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During preadolescent childhood, the growth of the mammary gland is isometric, keeping pace with the general growth of the child’s body.1 At thelarche, the onset of secondary breast development occurring at puberty, the female mammary gland undergoes significant further development of its previously primitive ductal and lobular structures. It is the deposition of adipose tissue within the mammary gland, however, that accounts for the majority of the increase in breast size associated with puberty.2 Body mass index (BMI) has been found to relate to both the timing of thelarche and the composition of the adult female breast.

Childhood Obesity and Thelarchal Age

Age at thelarche is known to vary by race and ethnicity. A recent prospective study of a cohort of more than 1200 girls in the United States reported the median onset of thelarche (Tanner stage 2) to be 8.8 years for African American girls, 9.3 years for Hispanic girls, and 9.7 years for both white non-Hispanic and Asian girls participating in the study.3 Excess childhood weight has also long been associated with earlier breast development, independent of race and ethnicity.4 Some studies have relied on visual inspection as a means to assess breast development, raising the potential for excess fatty tissue deposition among obese girls to be confused with true glandular tissue, thus confounding results. However, more recent studies have made use of palpation by trained examiners to determine the onset of glandular breast development, and these studies have likewise demonstrated correlation between higher BMI and younger age at thelarche. The Breast Cancer and the Environment Research Centers (BCERC) study found a significant correlation between higher BMI and onset of breast development with girls in all BMI categories above the 50th percentile progressively more likely to have reached thelarche than those below the 50th percentile, adjusting for race and ethnicity. The authors reported that while race accounted for 4.4% of the variance in thelarchal age, BMI was the strongest predictor of earlier age at thelarche of all covariates included in their statistical model, accounting for 14.2% of the variance.3

There is growing evidence of a decline in age at female puberty, specifically in the age at thelarche, over the past several decades.5,6 The concurrent trend of increasing prevalence of childhood obesity in Western populations further suggests a relationship between childhood BMI and pubertal breast development. Childhood obesity may have a direct causal impact on the timing of pubertal development through, for example, obesity’s influence on endogenous hormonal levels. Studies in animal models have demonstrated causal links between feeding and pubertal development and support the possibility of a direct causal relationship between increasing obesity and earlier age at thelarche.7 However, other potential mechanisms for the association between childhood BMI and timing of, and trends in, pubertal development have been proposed. One possible such mechanism is population ...

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