The HUNT study
The Nord-Trøndelag Health Survey (HUNT) is a longitudinal cohort study in which all inhabitants ≥ 20 years old in Nord-Trøndelag were invited to participate. Subjects were examined three times; from 1984 to 1986 (HUNT 1), 1995 to 1997 (HUNT 2) and 2006 to 2008 (HUNT3). The surveys covered a large number of health-related items, and participants were also invited to clinical consultations, which included blood samples and measurements of blood pressure, height and weight. HUNT 2 included questions on headache, whereas HUNT 3 also included question on religious attendance.
Sampling frame
Nord-Trøndelag is one of 19 counties in Norway, and is fairly representative for the rest of the country. During HUNT 3, 89.7% of the inhabitants were members of the evangelical Lutheran Church, 2.1% were members of other Christian churches, and 0.4% belonged to other religions [4]. Norwegians are registered at baptism as members of the Church of Norway.
HUNT 2 questionnaire
In HUNT 2 each person completed extensive questionnaires eliciting information on health problems. Among a wide range of topics in the first questionnaire (Q1) were education, physical activity, smoking, and anxiety and depression (measured by the Hospital Anxiety and Depression Scale (HADS)). Details of the phrasing of these questions have been described previously [5–8]. Educational level was categorized according to duration: ≤9 years, 10–12 years, and ≥13 years. Cigarette smoking was categorized as “current daily smoking”, “previous daily smoking”, and “never daily smoking”. Reponses to questions on physical activity were categorized according to duration and intensity of exercise per week: ≥3 hours hard physical activity, 1–2 hours hard physical activity, ≥3 hours light physical activity, 1–2 hours light physical activity and physical inactivity (0 hours). BMI was subdivided into three groups: <25 kg/m2, 25–29.9 kg/m2 and ≥30 kg/m2.
The headache questions in the second questionnaire (Q2) were designed principally to determine whether or not each person had headache and, if so, its frequency, and to diagnose migraine according to a modified version of the first version of the International Classification of Headache Disorders (ICHD-I) [9] when headache was reported. Subjects who answered “yes” to the screening question “Have you suffered from headache during the last 12 months?” were classified as having headache. Mutually exclusive diagnoses were made for migraine and non-migrainous headache [10]. Chronic daily headache (CDH) was defined as headache occurring on ≥ 15 days/month.
The validity of these questionnaire-based diagnoses was reported previously [6, 10]: for headache sensitivity was 85% and specificity 83% (kappa value 0.57); for migraine, sensitivity was 69% and specificity 89% (kappa value 0.59); for non-migraineurs, sensitivity was 61%, specificity 81% (kappa 0.43) and for CDH, sensitivity was 38%, specificity 97% (kappa 0.44).
HUNT 3 questionnaire
HUNT 3 was to a large extent a replication of HUNT 2 but included also a question on religious attendance (RA) in Q2 measured with the item “How often in the last 6 months have you been to church/prayer house?” with possible responses covering “never”, 1–6 times in the last 6 months”, “1-3 times/month”, and “more than 3 times/month”. In the prospective analysis, frequent religious attendees (fRA) were defined as those who answered “1-3 times/month” or “more than 3 times/month”. The individuals were also asked to respond to frequency of non-religious social activities using the question “How often in the last 6 months have you been to concerts, cinema and/or theatre” with similar response options. Frequent social activity attendees (fSA) were defined as those who answered “1-3 times/month” or “more than 3 times/month”.
The Q2 also included the same headache screening question that was used in HUNT 2. For the questionnaire-based status as a headache sufferer, a sensitivity of 88%, a specificity of 86%, and a kappa value of 0.70 were found [11].
Samples and procedures
In HUNT 2, 51,383 (56%) of 92,936 invited individuals answered the headache-related questions in Q2. Details of non-responders have been described previously [10]. In HUNT 3, 50,839 (54%) of the 94,194 invited adults answered Q1 and 37,383 (40%) answered the question in Q2 on RA. Details of non-responders have been described previously [11]. The mean time of follow-up was 11.3 (range 9–13) years. Among the 51,383 who answered the headache questions in HUNT 2, 6,608 had died and 3,009 had moved out of the county by the time of HUNT 3, leaving 41,766 eligible individuals for this analysis. Of these, 25,177 (60%) completed the questions regarding RA in HUNT 3 whereof 24,610 answered headache questions both in HUNT 2 and HUNT 3. The flow chart of participants in HUNT 2 and HUNT 3 is shown in Figure 1.
Statistical analysis
By multivariate analyses using logistic regression we estimated odds ratio (OR) with 95% confidence intervals (CI) evaluating the influence of headache status at baseline in HUNT 2 on the odds of being fRA in HUNT 3. The relationship between headache in HUNT 2 and the four RA categories was also evaluated, and these categories were treated as a single ordinal variable and incorporated in a two-sided test for trend to evaluate the probability of a linear relationship between RA categories and headache. The trend test was considered statistically significant at p < 0.05. We also evaluated potential confounding factors identified previously [5, 6, 8, 12–14]: in particular age (continuous variable), gender, education level, smoking status, physical activity, body mass index, chronic musculoskeletal complaints, systolic blood pressure (SBP) and anxiety and depression measured by total HADS score (continuous variable). Subjects with incomplete data for one or several variables were included (as a separate missing category) in all analyses to reduce the impact of response bias. In supplementary analyses, we evaluated the influence of headache status in HUNT 2 and HUNT 3 on the odds of being fRA in HUNT 3. Statistical analyses were performed with the Predictive Analytics SoftWare (PASW) Statistics version 17.0 by SPSS inc., an IBM Company (Chicago, IL).
Ethics
The Norwegian Data Inspectorate, the Norwegian Board of Health, and the Regional Committee for ethics in Medical Research had approved all HUNT studies, and the Regional Committee also approved the present analysis.