OBJECTIVE

A global epidemic of obesity is developing, but its causes are still unclear. In Denmark, two periods of steep increases in prevalence of obesity have occurred among young men born in the 1940s and 1960-70s. This study investigated the preceding changes in prevalence of obesity and in the entire body mass index (BMI = weight/height2) distribution by birth cohort, calendar time and age among Danish school boys.

METHODS

Children attending Copenhagen schools 1937-1983 had annual health examinations, from which we computerized 1,037,468 measurements of height (m) and weight (kg) of 161,314 boys aged 7-13 y. Obesity was defined as age-specific BMI exceeding the 95.0, the 99.0 and the 99.9 percentile among those born 1930-1934, the latter corresponding to the prevalence of obesity among the young men in these cohorts. The median, standard deviation, skewness, and the 5th, 25th, 75th and 95th percentiles of the age-specific BMI were estimated for each birth cohort.

RESULTS

The prevalence of obesity, defined by the 99.9 percentile, increased at all ages during the same birth years as among the young men, and, accordingly, at earlier calendar years. The prevalence of obesity, defined by the 95.0 percentile, showed a distinctly different pattern: a sharp increase, irrespective of age, during the calendar years 1947-1949, and thereafter a stable level until the 1970s, where a further modest increase began. The prevalence defined by the 99.0 percentile showed a mixture of the trends in those defined by the 99.9 and 95.0 percentiles. The median BMI showed small fluctuations, parallel at all ages. The standard deviation and right-sided skewness increased until birth year 1950, but were almost stable thereafter. The pattern of changes in the quartiles mostly reflected those in the median.

CONCLUSIONS

The prevalence of obesity defined by the 99.9 or 99.0 percentile has increased in Danish boys born in the 1940s and since the mid 1960s, without corresponding changes in the central part of the BMI distribution. When defining obesity by the 95.0 percentile, there was a sharp distinct age-independent increase in the late 1940s. The development of the obesity epidemic is a heterogeneous phenomenon that has involved changes in environmental influences starting at preschool ages and affecting different subsets of the population, either because of selective exposure or particular susceptibility.