In 1983, 1298 subjects participated in a school-based study. Thirty six schools, 18 vocational and 18 high schools, were selected to be representative of the whole country, and all pupils in the selected classes were invited to participate. No subjects refused to participate. Among these, every fourth pupil on the class lists were invited to participate in the present study. Only 2.3% refused to participate. In the first cross-sectional study in 1983, 133 boys and 172 girls, at that time 16–19 years of age, took part. In 1991, 98 men (73.7%) and 137 women (79.7%) from the original sample, now 23–27 years of age, participated in a second examination.
Variables include demographic and health habit variables, blood pressure, blood lipids, fatness and physical fitness. Body height was measured to the nearest cm, and body weight to within 100 g. Skinfolds were measured with a Harpenden skinfold calliper above the patella on both knee's, and 1/3 of the distance from umbilicus to spina iliaca anterior superior on both sides of the abdomen. The sums of these four skinfolds were used to calculate body fat% using the following equation:
Body fat (kg) = 0,184 × SFabd + 0,145 × SFknee - 0,66, where SFabd is the sum of right and left abdominal skinfolds and SFknee is the sum of the skinfolds above the left and right knee . Arterial BP was measured with a mercury sphygmomanometer after 20 min of supine rest followed by another 5 min in the sitting position. A fasting venous blood sample was drawn from a superficial vein in the arm. The plasma was enzymatically analyzed for total cholesterol, HDL-cholesterol (HDL-C) and triglycerides (Gilford 3500 Auto Analyzer).
Self-reported leisure physical activity was obtained as type and quantity of organized sport activities in clubs and at the schools, of unorganized sports activities, and as other physical activities including physical labor, walking, cycling, and dance. A physical activity index was constructed from the reported intensity and duration of the activities. Maximal oxygen uptake was directly measured during a maximal progressive cycle test. In the study from 1984, expired air was collected in Douglas bags and analysed with an Electrochemistry (S-3a) analyser, and in the study from 1991, in an open MedGraphics system. Detailed description of all methods has been published previously [9, 10].
All data were analysed using SPSS (version 11). Participants in each cross-sectional examination were ranked into quartiles by sex in four CVD risk factors all related to the metabolic syndrome. Risk factors were the ratio between total cholesterol and HDL, triglyceride, systolic BP and body fat. The upper quartile was defined as being at risk. Clustering of risk was defined in those subjects having two or more risk factors (~15% of the subjects). Logistic regression was used to calculate the odds ratio (OR) between quartiles of fitness for being a case at the first examination, and the same analysis was performed at the second examination. Logistic regression was also used to calculate tracking of clustered risk factors. The OR of being a "case" at the follow up for those who were a "case" at the first examination compared to those who were a "non-case" at baseline.