'Extreme' teenagers

Health researchers commonly use body mass index (BMI), calculated as weight in kilograms divided by the height in meters squared, in weight-related health studies. Sigrid Bjørnelv of the Norwegian University of Science and Technology and colleagues write that changes in these measurements across society over periods of time often reflect changes in nutrition. Better nutrition increases both height and weight, and reduces health problems connected with malnutrition. However, increases in BMI can also point to poor diet and lack of exercise.

Bjørnelv and her colleagues analyzed height, weight and calculated BMI data for 6774 14-18-year olds who participated in the Young-HUNT study in 1995-97. They compared the data with 8378 adolescents in the same age group collected by Norway's National Health Screening Service in 1966-69.

The researchers found significant changes between the two periods. Height and weight increased significantly in both sexes and all ages, while average (mean) BMI increased significantly in boys of all ages but only in 18-year old girls. Mean BMI did not change for girls aged 14-17 years. Critically, the team revealed a change in distribution of BMI, with an increase in the upper percentile values and a decrease in the lower percentile values.

While the increase in the highest percentile implies better nutrition and an increase in prevalence of obesity amongst adolescents in agreement with other studies, explains Bjørnelv, the decrease in the lower percentile values implies that the thinnest adolescents in 1995-97 had a lower BMI than their counterparts in 1967-69. This finding requires further study.