November 04, 2013 Girls in the United States are entering puberty at younger ages than they have in the past, but investigators have not been able to pinpoint the cause. Now, a longitudinal study, published online November 4 in Pediatrics, suggests a higher body mass index (BMI) plays a role in earlier breast development in white, non-Hispanic girls.
"This study demonstrates earlier maturation in white non-Hispanic girls, with greater BMI linked as a major factor," write Frank Biro, MD, the study's lead author and the director of research for adolescent and transition medicine at Cincinnati Children's Hospital Medical Center in Ohio, and colleagues.
"This article adds to studies providing the unsettling findings that the age of onset of breast development, in synch with, though not entirely explained by, the 'obesity epidemic,' has continued to drop," writes Dr. Herman-Giddens, an adjunct professor with the Department of Maternal and Child Health at the Gillings School of Global Public Health, University of North Carolina, Chapel Hill.
The whole distribution of puberty timing has shifted to a younger age, note Dr. Frank and colleagues. The fact that girls are undergoing earlier maturation has important clinical ramifications. For example, girls who reach puberty earlier than their peers may be at increased risk for depression and low self-esteem and be more likely to engage in substance abuse and intercourse at an earlier age than girls who reach puberty later.
In addition, the ripple effects of early maturation extend into adulthood, the authors write, including increased risk of breast and ovarian cancers.
The researchers enrolled 1239 girls from Greater Cincinnati, New York City, and the San Francisco Bay Area. The girls were between the ages of 6 and 8 years at the time of enrollment. Trained clinicians examined the girls at regular intervals for 7 years from 2004 to 2011, assessing breast development through observation and palpitation. They then determined BMI percentiles and z scores, using the 2000 growth charts from the Centers for Disease Control and Prevention.
At baseline, 39% of the black girls had a BMI in the 85th percentile or higher, as did 44% of Hispanic girls, 26% of non-Hispanic white girls, and 12% of Asian girls.
The investigators found that the onset of breast development, as defined by reaching breast stage 2 or greater, differed by clinic site, race/ethnicity, and BMI at baseline.
The mean and median age breast development varied by race and ethnicity. Black girls started developing breasts at a median age of 8.8 years, whereas Hispanic, non-Hispanic white, and Asian girls did so at median ages of 9.3, 9.7, and 9.7 years, respectively.
When the investigators examined timing of breast development and BMI, they found that as the girls' BMI increased above the 50th percentile, the likelihood of earlier breast development increased, relative to girls with a BMI below the 50th percentile (P value for trend = .001). The researchers adjusted for race/ethnicity and site.
In addition, the authors found that the onset of breast development in white girls occurred at younger ages than reported in previous publications but that "black girls continue to experience breast development earlier than white girls." Much of the difference in timing of development in non-Hispanic white girls, between this study and previous ones, is likely a result of the higher BMIs in the current study population, the authors note.
The authors conclude that having a higher BMI is the strongest predictor of earlier onset of stage 2 breast development.
Dr. Herman-Giddens notes that our society may have to live with uncertainty as to what factor or combination of factors is causing girls to start puberty earlier than they did decades ago.
The obesity epidemic is an important factor in the decline in age of onset for puberty in girls, she writes, but she also notes that the study found that BMI accounted for 14.2% of the variance of all covariates in the researchers' model."
Although obesity is implicated as a 'prime driver,' factors involved in these secular changes are far more complex," she writes. "Extensive interacting variables are known to be associated with earlier development in addition to weight and genetics: certain intrauterine conditions and exposures, preschool high-meat diets, dairy products, low fiber intake, isoflavones, high-stress families, absent fathers, certain endocrine disruptors, the microbiome as it influences weight, epigenetics, light exposure, hormone-laced hair products, insulin resistance, activity level, geographical location, and others."
Pediatrics. Published online November 4, 2013.