Birth cohort effect on the obesity epidemic in Denmark.

BACKGROUND: In Denmark, the obesity epidemic has developed in phases. To investigate if this has been a birth cohort phenomenon, we explored the secular trend in the prevalence of obesity among boys and young men.

METHODS: We calculated body mass index (BMI; kg/m) over time for 163,835 Danish boys examined at ages 7-11 years and for 708,342 male draftees examined at age 19 years. Obesity was defined according to age-specific criteria for boys and as BMI >or=31 kg/m for men. We examined trends in the prevalence of obesity from 1930 through 1999, expressing time as the subjects' year of measurement and as year of birth.

RESULTS: When expressed by year of measurement, 4 phases in the prevalence of obesity emerged for the boys and the young men, but they occurred at different times. However, when expressed by year of birth, the 2 stable periods and the 2 periods with sharp increases in obesity occurred in the same birth years. An increased prevalence of obesity was identified for birth cohorts from the early 1940s to the mid-1950s and from the late 1960s onward.

CONCLUSIONS: The trends in the prevalence of obesity were similar in boys and young men only when expressed by year of birth, which suggests that early life may be a susceptible period for developing obesity. Specifically, the increased prevalence of obesity observed for birth cohorts from the early 1940s to the mid-1950s and from the late 1960s onward suggests that changes in the early life of these cohorts resulted in an increase in obesity at later ages.