Introduction: This study aimed to examine the trends in adherence to the Physical Activity Guidelines (PAG) for aerobic activity and sedentary time and their effects on mortality and disease progression among US adults with chronic kidney disease (CKD). Methods: We studied individuals from the National Health and Nutrition Examination Survey 2007–08 to 2017–18 with a mortality file in 2015. Multivariate regression models were used to evaluate the association between adherence to PAG and sedentary time with mortality, estimated glomerular filtration rate (eGFR), and urine albumin-to-creatinine ratio. Results: For the CKD population, adherence rate increased from 48.2% in 2007–08 to 55.0% in 2017–18, and sedentary time peaked in 2013–14 (7.5 h/day) and then decreased afterward. There was no difference in the trends across the non-CKD and CKD population. For the CKD population, adherence to the PAG was significantly associated with all-cause mortality (HR, 0.49; 95% CI: 0.38–0.63), malignant neoplasm mortality (HR, 0.30; 95% CI: 0.17–0.52), and albumin-creatinine ratio (OR, −0.27; 95% CI: −0.39 to −0.15). Sedentary time was significantly associated with all-cause mortality (HR, 1.12; 95% CI: 1.08–1.15), heart disease mortality (HR, 1.13; 95% CI: 1.08–1.19), and eGFR (OR, −0.49; 95% CI: −0.72 to −0.26). Conclusions: Favorable trends were observed in adherence to the PAG and sedentary time. Adherence to the PAG and reduction in sedentary time reduced all-cause and cause-specific mortality and prevented disease progression differently. Efforts are needed to decrease sedentary time rather than adhering to the PAG for aerobic activity alone.

© 2022 The Author(s). Published by S. Karger AG, Basel

Introduction

Physical activity consists of all body movements during daily living, and its beneficial effects on the general population are well documented. Reduced physical activity is an important risk factor for all-cause death, resulting in 9% of premature mortality globally [1]. The USA has the highest economic burdens from physical inactivity worldwide, with an estimated cost of USD 24.7 billion in health care annually [2]. The US Department of Health and Human Services released the second edition of the federal Physical Activity Guidelines (PAG) for Americans in 2018 [3]. Consistent with the first edition [4], the second edition recommends that adults should engage in at least 150 min of moderate-intensity aerobic physical activity or at least 75 min of vigorous-intensity aerobic physical activity a week, or an equivalent combination of both. In addition, the PAG recommends that adults should reduce sedentary time. However, the proportion of adults adhering to the recommended PAG for aerobic activity in the USA has not improved from 2007–08 to 2015–16, while the sedentary time increased significantly during the same period [5].

Chronic kidney disease (CKD) is highly prevalent worldwide and in the USA [6, 7]. Individuals with CKD are associated with poor levels of physical activity and exercise behaviors during the whole course of the disease [8]. Similar to the PAG for Americans, the guidelines from the UK Kidney Research Consortium Clinical Study Group suggested that nondialysis CKD patients should aim to accumulate 150 min of moderate-intensity aerobic activity or 75 min of vigorous-intensity activity per week or a combination of both [9]. Similarly, the Kidney Disease Improving Global Outcomes Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease lifestyle section encouraged individuals with CKD to undertake at least 150 min of physical activity per week [10]. However, these recommendations were extrapolated from the general population data, and specific evidence-based guidance for CKD patients is lacking [11]. Moreover, though the beneficial effects of physical activity and exercise on outcomes in individuals with CKD have been extensively investigated, the conclusions are limited by their consistency [9]. A recent meta-analysis showed that self-management intervention (including exercise) for CKD was not associated with renal outcomes and all-cause mortality [12]. One further multicenter cohort study also proved that a higher level of self-reported physical activity was not associated with a reduction in long-term mortality [13]. Another recent meta-analysis which included 31 trials failed to demonstrate a significant relationship between aerobic exercise with serum creatinine [14]. Therefore, further study is needed to clarify the relationship between physical activity, specifically adherence to the PAG, with outcomes for the CKD population. In this study, we analyzed the data from the National Health and Nutrition Examination Surveys (NHANES) between 2007–08 and 2017–18 to examine the trends in adherence to the PAG for aerobic activity and sedentary time among US adults with CKD and to investigate their associations with mortality and disease progression.

