The Relationship between Body Mass Index and Body Size Dissatisfaction in Young Adolescents: Spline Function Analysis

This study assessed how body size dissatisfaction (BSD) varies in relationship to specific body mass index (BMI) values in a sample of preadolescents. A novel statistical approach based on spline function, suitable to assess in detail how two variables are related, was used. The study was conducted between December 2004 and March 2005. Students (aged 11 to 14 years) from seven selected secondary schools in Verona, Italy, were invited to participate. The final study group included 678 subjects. BSD was assessed using the Body Image Assessment Procedure. BMI values were expressed as z scores. It was found that, in the total sample, slightly underweight subjects (BMI z scores=−0.5) had no BSD. BSD progressively increased (current body size > ideal body size) for BMI z scores >−0.5 and became negative (ideal body size > current body size) for BMI z scores <−0.5. In boys, average weight subjects had no BSD. BSD progressively increased for BMI z scores >0 and became negative for BMI z scores <0. In girls, moderately underweight subjects (BMI z scores=−1) had no BSD. BSD progressively increased for BMI z scores >−1 and became negative for BMI z scores <−1. Although sex significantly moderated the relationship between BMI and BSD (P<0.001), socioeconomic status did not (P=0.459). Because average weight and slightly underweight young girls desired a thinner body, our study suggests that these subgroups should receive particular attention in public health programs as well as in dietetics clinical practice.