Methods
This study was based on data from the public files for the 2006 follow-up of participants of the 1999–2000 and 2001–2002 cycles of the National Health and Nutrition Examination Survey (NHANES). A multistage, stratified sampling design was used to generate a sample of participants who were representative of the non institutionalized civilian US population. The response rates for the interviewed and examined samples of the entire survey were 82% (9965/12160) and 76% (9282/12160), respectively, for 1999–2000 and 84% (11039/13156) and 80% (10477/13156), respectively, for 2001–2002. After an interview at home, participants were invited to complete additional questionnaires, undergo a set of tests, and provide blood and other biological specimens in the mobile examination center. Methodological details about the NHANES and the linked mortality files have been published. The National Center for Health Statistics Research Ethics Review Board granted approval for the conduct of the study, and participants were asked to sign an informed consent form.
The mortality status of participants aged ≥ 20 years through 2006 was determined by using the National Death Index. Several studies have shown that the National Death Index identifies over 90% of deaths. Participants who were not deemed to have died as of December 31, 2006 were considered to be alive. The International Classification of Diseases, 10th Revision (ICD-10) codes I00-I99 were used to identify deaths from diseases of the circulatory system.
Participants were asked "Over the past 30 days, on a typical day how much time altogether did you spend on a typical day sitting and watching TV or videos or using a computer outside of work? Would you say…". Response options were none, less than 1 h, 1 h, 2 h, 3 h, 4 h, or 5 h or more. The time spent watching TV or videos or using a computer will also be referred to as screen time.
Study covariates included age, gender, race or ethnicity (white, African American, Mexican American, and other), educational attainment (< high school, high school graduate or equivalent, > high school), smoking status (current, former, never), leisure-time physical activity (continuous), Healthy Eating Index score (continuous), alcohol use (continuous), self-reported health status, health insurance coverage, histories of cardiovascular disease and cancer, body mass index, systolic blood pressure, and concentrations of high-density lipoprotein cholesterol, non-high-density lipoprotein cholesterol, and HbA1c. Participants who had smoked 100 cigarettes during their lifetime and reported smoking at the time of the interview were classified as current smokers. Participants who had smoked 100 cigarettes during their lifetime and reported not smoking at the time of the interview were classified as former smokers. Participants who had never smoked 100 cigarettes during their lifetime were classified as never smokers. Participants were asked about partaking in moderate and vigorous physical activities in leisure-time and, for those who did, the time spent being physically active was calculated from their responses to the frequency and duration of the reported moderate and vigorous activities with the time spent being vigorously active being weighted by a factor of 2. The Healthy Eating Index is a score that ranges from 0 to 100 and has 10 subcomponents: grains, fruits, vegetables, dairy, meats, fats, saturated fat, cholesterol, sodium, and variety. The index was determined from dietary information collected by a single 24-h recall administered in person to participants attending the medical examination. The intake of alcohol was obtained from information provided during a single 24-h dietary recall.
Self-reported health status was determined from the question "Would you say your health in general is excellent, very good, good, fair, or poor?". Health insurance coverage (yes/no) was derived from the question "Are you covered by health insurance or some other kind of health care plan?". Participants who reported ever being told by a doctor or other health professional that they had congestive heart failure, coronary heart disease, angina pectoris, heart attack, or stroke were considered as having a history of cardiovascular disease. Participants who reported ever being told by a doctor or other health professional that they had diabetes were considered to have diabetes. Participants who reported ever being told by a doctor or other health professional that they had cancer were considered to have cancer.
Body mass index (kg/m) was calculated from measured weight and height. Up to four attempts were made to measure blood pressure. The average of the last two measurements of blood pressure for participants who had three measurements, the last measurement for participants with only two measurements, and the only measurement for participants who had one measurement were used. Serum total cholesterol and high-density lipoprotein cholesterol were measured enzymatically on a Hitachi 704 Analyzer (Roche Diagnostics, Indianapolis, IN) at Johns Hopkins University. Non-high-density lipoprotein cholesterol was calculated by subtracting the concentration of high-density lipoprotein cholesterol from that of total cholesterol. Concentrations of HbA1c were measured on Primus Automated HPLC systems, models CLC330 and CLC385 (Primus Corp., Kansas City, MO) at the University of Missouri-Columbia.
The analyses included participants who were aged ≥ 20 years and nonpregnant women. Chi-square tests and t-tests for independent samples were used to examine differences in percentages and means, respectively. The Cochran-Mantel-Haenzel test was used to test for differences in the distribution of categorical variables after stratification by age groups. Age-adjusted mortality rates per 1,000 person-years of follow-up were calculated. Age-adjustment was performed using the direct method with the projected year 2000 US population. Proportional hazards analysis was used to estimate hazard ratios. Using Schoenfeld residuals, the proportional hazards assumption was met. Several proportional hazards models were run that included varying sets of covariates selected from age, gender, race or ethnicity, educational attainment, smoking status, leisure-time physical activity, Healthy Eating Index score, alcohol use, self-reported health status, health insurance coverage, and histories of cardiovascular disease and cancer. The statistical softwares SAS and SUDAAN were used to generate the results.