Methods

The NHANES is a large, cross-sectional survey conducted continuously in 2-year cycles in the USA. It focuses on health conditions and behaviors, physical examination findings, and laboratory results. With a complex, multistage probability design that samples individuals in strata defined by geographic location and race/ethnicity, the NHANES seeks to create nationally representative estimates for the nonmilitary, noninstitutionalized US population. During each survey, participants undergo a household interview and then a clinical examination in the mobile examination center [15]. The study protocols were approved by the Ethics Review Board of the National Center for Health Statistics, and written informed consent was obtained from each participant. In the current study, we included data from 2007–08 to 2017–18 as NHANES used a different questionnaire to evaluate physical activity before 2017. We limited our participants to nonpregnant adults aged 18 years or older, and participants with missed information on physical activity or sedentary time were excluded.

Data Collection

Variables previously shown to be associated with CKD and mortality were included as covariates [16-18]. The demographic and health-related information was collected by standardized questionnaires. Race/ethnicity was classified as Mexican American, non-Hispanic white, non-Hispanic black, and others. Education was classified as less than high school, high school graduate, some college, and college graduate or higher. Income was measured as the income-to-poverty ratio, which was defined as annual family income divided by the poverty threshold adjusted for family size and inflation. Current smoking and alcohol use were based on questionnaires about whether participants were currently smoking and drinking or not. Depression was assessed using the 9-item Patient Health Questionnaire, and we defined the major depression as a summed score of 10 potions or greater [19]. Eating behaviors were assessed based on the amount of fruit and vegetables that participants reported consuming in the 24-h period preceding the survey examination. We defined an unhealthy diet as consumption of less than 3 distinct portions of fruit/vegetable [20]. History of cardiovascular disease and cancer was based on self-report, and cardiovascular disease included stroke, congestive heart failure, angina, and myocardial infarction.

During the examination, weight and height were measured, and the body mass index was calculated as weight in kilograms divided by height in meters squared. Blood pressure was measured by trained staff, and mean values were determined as the mean of three or four readings according to the standardized protocol. Blood samples were collected and sent to the central laboratories for the measurement of hemoglobin A1c, total cholesterol, and serum creatinine. Urine albumin and creatinine were measured in the same laboratory during the surveys.

Obesity was defined as body mass index of 30 or greater; hypertension as systolic blood pressure ≥130 mm Hg, diastolic blood pressure ≥80 mm Hg, or the use of antihypertensive medications; diabetes as hemoglobin A1c ≥6.5% or the use of antidiabetic medications; dyslipidemia as total cholesterol ≥240 mg/dL or the use of lipid-lowering medications.

The estimated glomerular filtration rate (eGFR) was calculated based on serum creatinine by using the Chronic Kidney Disease Epidemiology Collaboration equation [21]. Measurements of serum creatinine were calibrated according to the recommendation of NHANES if necessary. CKD was defined as an eGFR of 15–59 mL/min/1.73 m2 or one-time urine albumin-to-creatinine ratio ≥30 mg/g. Participants with an eGFR less than 15 mL/min/1.73 m2, which corresponds to CKD stage 5, were excluded owing to the lack of information on dialysis.

Assessment of Physical Activity

Physical activity was assessed by the Global Physical Activity Questionnaire in the NHANES 2007–08 to 2017–18. It measured 3 domains of physical activity, including leisure-time, work-related, and transportation-related physical activity. The first 2 domains included questions to assess the intensity (moderate vs. vigorous), frequency (per week), and duration (minutes) of the physical activity in a typical week. The last domain included questions to assess the number of days in a typical week and the mean duration per day that they participated in the activity. The NHANES guidelines suggest a metabolic equivalent score of 4.0 for transportation-related physical activity, 4.0 for moderate-intensity activity, and 8.0 for vigorous-intensity activity. Therefore, the transportation-related activity was counted as moderate-intensity activity. As validated, the total amount of physical activity was calculated as minutes of moderate-intensity activity plus twice the minutes of vigorous-intensity activity of all 3 domains [22]. Adherence to the PAG for aerobic activity was defined as participating in at least 150 min per week of moderate-intensity activity. Sedentary time was evaluated as the reported hours spent sitting per day in a typical week.

Ascertain of Mortality

Mortality information was ascertained by linkage to the National Death Index through December 31, 2015. The International Classification of Diseases-10 was used to determine disease-specific mortality. From NHANES 2007–08 to 2013–14, only all-cause mortality, as well as heart disease and malignant neoplasm mortality, was reported [23].

