Development of the obesity epidemic in Denmark: Cohort, time and age effects among boys born 1930-1975

OBJECTIVE: A global epidemic of obesity is developing, but its causes are s till unclear. In Denmark, two periods of steep increases in prevalence of o besity have occurred among young men born in the 1940s and 1960 - 70s. This study investigated the preceding changes in prevalence of obesity and in t he entire body mass index (BMI = weight/height(2)) distribution by birth co hort, calendar time and age among Danish school boys. METHODS: Children attending Copenhagen schools 1937-1983 had annual health examinations, from which we computerized 1037 468 measurements of height (m ) and weight (kg) of 161314 boys aged 7-13y. Obesity was defined as age-spe cific 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, 25ht, 75th and 95th percentiles of the age-specific BMI were e stimated for each birth cohort. RESULTS: The prevalence of obesity, defined by the 99.9 percentile, increas ed at all ages during the same birth years as among the young men, and, acc ordingly, at earlier calendar years. The prevalence of obesity, defined by the 95.0 percentile, showed a distinctly different pattern: a sharp increas e, 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 i n those defined by the 99.9 and 95.0 percentiles. The median BMI showed sma ll fluctuations, parallel at all ages. The standard deviation and right-sid ed skewness increased until birth year 1950, but were almost stable thereaf ter. 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 percenti le 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 ag e-independent increase in the late 1940s. The development of the obesity ep idemic is a heterogeneous phenomenon that has involved changes in environme ntal influences starting at preschool ages and affecting different subsets of the population, either because of selective exposure or particular susce ptibility.