Background: Improvements in environmental conditions may result in an earlier onset of thelarche. However, its onset is not homogeneous among different population groups.
Aim: To assess the relationship between nutritional status, ethnicity and age of thelarche.
Material and methods: Cross-sectional study of girls in second through sixth grade, attending 165 schools located in Chile's Araucania region. Of these, 231 girls who presented thelarche (breast button) were selected. The girls' surnames were used to identify their ethnicity: indigenous had three or four last names of Mapuche indigenous origin (n =113), while non-indigenous were those who only had Chilean-Spanish last names (n =118). Weight, height, waist circumference and skinfold thicknesses were measured and socio-economic background information was collected through a home interview.
Results: The median age of thelarche was 10 years and 4 months, regardless of ethnic group. Multivariant models showed that an increase of one z score unit of body mass index (BMI) decreased the median age of thelarche by 5.6 months (CI: -7.24 to -3.90), controlling for the effect of different covariables. A decrease in one z score of height retards the median age of thelarche by 5.5 months (CI: 4.02 to 6.98). Ethnicity did not influence the age of thelarche.
Conclusions: The age of thelarche found by us is similar to that reported in international studies, it comes earlier as weight increases, is delayed as height decreases and is not related to ethnicity.