Statistical Analysis

To account for the oversample population, appropriate 12-year sampling weights were constructed according to the NHANES recommendation. We conducted the statistical analysis with R version 4.1.0 using the “Survey” package. Sampling weights, clustering, and stratification were incorporated in all analyses to account for the complex sampling design. All statistical tests were 2 sided, and p < 0.05 was considered statistically significant. The urine albumin-to-creatinine ratio was right skewed and log transformed. Continuous variables were expressed as means (standard error) and categorical variables as percentages (standard error).

The logistic regression model was applied to test the trends in adherence to the PAG for aerobic activity across time, with the survey cycle as an independent variable. The linear regression model was applied to test the trends in sedentary time in a similar manner. We first tested the nonlinearity of the trend by adding a quadratic term of the survey cycle into the model, and then we tested the linearity of the trend if it was insignificant. We further tested the significance of the trend by adding the aforementioned covariates in the regression model. To test whether the trends differ between the CKD and non-CKD populations, a two-way interaction term between survey cycle and CKD status was added to the model. Similar interaction tests were conducted to test whether the trends differed across subgroups by age (18–44 years, 45–64 years, and 65 years or older), gender, race/ethnicity, education, and income.

Cox proportional hazards regression models were used to calculate the multivariate-adjusted hazard ratios (HRs) for the association between adherence to the PAG for aerobic activity and sedentary time with all-cause and cause-specific mortality. The proportional hazards assumption was tested before all the analyses. The multicollinearity was evaluated for the regression model, and a variance inflation factor less than 5 was considered acceptable. We developed 3 different regression models. Model 1 was adjusted for age, gender, race/ethnicity, education, and income. Model 2 was adjusted for obesity, diabetes, hypertension, dyslipidemia, depression, history of cardiovascular disease, and history of cancers on the basis of model 1. Model 3 was adjusted for current smoke, current alcohol use, and unhealthy diet based on the basis of model 2. A two-way interaction term between adherence to the PAG for aerobic activity (or sedentary time) and CKD status was added to the models to test whether the non-CKD and CKD populations had a different change in mortality. The linear regression models were used to calculate the multivariate-adjusted odds ratios (ORs) for the association between adherence to the PAG for aerobic activity and sedentary time with eGFR and urine albumin-to-creatinine ratio. The aforementioned 3 different models were applied in the linear regression analysis.

We performed several sensitivity analyses to test the robustness of the results. First, to account for the difference in the definition of CKD, we redefined the CKD as an eGFR of 15–59 mL/min/1.73 m2 alone without considering the urine albumin-to-creatinine ratio. Second, to account for bias from the short period of follow-up, we excluded participants with follow-up of fewer than 2 years. Second, we performed meta-analyses to calculate the summarized HRs or ORs with 95% confidence intervals (CIs) based on the results from the single survey cycle or combined 2 survey cycles.

Results

The baseline characteristics of participants from 2007–08 to 2017–18 were presented in Table 1. Our final dataset comprised 32,429 participants, and 5,916 of them were identified as having CKD. In general, the CKD population was more likely to be older, received less education, had lower income, less likely to smoke and drink currently, less likely to have an unhealthy diet, and more likely to have obesity, hypertension, diabetes, dyslipidemia, depression, history of CVD, and history of cancer.

Table 1.

Characteristic of the non-CKD and CKD population, NHANES 2007–08 to 2017–18

Trends in Adherence to the PAG for Aerobic Activity and Sedentary Time

The rate of adherence to the PAG for aerobic activity was 48.9% in the CKD population, which was significantly lower than that in the non-CKD population (67.9%). The adherence rate increased from 48.2% in 2007–08 to 55.0% in 2017–2018 in the CKD population (p for linear trend 0.004) and from 67.6% in 2007–08 to 70.8% in 2017–18 in the non-CKD population (p for linear trend 0.049) (Fig. 1). There was no difference in the trends across these two groups (p for interaction 0.064). In addition, the trends were similar across different subgroups by age, gender, race/ethnicity, education, and income among the CKD population. After adjusting for the covariates, the trend in adherence to the PAG remained significant in the CKD population (p for linear trend 0.007).

Fig. 1.

Crude weighted trends in adherence to PAG for aerobic activity among the non-CKD and CKD population, NHANES 2007–08 to 2017–18. Error bars indicate 95% CI. CKD, chronic kidney disease; NHANES, National Health and Nutrition Examination Survey.

