Secular trends in body mass index by birth cohort in eastern Finland from 1972 to 1997

OBJECTIVE: To describe changes in body mass index (BMI) with age over a 25 y period, and to investigate whether those changes differed by birth cohort, gender or education. DESIGN: Six cross-sectional population surveys repeated in eastern Finland every fifth year between 1972 and 1997. Synthetic birth cohorts were created by dividing subjects into twelve 5 y groups according to birth year. SUBJECTS: Independent random samples from the national population register including men and women aged 25 - 64 y, the final sample comprising 18 439 men and 19 501 women. The oldest birth cohort included subjects born in 1913 - 1917 and the youngest were born in 1968 - 1972. MEASUREMENTS: Weight and height were measured, and data on educational level were collected by a self-administered questionnaire. RESULTS: A BMI increase with age was more prominent in women (4.2 kg/m(2) in 25 y) than in men (3.3 kg/m(2) in 25 y), and was very similar in all birth cohorts. In men, the BMI increase with age varied across cohorts. Among birth cohorts participating in each survey, the BMI increase over the 25 y period was most prominent (3.9 kg/m(2) in 25 y) in men born between 1943 and 1947. Among all birth cohorts, the strongest upward trend (0.2 kg/m(2) per year) was observed in men born between 1953 and 1957. CONCLUSIONS: Weight gain with age has remained unchanged in women over the 25 y period, whereas in men, the younger the birth cohort, the more prominent was the BMI increase with age.