time has become a major public health problem. At present, the United States and the United Kingdom are offering suggestions to their citizens for reducing long-term sedentary behavior28, but there is at present no Chinese guideline in relation to sedentary time for the population, and there is still no common standard or recommendation on sedentary time around the world. Due to the adverse health outcomes and serious disease burden resulting from prolonged sedentary time observed in this study and other studies (6, 8, 29), future public health strategies should not only advocate appropriate physical activity but also emphasize that reducing or avoiding prolonged sedentary time may delay or reduce the occurrence of chronic diseases and mortality.
Our study is the first investigation to clarify an association between sedentary time and all-cause mortality in a relatively large rural population of adults with a 6-year follow-up. This study not only used Cox’s proportional hazards regression models to explore the association between different sedentary time groups and all-cause mortality, but also fitted a dose-response relation by restricted cubic spline models, more intuitively and accurately reflecting the sedentary time-mortality association. Further, we performed several sensitivity analyses to verify the stability of the main findings.
This study nevertheless has some limitations. First, information on the total daily time spent watching TV, eating, reading, and reclining during the workday and on days off was obtained instead of separate information being collected on different types of sedentary behaviors. This made it impossible to analyze the mortality risks associated with different types of sedentary behavior. Second, information on sedentary behavior was obtained via questionnaire rather than via objective measurement, which may have introduced measurement bias and led to underestimation of associations between sedentary time and mortality risk. In the future, more studies based on objective measurement of sedentary time by accelerometer and other methods are needed to confirm the results of this study. Finally, the participants of this study were all from rural areas of China, limiting the generalizability of the results of this study to other ethnic groups or urban populations.
Conclusion
In summary, prolonged sedentary time was positively associated with increased risk of mortality in the adult rural Chinese population, and there was a significant nonlinear dose-response association. Further evidence, acquired through using accelerometers to objectively measure sedentary time and conducting intervention trials, is needed to clarify the biological mechanisms underlying this association and to develop effective intervention strategies to reduce risk of mortality.
There are at least three practical implications. First, with the development of technology, people need more physical activity to reduce sedentary time and to reduce the risk of all-cause mortality. Second, rural women in China should take more physical exercise. Third, more effective forms of physical activity (energy expenditure≥1.5 METS) need to be studied and recommended in order to reduce sedentary time in the whole population.
Funding
This study was funded by the National Natural Science Foundation of China (grant nos. 81402752 and 81673260) and the Natural Science Foundation of Guangdong Province (grant no. 2019A1515011183).
Ethical approval
This study conformed to the ethical guidelines of the 2008 Declaration of Helsinki and was approved by the Institutional Review Board of the Shenzhen University Health Science Center. All participants provided written informed consent prior to being administered the survey.
Conflict of Interest
We declare that we have no financial or personal relationship with other people or organizations that can inappropriately influence our work. In case animals were involved, all applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
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