Cohort profile: The Tromso Study

Abstract

The Tromsø Study was initiated in 1974 in an attempt to help combat the high mortality of cardiovascular diseases in Norway, that was particularly pronounced among middle-aged men. In the mid-1970s, Norwegian men had a 20% risk of dying of myocardial infarction (MI) before the age of 75 years. The situation in Northern Norway was even worse.1 The primary aim of the Tromsø Study was to determine causes of the high cardiovascular mortality, and also to develop ways of preventing heart attacks and strokes. This was reflected through the first name of the study: The Tromsø Heart Study. However, during the 37 years since the first examination of the Tromsø Study took place, increasing emphasis has been put on other chronic diseases and conditions, in particular atrial fibrillation, venous thromboembolism, diabetes mellitus, osteoporosis and fractures. It has been a deliberate policy to invite a wide range of faculty research groups to join in with subprojects in the surveys, and there are currently some 100 different ongoing research projects based on the data from the consecutive six surveys. The study was initially funded by the University of Tromsø, and has been so for the entire period since 1974, but there have also been substantial contributions, directly and indirectly from, for example, the National Screening Services, the Research Council of Norway, Northern Norway Regional Health Authority, Norwegian Council on Cardiovascular Diseases and Norwegian Foundation for Health and Rehabilitation. Teams of investigators approach public research programmes for funding of the different examinations conducted. Tromsø is the largest city in Northern Norway. It is situated ∼400 km north of the Arctic Circle, and has approximately 67 000 inhabitants. The physical living conditions are dominated by dramatic changes in the light with 2 months of midnight sun and 2 months of the polar night. However, due to the Gulf Stream, the climate is relatively mild, the latitude (69°N) taken into account. The Tromsø Study consists of six surveys (referred to as Tromsø 1–6) that have been conducted in the municipality of Tromsø from 1974 to 2008 (Table 1 and Figure 1). The Tromsø Study population includes subjects who have attended at least one of the six surveys, 40 051 subjects in total. As detailed below, the attendance rates have been high (>75% in surveys 1–5), but somewhat lower in the last survey (66%) conducted in 2007–08, due to lower attendance rate among the relatively young and those who never had participated in the previous Tromsø Study surveys. The Tromsø Study. Invitation by birth cohort and attained age in Tromsø 1–6. Invitation of total birth cohorts is marked as bold, shading indicates that samples of birth cohorts were invited. aAdjusted for deaths, emigration from Tromsø during the survey period etc. bMen only. c10% of total birth cohort and offspring of high-risk men who participated in a family intervention trial after the second survey. dRestricted to those who participated in the second visit in Tromsø 4. e 40% of the total birth cohorts. f10% of the total birth cohorts The Tromsø Studya aExamination year, age groups included and attendance rate. Number of subjects (n), mean age in the six different surveys according to gender and attendance. b20–49 years in women. cAll men aged 20–61 years and women aged 20–56 years were invited (see text). The Tromsø Studya aExamination year, age groups included and attendance rate. Number of subjects (n), mean age in the six different surveys according to gender and attendance. b20–49 years in women. cAll men aged 20–61 years and women aged 20–56 years were invited (see text). The six surveys had the same general design. Based on the official population registry, residents of the municipality of Tromsø were invited to take part in the survey. A personal invitation was mailed about 2 weeks before a suggested time of appointment. The subjects were free to attend whenever suitable within the time frame of the study (∼1 year). The invitation leaflet included information about the survey and the examination. Non-attendees were given one reminder. Tromsø 4–6 also included a second visit with a more extensive examination of the participants. Subjects in the population who were eligible for the second-visit examinations were identified before they were to attend the first visit of the survey. If they attended this, they were invited to the second-visit examination 2–4 weeks later. Table 1 gives the basic features of the Tromsø Study with regard to year of examination, the number of subjects who attended, age groups included and attendance rate. The table also gives the mean age according to gender and attendance in each of the six surveys. The aim has been to include large, representative samples of the Tromsø population, with invitation of whole birth cohorts and random samples. We have obtained repeated measurement from the same population. Figure 1 shows the general outline