Study population
This prospective cohort study used data from the US National Center for Health Statistics’ National Health and Nutrition Examination Survey (NHANES), conducted on 2-year cycles since 1999 by the Centers for Disease Control and Prevention (CDC) and the National Centers for Health Statistics (NCHS) in the USA[8]. The study design, protocol, data collection and analysis procedures have been reported in detail. All the NHANES protocols were approved by the National Center for Health Statistics ethics review board, and written informed consent was obtained from all participants[7]. This study followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) Statement guidelines[9]. In this study, we integrated data for analysis from six two-year NHANES cycles (2007–2018). The inclusion criteria for all individuals were men aged ≥ 40 years old. Ultimately, after excluding participants with missing data for important covariates, a total of 617 participants were enrolled (Figure. 1).
Diagnosis of prostate cancer
During the in-person interview, data about cancer diagnosis and cancer type were gathered. This information included up to three cancer types per person and the age at each diagnosis. Prostate cancer was identified based on two questions: "Have you ever been told by a doctor or other health professional that you had cancer or a malignancy of any kind?" and "What kind of cancer was it?" Individuals with prostate cancer were determined based on the primary cancer site. The current analysis focused on individuals aged 40 years or older at the time of the survey because most cases of prostate cancer are diagnosed after age 40.
Ascertainment of mortality
The NCHS provided mortality data, which were linked to the National Death Index up until December 31, 2019. The underlying causes of death were recorded using the International Classification of Diseases, 10th Revision (ICD-10). Cancer-related deaths were categorized under malignant neoplasms (ICD-10 codes C00-C97), while non-cancer deaths were classified under other causes. The follow-up duration was measured in months from the survey date (2007 to 2018) to the date of death or December 31, 2019, for those who were censored[10]. To minimize the risk of reverse causation, deaths occurring in the first year of follow-up were excluded[11].
Leisuretime physical activity and daily sitting time
Total daily sitting time and LTPA were self-reported by participants responding to the Global Physical Activity Questionnaire (GPAQ). The GPAQ has previously undergone validation for assessing information pertaining to daily activities, leisure-time activities, and sedentary behaviors[12]. During the in-person interview, participants were asked to report their moderate- and vigorous-intensity recreational activities over a typical week[8]. The total LTPA was calculated by summing the minutes of moderate-intensity recreational activities and doubling the minutes of vigorous-intensity recreational activities[13]. Based on the 2018 Physical Activity Guidelines for Americans, participants were classified as follows based on their LTPA in the previous week: those with no LTPA were classified as inactive, those with more than 0 minutes but less than 150 minutes per week (min/week) were classified as insufficiently active, and those with 150 min/week or more were classified as sufficiently active[14]. Additionally, participants were asked, "On a typical day, how much time do you usually spend sitting at school, at home, getting to and from places, or with friends, including time spent sitting at a desk, traveling in a car or bus, reading, playing cards, watching television, or using a computer?" [8] Based on recent studies, participants' responses were converted into hours per day (h/d) and categorized into three groups: 0 to less than 6 hours per day, 6 to 8 hours per day, and 8 or more hours per day[7, 15, 16].
Covariates
Several potential covariates were evaluated, including age, gender, race/ethnicity, education level, marital status, smoking habits, body mass index (BMI), and comorbidities including diabetes, stroke, coronary heart disease, and hypertension[17]. Race/ethnicity was classified into the following categories: non-Hispanic White, non-Hispanic Black, Mexican American, and other races[18]. Marital status was categorized as married, never married, living with a partner, or other, which includes widowed, divorced, or separated individuals[18]. Education level was categorized into three groups: less than 9th grade, high school or equivalent, and above high school, family income was categorized into three groups by the poverty income ratio (PIR): low (PIR ≤ 1.3), medium (PIR > 1.3 to 3.5), and high (PIR > 3.5)[19]. Smoking status was classified as follows: never smokers (smoked fewer than 100 cigarettes), current smokers, and former smokers (quit smoking after smoking more than 100 cigarettes)[19]. Alcohol consume was classified as follows (never, former, mild, moderate, heavy ). Body mass index (BMI) was calculated as weight (kg)/(height [(m])2)[20]. Hypertension was either self-reported by participants who had received a diagnosis from a health professional or determined by NHANES-measured blood pressure (≥ 130 mm Hg[systolic] or ≥ 80mmHg [diastolic])[7]. A history of cardiovascular disease (CVD) and/or diabetes was self-reported by participants who had received either or both of these diagnoses from a health professional or determined by a prescription history for medications used to treat these conditions[10].
Statistical analyses
This study was a secondary analysis of publicly available datasets. We used dietary weights for the weighted analysis. All analyses adhered to NHANES analytic guidelines, accounting for unequal selection probabilities, oversampling of specific subpopulations, and adjustments for non-response to ensure nationally representative estimates. The study utilized data from six biennial cycles of the NHANES spanning from 2007 to 2018. All analyses were conducted using sample weights provided by NHANES, which account for the complex sampling design, oversampling of specific populations, and adjustments for non-response to ensure nationally representative estimates. Categorical data were expressed as unweighted numbers (weighted percentages). Sample sizes and weighted percentages were estimated based on participants' sociodemographic and lifestyle factors. In prostate cancer survivors, we used a multivariable Cox proportional hazards regression model to estimate how LTPA and daily sedentary time are associated with overall mortality and cancer-specific mortality. We reported the results as hazard ratios (HR) with their corresponding 95% confidence intervals (CI). To examine joint associations, participants were classified based on LTPA and sitting time to estimate mortality risks using multivariable Cox proportional hazards regression models. Model1 was adjusted for sociodemographic characteristics, including age, race/ethnicity, marital status, education level, and family income. Model2 was adjusted for sociodemographic characteristics and the factors that p values were less than 0.05 in the univariate analysis. Model3 was fully adjusted, including sociodemographic characteristics, smoking status, hypertension, diabetes, stroke, coronary heart disease, BMI. All analyses were conducted in prostate cancer survivors. Stratified analyses were conducted by sex, BMI, smoking status, history of diabetes and CVD among prostate cancer survivors.