The CKD population was reported to have a longer sedentary time than the non-CKD population (6.4 h/day vs. 6.2 h/day, p for difference <0.001). Similar significant nonlinear changes (p for interaction 0.217) in sedentary time were observed in both the CKD and non-CKD populations. In the CKD population, the sedentary time increased from 5.71 h/day in 2007–08 to 7.50 h/day in 2013–14 and then decreased to 6.31 h/day in 2017–18 (p for nonlinear trend <0.001) (Fig. 2). In the non-CKD population, the sedentary time increased from 5.55 h/day in 2007–08 to 7.06 h/day in 2013–14 and then decreased to 5.71 h/day in 2017–18 (p for nonlinear trend <0.001). There was no difference in the trends in sedentary time across the CKD and non-CKD populations (p for interaction 0.217). Moreover, the trends were similar across different aforementioned subgroups among the CKD population. The trend in sedentary time remained significant after adjusting for the covariates (p for nonlinear trend <0.001).

Fig. 2.

Crude weighted trends in sedentary time among the non-CKD and CKD population, NHANES 2007–08 to 2017–18. Error bars indicate 95% CI. CKD, chronic kidney disease; NHANES, National Health and Nutrition Examination Survey.

The Association between Adherence to the PAG for Aerobic Activity with Mortality

The number of participants, overall length of follow-up (person-years), number of events, and mortality rate (number of events per 1,000 person-years) in different groups were presented in Table 2. Figure 3 shows the Kaplan-Meier survival curves of adjusted mortality for different groups. Among the non-CKD population, the HR for the association between adherence to the PAG with all-cause mortality was 0.64 (95% CI: 0.52–0.80), 0.73 (95% CI: 0.58–0.92), and 0.78 (95% CI: 0.62–1.00) in model 1, model 2, and model 3, respectively. Among the CKD population, the HR was 0.43 (95% CI: 0.34–0.54), 0.51 (95% CI: 0.39–0.65), and 0.49 (95% CI: 0.38–0.64) in model 1, model 2, and model 3, respectively. For the heart disease mortality, HR was not significant in model 3 among both the non-CKD and CKD populations. For the malignant neoplasm mortality, the HR was not significant in all the 3 models among the non-CKD population, while it was significant in all the 3 models among the CKD population. The CKD population had a greater reduction in the all-cause mortality (model 3, p for interaction 0.005) and malignant neoplasm mortality (model 3, p for interaction 0.009) than the non-CKD population. In the sensitivity analyses for the CKD population, the results were similar to the overall analysis except for the meta-analysis in which a significant association between adherence to the PAG and heart disease mortality was observed (online suppl. Table 1 in the online suppl. Material; see www.karger.com/doi/10.1159/000526956 for all online suppl. material).

Table 2.

The HR for the association between adherence to the PAG for aerobic activity with all-cause and cause-specific mortality in the non-CKD and CKD population

Fig. 3.

Estimated probability of survival. a1 All-cause mortality in the CKD population; b1 all-cause mortality in the non-CKD population; a2 heart disease mortality in the CKD population; b2 heart disease mortality in the non-CKD population; a3 malignant neoplasm mortality in the CKD population; b3 malignant neoplasm mortality in the non-CKD population. CKD, chronic kidney disease; PAG, Physical Activity Guidelines for Americans.

The Association between Sedentary Time with Mortality

Among the non-CKD population, the HR for the association between sedentary time with all-cause mortality was 1.06 (95% CI: 1.03–1.08), 1.04 (95% CI: 1.01–1.07), and 1.03 (95% CI: 1.00–1.06) in model 1, model 2, and model 3, respectively. In the CKD population, the HR was 1.12 (95% CI: 1.09–1.15), 1.11 (95% CI: 1.08–1.15), and 1.12 (95% CI: 1.08–1.15) in model 1, model 2, and model 3, respectively (Table 3).

Table 3.

The HR (95% CI) for the association between sedentary time with all-cause and cause-specific mortality in the non-CKD and CKD population

For the heart disease mortality, the HR was not significant in all the 3 models among the non-CKD population, while it was significant in all the 3 models among the CKD population. For the malignant neoplasm mortality, the HR was not significant in all the 3 models among both the non-CKD and CKD populations. The CKD population had a greater increase in the all-cause mortality (model 3, p for interaction <0.001) and heart disease mortality (model 3, p for interaction 0.001) than the non-CKD population. The association between sedentary time and mortality was similar to the overall analysis in the sensitivity analyses for the CKD population (online suppl. Table 1 in the online suppl. Material).

