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.