Study population
The Northern Finland Birth Cohort 1966 study (NFBC1966) includes subjects in Northern Finland whose dates of birth fell in the year 1966 (n = 12,058 live births) [23]. Information about these individuals has been recorded regularly since their birth through health care records, questionnaires, and clinical examinations, and data has been collected on their parents and offspring. The study was approved by the Ethical Committee of the Northern Ostrobothnia Hospital District in Oulu, Finland (94/2011), and it has been performed in accordance with the Declaration of Helsinki. The subjects and their parents provided written consent for the study. Personal identity information was encrypted and replaced with identification codes to provide full anonymity. This study analyzed the data obtained from the most recent time point, at which time participants were 46 years old (n = 10,321). Participants attended clinical examinations (n = 5852), where trained nurses thoroughly assessed their medical condition, their height and weight were measured, and their BMI (body mass index) calculated.
Questionnaire
Postal questionnaires were sent to all participants with known addresses in 2012–2014 (PA questionnaire response rate 62%, n = 6384). Questionnaires included items about participants’ health, behavior, and social background. Their marital status, education, employment status, annual household income, and prevalence of diagnosed diseases were addressed. Smoking status (former, current, non-smoker) and alcohol consumption (g/day) were composed based on multiple questions about drinking and smoking habits.
Participants were asked about their PH with the question “How would you describe your health at the moment?” The response alternatives were 1) very good, 2) good, 3) fair, 4) poor, and 5) very poor. The PH responses were dichotomized as good (very good and good) and other (fair, poor, and very poor) [2].
LTPA was self-reported with questions on the frequency and duration of light and brisk physical activities or exercises during leisure time, as was done in the 31-years follow-up study (24). Brisk PA was described as causing at least some sweating and breathlessness, while light PA was defined as causing no sweating or breathlessness. PA frequency had six response options: 1) once a month or less often, 2) 2–3 times a month, 3) once a week, 4) 2–3 times a week, 5) 4–6 times a week, and 6) daily. PA duration had the following response options: 1) not at all, 2) less than 20 min, 3) 20–39 min, 4) 40–59 min 5) 1–1.5 h, and 6) more than 1.5 h [24]. Weekly averages of metabolic equivalent of a task (MET) minutes of light and brisk PA were calculated by multiplying the PA volume (duration*frequency) by its intensity (light PA 3 METs and brisk PA 5 METs) [25].
Daily sitting time (ST) was assessed with the question “How much time do you spend sitting on a normal weekday?” The response was divided to describe the amount of sitting in five domains (at work, at home watching TV or video, at home in front of computer, in a vehicle, and in another place) [26]. Average daily ST (min/day) was calculated as a sum of durations of these sedentary behaviors. Those reporting ST higher than 18 h/day (n = 27) were excluded from analyses concerning ST.
Accelerometer-measured physical activity
PA was objectively measured with wrist-worn activity monitors (Polar Electro Oy, Kempele, Finland) for 14 days. Polar Active is a waterproof accelerometer providing MET values every 30 s based on daily PA [27]. Polar Active has been shown to correlate (R2 = 0.74) with the double-labelled water technique when measuring energy expenditure during exercise [28]. It uses height, weight, gender, and age of the user as predefined inputs. Each activity monitor was blinded, giving no feedback to the user, and the monitors were given to the participants during clinical examinations, with participants being instructed to mail them back after the measurement period. Participants were asked to wear the Polar Active monitor 24 h/day for at least 14 days on the non-dominant hand. Measured PA was classified as five levels (very light: 1–2 MET, light: 2–3.5 MET, moderate: 3.5–5 MET, vigorous: 5–8 MET, and vigorous+ ≥8 MET) [29]. Daily averages of duration spent in activity levels (min/day) were calculated for all participants. All activity performed with the intensity of 3.5 MET or higher was assessed as MVPA (min/day), and MVPA MET-minutes were calculated by multiplying each MET value with its duration (30s). The first day when the activity monitor was given to the participant was excluded from the analysis. Participants with four or more eligible days (wear time of at least 600 min/day) were included in the analyses (Fig. 1) [30].
Statistics
The descriptive data is presented as numbers and percentages, means, standard deviations (SD) or medians, and 25th and 75th percentiles for skewed data. Accelerometer-measured moderate, vigorous, and very vigorous intensity PA and MVPA were natural log-transformed to obtain normal distribution. Non-transformed values are presented in the tables. Univariate associations between continuous variables and gender and PH were analyzed using the independent-samples t-test, with Tukey post hoc tests for normally distributed variables and with the independent-samples Mann-Whitney U test for skewed data. The agreement between objectively measured MVPA and questionnaire based LTPA as MET-minutes was evaluated with Spearman’s rank correlation coefficient (r). The associations of demographic, socioeconomic, and health characteristics to PH were studied through binary logistic regression analysis. PH was controlled for gender, marital status (married/cohabiting, unmarried), employment status (working, unemployed, other), education (vocational or no vocational education, polytechnic/university degree), prevalence of diagnosed diseases (cardiovascular disease, diabetes mellitus, cancer, musculoskeletal diseases, or mental disorder), smoking (former, current, non-smoker), heavy alcohol consumption (men ≥40 g/day, women ≥20 g/day), BMI (body mass index kg/m2, as continuous variable), income (quartiles of yearly household income), and ST (min/day, quartiles). Model 1 included in addition accelerometer-measured MVPA (min/week, quartiles), while Model 2 included self-reported LTPA (min/week, quartiles) instead of MVPA. Odds ratios (OR) and 95% confidence intervals (95% CI) for good PH were calculated, and ORs were adjusted for all the variables. The significance of the models was evaluated using the likelihood ratio test (χ2) and Nagelkerke R2 value. Statistical significance was set to p < 0.05, and statistical analyses were performed with IBM SPSS Statistics for Windows, version 24.0 (IBM Corp., Armonk, USA).