The Association between the Adherence to the PAG for Aerobic Activity and Sedentary Time with Disease Progression among the CKD Population

The OR for the association between adherence to the PAG with eGFR was not significant in model 1, model 2, and model 3 (Table 4). The OR for the association between adherence to the PAG with urine albumin-creatinine ratio was significant in model 1 (−0.33, 95% CI: −0.44 to −0.21), model 2 (−0.23, 95% CI: −0.34 to −0.12), and model 3 (−0.27, 95% CI: −0.39 to −0.15). The OR for the association between sedentary time with eGFR was significant in model 1 (−0.54; 95% CI: −0.73 to −0.35), model 2 (−0.43; 95% CI: −0.63 to −0.22), and model 3 (−0.49; 95% CI: −0.72 to −0.26), while it was not significant for the association between sedentary time with urine albumin-creatinine ratio in all the 3 models. Similar results were observed in the sensitivity analyses (online suppl. Table 2 in the online suppl. Material).

Table 4.

The standardized β-coefficients for the association between adherence to the PAG for aerobic activity and sedentary time with disease progression in the CKD population

Discussion

According to the most recent report, more than 1 in 7, that is, 15% of US adults or 37 million people are estimated to have CKD [7]. Although a majority of nephrologists are of the opinion that their CKD patients should exercise regularly to improve health [24], rates of physical activity and exercise counseling remain low [25, 26]. In addition, adherence to the PAG was rather low [27]. In this nationally representative study, we investigated the trends in adherence to the PAG for aerobic activity and sedentary time in US adults with CKD, which have not been reported before. The current study offers welcome findings. The proportion of individuals with CKD adhering to the PAG for aerobic activity increased significantly from 2007–08 to 2017–18. While the sedentary time for the individuals with CKD increased from 2007–08 to 2013–2014, it decreased for 2 consecutive survey cycles after 2013–14. These trends were consistent across CKD and non-CKD populations and across different subgroups of the CKD population. The reason for these favorable trends remains further investigation, especially under the fact that the rate of awareness of the PAG is still low now [28].

According to the previous findings, the sedentary time was also highest in 2013–14 among the general population and then decreased during the following years [5, 29]. While the reasons for this remain unclear, we believe that the great efforts by the US government as well as by the public play an important role. The Centers for Disease Control and Prevention launched several surveillance systems, including National Health Interview Survey, NHANES, and Behavioral Risk Factor Surveillance System, to estimate and track the prevalence of physical activity for adults in the US. These systems are used to track progress on many different national health objectives, including the physical activity objective in Health People 2010 and Health People 2020 [30]. As the leading health indicator for the Healthy People objectives, physical activity is monitored continuously and used to inform policy decisions related to physical activity. Moreover, due to the increasing amount of attention as a public health problem, the second edition of PAG started to address the health effects of sedentary behavior which was not addressed in the first edition [3]. Thus, these efforts would lead, at least in part, to the reduction of sedentary behavior among the US population.

CKD is associated with physical inactivity. In patients with CKD not requiring renal replacement therapy, physical activity worsens with GFR lowering and is associated with frailty and disability [31]. Individuals with CKD have significantly reduced peak oxygen consumption, self-reported physical functioning, and physical performance compared to those with normal kidney function, which might be caused by several factors, including renal anemia, malnutrition, uremic toxins, vitamin D deficiency, and abnormal potassium metabolism [32, 33]. Nonetheless, the functional limitation in the CKD population could be improved with increased physical activity. In a meta-analysis with 31 randomized controlled trials that studied aerobic exercise in adults with CKD, the author found a significant improvement in peak oxygen consumption, exercise duration, physical role, and general health after aerobic training [14]. Thus, physical function in the CKD population could be improved by physical activity.

