Association Between Headache and Serum 25-Hydroxyvitamin D

The Tromso Study: Tromso 6

Marie Kjærgaard, MD; Anne Elise Eggen, PhD; Ellisiv B. Mathiesen, PhD; Rolf Jorde, PhD


Headache. 2012;52(10):1499-1505. 

In This Article


The Tromsø Study

The Tromsø Study, initiated in 1974, is an epidemiologic, prospective study of health problems and chronic diseases, and a resource for the surveillance of disease risk factors.[6] The main focus was initially cardiovascular disease, however, several other chronic diseases and conditions have been included through the years, like vitamin D status and chronic pain. The purpose of the sixth survey (Tromsø 6) performed in 2007–2008, was to collect novel and repeated measurement of exposure data and to assess levels of disease-risk factors. The Tromsø Study design and population are described by Jacobsen et al.[7] In the sixth survey 19,762 subjects were invited; 12,982 attended. The age of the participants ranged from 30 to 87 years. The study was conducted by the Department of Community Medicine, University of Tromsø.


The participants completed 2 questionnaires on lifestyle variables and health. One was filled in before the first visit and handed in at attendance, the other after the first visit and was either handed in before they left the examination site or returned by mail later. An English translation of the questionnaires is available at The answers from the 2 questionnaires were combined for the analyses and used as either dichotomized variables (smoking yes/no and education, tertiary level yes/no [college and/or university] ) or continuous variables (alcohol consumption, physical exercise, and self-reported chronic diseases [asthma, diabetes, stroke, hypertension] ). The physical exercise estimate was calculated in hours/week, giving hard activity double weight and light activity half weight compared to moderate activity. Weekly alcohol consumption was estimated in units/week. Each of the 4 chronic diseases was given equal weight as a confounder in the analyses and was therefore summarized to a continuous score.


The participants were asked in the questionnaire if they had experienced headache in the last year (yes/no). If the answer was yes, they were asked to specify if it was of migraine or other type of headache (not specified further), average frequency per month (less than 1 day/1–6 days/7–14 days/more than 14 days), average duration (less than 4 hours/4–24 hours/1–3 days/more than 3 days), intensity (mild/moderate/severe), type (pulsating/pressure [tension]/unilateral), and if there were any worsening of symptoms with physical activity. The latter 4 questions were used to evaluate whether the criteria for a diagnosis of migraine was fulfilled according to the International Classification of Headache Disorders (ICHD-II).[8] Migraine was defined as an answer of "yes" to the question on migraine type of headache, combined with 1; headache with a duration of 4–72 hours and 2; at least 2 of the following 4 criteria: intensity moderate or strong, pulsating character, unilateral distribution, and worsening by physical activity. All other types were classified as non-migraine headache. Thus, persons who answered "no" to the migraine question were classified as non-migraine even if they fulfilled the other migraine criteria.

25(OH)D Analyses

Non-fasting blood samples were drawn, and serum 25(OH)D3 was measured by immunometry (electrochemiluminescent immunometric assay) using an automated clinical chemistry analyzer (Modular E170; Roche Diagnostics GmbH, Mannheim, Germany). According to the producer, the assay has, for total analytical precision, a coefficient of variation ≤7.8% as judged in any of 3 different concentrations (48.6–73.8–177.0 nmol/L). The cross-reactivity with 25(OH)D2 was <10%, and the analytical sensitivity was 10 nmol/L. In Norway, all food fortification and ordinary supplements are vitamin D3. Vitamin D2 preparations are only sold by prescription and to highly selected patients. At present, the laboratory has no reference values for 25(OH)D, but the manufacturer provides a population-based reference range of 27.7–107.0 nmol/L for adults as a guideline. This analysis has been approved by Norwegian Accreditation. After the study was completed, this method was found to overestimate the serum 25(OH)D in smokers.[9] To adjust for this, we chose to divide the study population into current smokers and non-smokers. Serum 25(OH)D values in former smokers are shown to be at the same level as non-smokers, and therefore the former smokers are included in the non-smokers' group. Subjects without information about smoking status or missing serum 25(OH)D values were excluded.

Statistical Analyses

Data were analyzed stratified according to smoking status. Interaction was analyzed by including 25(OH)D*gender in the regression analyses and was not found to be significant. Therefore, men and women were pooled in the analyses, and adjustment was done accordingly. Due to the binary nature of the headache data, logistic regression analyses were used to evaluate the association between serum 25(OH)D using the presence of headache (any type) as a dichotomous variable. Normal distribution of the variables was evaluated with visual inspection of histograms and determination of skewness and kurtosis. Serum 25(OH)D was either used as a continuous variable or divided into quartiles for OR analyses. Adjustment for season was performed by dividing the serum 25(OH)D-values into quartiles for each month individually, thus adjusting for season without use of dummy variables. Age, body mass index (BMI), physical exercise, number of chronic diseases, and alcohol consumption were used as continuous variables, and gender and education as dichotomous variables.

Trend analyses in the logistic regression were performed using serum 25(OH)D quartiles as a continuous variable. T-test, Mann-Whitney, and chi-square test were used comparing means, medians, and proportions. The data are shown as mean (SD) unless otherwise indicated. The number of self-reported chronic diseases was not normally distributed but is presented as mean and SD to lessen loss of information since the median value is zero.

All tests were performed 2-sided, and a P value < .05 was considered statistically significant. Statistical analyses were performed with PASW (SPSS) version 18.0 software (SPSS Inc., Chicago, IL, USA).


All participants gave written consent before participating in the study. The study was approved by the Regional Committee for Medical and Health Research Ethics and The Data Inspectorate of Norway.