Study population
The CHNS is considered as a national, representative study aimed at exploring the impact of social-economic transformation on Chinese health and nutritional [16]. It includes multiple samples and cohorts over nine rounds of surveys in nine provinces and three megacities between 1989 and 2011[17]. The initial round of the CHNS was conducted in 1989, and nine follow-up rounds were carried out respectively in 1991, 1993, 1997, 2000, 2004, 2006, 2009, 2011, and 2015. More details of study design, sampling method as well as eligibility criteria have been published and updated recently [18].
As adults’ sedentary leisure time measurements were available after the 2004 survey, the present study included >18-year-old adults from the four surveys conducted between 2004 and 2011 (2004, 2006, 2009, and 2011). Our study included participants aged 18–63 years at baseline and 25–70 years at follow-up. The following participants were excluded: 875 participants under the age of 18 and 1,203 participants with high blood pressure in the 2004 survey, 4,908 participants whose metabolic equivalent of energy (MET) values were outside the normal range or lost, and 1,792 participants who participated in only one survey. A total of 11,162 participants with PA measurements available from two to four surveys were included in our study. In 2004, 2006, 2009, and 2011 survey years, the number of participants was 8,293, 8,924, 9,211, and 8,424, respectively.
PA Measurement
In each survey, self-reported PA was collected using a standardized questionnaire [19]. Participants were surveyed the frequency of participation and time spent in different types of PA, which included occupational and domestic activities (such as cleaning, cooking, or washing), leisure activities (various forms of sports), and travel activities, and sedentary leisure activities (such as sleeping, watching TV, reading, writing or drawing, playing video games or computer games, and browsing or chatting online). The intensity of each activity was expressed as METs, with one MET is defined as the ratio of a person’s working metabolic rate to resting metabolic rate [20]. Vigorous activities (≥6 METs) included running, ball sports, bicycling, dance or wushu classes, and other strenuous exercise. Moderate-intensity activities (3–5 METs) included walking, driving, doing housework. Light activities (0.9–3 METs) included sleeping, watching TV, reading, and other sedentary activities [21].
The PA level is the product of the specific MET values multiplied by the time spent in each activity [22]. We multiply the number of minutes spent by each activity by the METs of the activity to calculate each PA score, and defined the total PA score as the sum of METs for all activities [23]. The total PA score ranges from 3,024 to 51,627 METs. The complete questionnaire and scoring system used to calculate the total PA score has been reported in detail elsewhere [19].
Assessment of incident hypertension
Self-reported of a history of hypertension diagnosis and/or consumption of antihypertension medication at baseline is defined as having hypertension [24]. The incident hypertension cases in the 2006, 2009, and 2011 survey years were collected.
Other measurements
Information on age, body mass index (BMI), carbohydrate, energy, fat and protein intake, urbanization index, education, smoking, drinking, and urban or rural status was collected through a questionnaire in all surveys. Doctor using standard protocols to measured height and weight. The weight and height of people were measured to the nearest 0.1 kg and 0.1 cm, respectively. The BMI was calculated by dividing the weight (kg) by the square of the height (m).
The researchers took 12 milliliters of blood from participants who had fasted for one night. The fasting plasma glucose (FPG), hemoglobin A1c (HbA1c), high-sensitivity C-reactive protein (hs-CRP), uric acid (UA), triglyceride (TG), and high-density lipopolysaccharide-cholesterol (HDL-C) levels were estimated.
Statistical analysis
GBTM was used to define the longitudinal discrete trajectories of PA over the participants’ life course by SAS PROC TRAJ [25], which is available at www.andrew.cmu.edu/user/bjones/ [26]. Model fit was based on the Bayesian information criterion (BIC), whereby the model with the lower BIC was favored [27].
Participant-years of follow-up were calculated from the date of the initial baseline interview until the date when participants were diagnosed with hypertension, the date of death, or the end of follow-up, whichever occurred first.
Distributions of covariates at baseline for each PA trajectory group membership were described. Categorical variables were described as percentages (%) and were compared using chi-square tests. Continuous variables were described as the mean ± standard deviation and were compared using one-way analysis of variance. A generalized linear model was used to test differences across PA trajectories.
A Cox proportional hazard model with hazard ratio (HR) and 95% confidence intervals (CI) was used to investigate the relationship between the trajectory group membership and the incident of hypertension. Model 1 was adjusted according to age. Model 2 was adjusted according to smoke, drink, degree of education, urban and rural, and province. Model 3 was further adjusted according to BMI. Model 4 was further adjusted according to protein, energy, fat and carbohydrate intake. Sensitivity analysis excluding participants with hypertension during the first 2 years of follow-up was conducted to assess whether the results were affected by reverse causation.