Research regarding the beneficial effects of physical activity on mortality and disease progression in individuals with CKD is limited by its consistency and nature of study design [9]. The current study investigated the impact of adherence to the PAG for aerobic activity on mortality among individuals with CKD. In this prospective cohort, we found that adherence to the PAG decreased the all-cause and malignant neoplasm mortality after adjusting for all potential covariates. A recent study based on National Health Interview Survey found that the recommended physical activity greatly reduced the risk of all-cause and cause-specific mortality [34]. However, this study targeted the general population and included only leisure time aerobic activity. Another study investigated the recommended physical activity and mortality in individuals with CKD based on data from NHANES III, and the results indicated that physical activity was associated with decreased mortality [18]. However, this study also included only leisure time activity, and the evaluation of adherence to physical activity was based on a different recommendation [35].

Consistent with previous findings suggesting that prolonged sedentary time is associated with poor health and functional capacity [36], the current study also indicated that prolonged sedentary time increased the risk of all-cause and heart disease mortality in the CKD population. Even though highly related, sedentary behavior has proved not to be just the opposite part of physical activity [37]. Evidence shows that high levels of moderate-intensity aerobic activity are needed to eliminate the increased risk of death induced by prolonged sedentary time. Thus, advanced efforts should be made to reduce the total sedentary time and then increase physical activity.

We found that the CKD population had a greater reduction in their all-cause and malignant neoplasm mortality when adhering to the PAG for aerobic activity than the non-CKD population. Similarly, a greater increase in the all-cause and heart disease mortality was observed along with prolonged sedentary behavior in the CKD population than in the non-CKD population. Therefore, these results indicated that adhering to the PAG for the aerobic activity or reducing sedentary time could provide more survival benefits in the CKD population than in the non-CKD population. To our knowledge, this is the first study that provides such a finding, and it suggests that increasing activity and reducing sitting time might be more important for the CKD population than for the non-CKD population.

The current study indicated that physical activity and sedentary behavior had a different impact on the cause-specific mortality and disease progression among the CKD population. Adherence to PAG for aerobic activity decreased the risk of malignant neoplasm mortality, but not heart disease mortality. It also decreased the level of urine albumin-creatinine ratio, while having no significant effect on the level of eGFR. This result was consistent with the previous 2 meta-analyses [12, 38]. Conversely, prolonged sedentary time increased the risk of heart disease mortality, but not malignant neoplasm mortality. It also decreased the level of eGFR, while having no significant effect on the level of urine albumin-creatinine ratio. This evidence highlights the importance of reducing sedentary time among the CKD population rather than being physically active alone.

Strengths and Limitations

The strengths of this study included using the nationally representative data from NHANES to allow the generalization of the results to the entire US noninstitutionalized adults. Second, the measurement of serum creatinine was adjusted according to the recommendation of NHANES for a comparable assessment across different cycles. Third, 3 sensitivity analyses were conducted to validate the robustness of our results.

The major limitation of this study was that the information on physical activity was self-reported. However, self-report instruments are widely used for physical activity assessment despite shortcomings such as recall bias and misinterpretation of questions [39]. Second, muscle strengthening has been suggested in the PAG to improve muscle function, while the information on muscle strengthening activity has not been recorded in NHANES 2007–18. Third, the casual interferences between adherence to the PAG and sedentary time with disease progression could not be made as this information was collected cross-sectionally.

Conclusion

In this continuous survey weighted to be representative of the US adults with CKD, the trends in adherence to PAG for aerobic activity increased from 2007–08 to 2017–18, and sedentary time peaked in 2013–2014 and then decreased afterward. Adherence to the PAG and reduction in sedentary time reduced all-cause and cause-specific mortality and prevented disease progression differently. Efforts are needed to decrease sedentary time rather than adhering to the PAG alone.

Statement of Ethics

The NHANES is a large, cross-sectional survey conducted continuously in 2-year cycles in the USA. Users can download relevant data for free for research and publish relevant articles. Our study is based on open source data, and there is no need to obtain participant consent to have their data included in this study. Therefore, there are no ethical issues and other conflicts of interest.

Conflict of Interest Statement

The authors have no conflicts of interest to declare.

Funding Sources

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Author Contributions

Jigang Chen designed the study and prepared the data. Jigang Chen, Lijun Hou, Xin Tong, and Junhui Zhang performed the statistical analysis and prepared the manuscript. Xiaolin Qu and Xiaoxiang Hou reviewed the results and revised the manuscript.

Data Availability Statement

The data are from the National Health and Nutrition Examination Survey and openly available at https://www.cdc.gov/nchs/nhanes/index.htm, reference number NHANES 2007–2008, NHANES 2017–2018.

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