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 pubertal stage in subjects similar in age. Design, subjects and measurements: Height and weight were recorded cross-sectionally in school subjects from three provinces in central Italy. The subjects were subdivided into three groups: (1) 4271 school subjects (2125 males and 2146 females: 8.-15.5 y old). in whom the pubertal development was also recorded, were selected to subdivide BMI values according to pubertal stage 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, separately; and (3) 1919 females (10.5-11.5 y old), who had presented their menarche within the previous 6 months, were selected to subdivide short-term post-menarche BMI values according to age. Results: The medians and interquartile ranges of BMI varied according to age 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 proved to be significant between stage I and stage II (P < 0.05) in IO-v-old females. but not in 10-11-y-old males. The Kruskal-Wallis test performed in subjects similar in pubertal stage demonstrated that significant differences among the medians of BMI at different ages existed only in females at stages II and III. A significant positive trend was observed in both genders according 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 significant 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 age in subjects similar in pubertal stage. BMI values were significantly higher in post-menarche girls as compared to pre-menarche girls similar in age (P < 0.001). However. at partial regression analysis BMI values were influenced by pubertal stage and, to a lesser extent, by age, but not by menarcheal status. An inverse association between short-term post-menarche BMI and age was observed. with the highest values in girls presenting menarche at 11 y of age (P < 0.05). The negative trend was demonstrated at the z-test for 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 evaluated in adolescents.