Body mass index in children and adolescents according to age and pubertal stage

Objective: To evaluate the dependence of body mass index (BMI) values on pu bertal stage in subjects similar in age. Design, subjects and measurements: Height and weight were recorded cross-se ctionally in school subjects from three provinces in central Italy. The sub jects were subdivided into three groups: (1) 4271 school subjects (2125 mal es and 2146 females: 8.-15.5 y old). in whom the pubertal development was a lso recorded, were selected to subdivide BMI values according to pubertal s tage and age; (2) 6345 females (10.5-14.5 y old), who were asked whether or not they had had their first menstrual period, were selected to subdivide BMI values according to age in pre-menarche and post-menarche girls, separa tely; and (3) 1919 females (10.5-11.5 y old), who had presented their menar che within the previous 6 months, were selected to subdivide short-term pos t-menarche BMI values according to age. Results: The medians and interquartile ranges of BMI varied according to ag e and pubertal stage. Kruskall-Wallis test performed in subjects similar in age demonstrated that significant differences existed among the medians of BMI values of subjects at different pubertal stages in 12-14-y-old males ( P < 0.05), and in 11-14-y-old females (P < 0.001). The difference also prov ed to be significant between stage I and stage II (P < 0.05) in IO-v-old fe males. but not in 10-11-y-old males. The Kruskal-Wallis test performed in s ubjects similar in pubertal stage demonstrated that significant differences among the medians of BMI at different ages existed only in females at stag es II and III. A significant positive trend was observed in both genders ac cording to pubertal stage for BMI values of subjects similar in age (z-test for trend, P < 0.01). On the contrary, a negative age trend proved to be s ignificant in females at stages I (P < 0.01), II (P < 0.01) and III (P < 0. 001), but not in males when the subdivision of BMI was made according to ag e in subjects similar in pubertal stage. BMI values were significantly high er in post-menarche girls as compared to pre-menarche girls similar in age (P < 0.001). However. at partial regression analysis BMI values were influe nced by pubertal stage and, to a lesser extent, by age, but not by menarche al status. An inverse association between short-term post-menarche BMI and age was observed. with the highest values in girls presenting menarche at 1 1 y of age (P < 0.05). The negative trend was demonstrated at the z-test fo r trend (P < 0.001). Conclusions: BMI values depend on pubertal degree of maturation, especially in girls. This influence should be taken into account when BMT is evaluate d in adolescents.