Background. In 1991, reference curves of Body Mass Index (BMI) for children and adolescents were published by Hammer et al using data from the first National Health and Nutrition Survey collected from 1971-74. Due to small numbers of other groups, these curves included only whites. In 1994, the curves were proposed as guidelines for identifying overweight in adolescents. There are two potential problems with this. First, adolescents in the 1990s are fatter than their 1970s counterparts. Have the BMI curves maintained their shape but shifted upwards or are the trajectories different? Second, do curves developed for whites describe the BMI trajectories for blacks?

Methods. The Cardiovascular Health in Children (CHIC) study is a longitudinal study begun in 1990 to examine the emergence of cardiovascular disease risk factors in North Carolina youth. In these analyses, data from 10,484 CHIC subjects ages 8-16 years were used to generate smoothed percentile curves of BMI by age, using quadratic fitting procedures. Subjects were 51% female, 49% male, 78% white and 22% black. To examine differences in the 1970s versus 1990s, we compared the Hammer curves with CHIC curves using white subjects. To examine racial differences in the curves we compared black and white subjects within CHIC, by gender.

Results. For females, not only were all the percentile curves shifted upwards, indicating greater BMIs in the CHIC females, but also the span from the 5th to the 95th percentiles was wider, indicating a greater range of BMI. The same results were seen for males with an even greater widening of the span.

After age 9-10 all the curves were higher for black girls, with a wider span than for whites. Above the 50th percentile, the upward shift for black females was quite steep. For males, all the curves were shifted upwards for blacks, especially above the 50th percentile. The span is similar in both races.

Conclusions. Youth in the 1990s are fatter than in the 1970s but the curves are not merely shifted upwards. The widening span indicates that some are resisting this trend and the differing shape of the curves indicates that the way BMI increases through adolescence is different. There are racial differences in both the span and shape of the curves as well as the levels, indicating that the Hammer curves do not accurately describe the developmental trajectory of BMI for blacks.