Increase in weight in all birth cohorts in a general population: The Tromsø Study, 1974-1994.

BACKGROUND Obesity is a risk factor for a number of chronic diseases. Few longitudinal studies have examined changes in body mass index (BMI [calculated as weight in kilograms divided by the square of the height in meters]). OBJECTIVE To investigate the changes in mean BMI and the prevalence of obesity in a large cohort examined several times during a 20-year period. METHODS Mean BMI, the percentage of subjects with low BMI (<20 kg/m(2)), and the percentage who were obese (BMI > or =30 kg/m(2)) were determined in a large population of men and women who were examined up to 4 times during a 20-year period (1974-1994/1995). In a longitudinal design, we observed 3541 men who attended all 4 screenings (1974-1994/1995) and 4993 women who attended the last 3 screenings (1979/1980-1994/1995). RESULTS The age- (25-49 years) and sex-adjusted mean BMI increased 1 kg/m(2) in men from 1974 to 1994/1995 and 0.9 kg/m(2)in women from 1979/1980 to 1994/1995. In the last survey, subjects aged 25 to 85 years were included. In most age groups, the mean BMI exceeded 25 kg/m(2) and the prevalence of obesity was 10% or higher in men and women aged 45 years or older. In the longitudinal analysis, the mean BMI in men aged 20 to 49 years increased 2.0 kg/m(2) during 20 years of observation and increased 2.4 kg/m(2)in women aged 20 to 49 years during 15 years of observation. The increase in BMI was larger in younger men than in older men. CONCLUSIONS Body mass index increased in every examined birth cohort (1925-1964) during the 15- to 20-year observation period. Primary prevention of further increased body weight should be a priority.