最近の乳房発育の早期化は思春期開始時期の早期化であり、bisphenol-A など様々な化学物質との関連が懸念される。Dr. Aksglaede は短期間での急激な変化の原因は不明であり、発達の変化が人類全体に及ぼす影響がどうなるのか問題である。
May 4, 2009, 6:00 am
Earlier Puberty in European Girls
INSERT DESCRIPTIONRuby Washington/The New York Times. Young girls appear to be maturing sooner than earlier generations.
A 15-year study of young girls in Denmark found that the average age of breast development has fallen by a full year compared to girls studied in the early 1990s.
The findings, published this month in the journal Pediatrics, add to a growing body of evidence that the timing of puberty is changing, possibly related to environmental exposure to endocrine-disrupting chemicals that mimic estrogen in the body.
While the conventional wisdom has long been that girls today mature sooner than their counterparts from earlier generations, the issue remains a source of debate in the scientific community. Part of the problem is a lack of historical data on the age of breast development, one of the earliest signs of puberty. And while studies in the United States have shown an earlier onset of puberty in recent decades, studies in Europe have failed to show similar trends. This discrepancy has led to speculation that the changes observed in the United States may really be due to differences in data collection methods among large-scale studies and changing ethnic demographics in that country.
But in the latest report, Danish researchers showed a pronounced change in the onset of breast development in just a 15-year period. The University of Copenhagen scientists tracked breast development among 2,095 girls between the ages of 5-and-a-half and 20. They compared 1,100 girls studied between 1991 and 1993 to a set of 995 similarly-aged girls studied between 2006 and 2008.
The average age for breast development among the current generation of girls was 9.86 years, compared to an average age of 10.88 years among the children studied in the early 1990s. While it has long been speculated that an increase in childhood obesity could explain earlier breast development, the difference in breast development remained significant even after controlling for body mass index, according to the study.
It’s important to note that while puberty, in the form of early breast development, appeared to be starting sooner in the Danish girls, the average age of menarche, the time when girls begin menstruating, did not change as quickly. In the study, the average age of menstruation in the girls studied between 2006 and 2008 began only three-and-a-half months earlier compared to the girls studied from 1991 to 1993. This suggests that the age at which girls fully mature hasn’t changed much in the past 40 years, but because puberty starts earlier, the duration of puberty is longer than for past generations.
Historical records show that puberty in girls in the United States and Europe changed dramatically from the mid-19th to mid-20th centuries. In the first part of the 20th century, the average age at menarche declined by two to three months each decade, falling from about 17 to 13, in both the United States and Europe. However, those changes are largely attributed to better health, nutrition and medical care among children.
The average age of menstruation hasn’t changed much since the 1970s. Today, the average age of menarche in the United States is 12.6 among white girls, 12.1 for black girls and 12.2 for Mexican-American girls. Although rates vary slightly between countries, European girls, on average, also begin menstruating between the ages of 12 and 13, according to a 2007 report from the Breast Cancer Fund.
While historical shifts in puberty appear largely due to improved health and living conditions, the more recent changes in earlier breast development are more worrisome. Studies have documented that a number of chemicals, such as bisphenol-A used to make hard clear plastic containers, may act as endocrine disruptors and have estrogenic effects on the body.
The study’s lead author, Dr. Lise Aksglaede of Copenhagen University Hospital, said it’s not clear why onset of puberty has appeared to change in such a short amount of time among the Danish school girls.
“Changes in development, such as early puberty, might reflect changes in the general health in a population,’’ Dr. Aksglaede said. “Certainly, it is worrying that we are seeing these marked changes in age at breast development over such a short period of time. We do not know why and what it might mean for the individual girl.’’
The concern is that early puberty is linked with higher breast cancer risk in adulthood. Early puberty has also been linked with social problems and depression, and is associated with high-risk behaviors in adolescence such as alcohol and drug use and unprotected sex. It’s not clear whether these concerns are the result of physiological changes that influence behavior or are explained by the social pressures girls encounter when their bodies mature.
“Whether or not these associations apply to girls who develop breasts at younger ages remain speculative,’’ Dr. Aksglaede said. “Probably, the majority of these girls will mature without any side effects. The problem is that we do not know.’’
PEDIATRICS Vol. 123 No. 5 May 2009, pp. e932-e939 (doi:10.1542/peds.2008-2491)
Recent Decline in Age at Breast Development: The Copenhagen Puberty Study
Lise Aksglaede, MDa, Kaspar Sørensen, MDa, Jørgen H. Petersen, PhDa,b, Niels E. Skakkebæk, MD, DMSca and Anders Juul, MD, DMSca
a University Department of Growth and Reproduction, Rigshospitalet, Copenhagen, Denmark
b Department of Biostatistics, University of Copenhagen, Copenhagen, Denmark
OBJECTIVE. Recent publications showing unexpectedly early breast development in American girls created debate worldwide. However, secular trend analyses are often limited by poor data comparability among studies performed by different researchers in different time periods and populations. Here we present new European data systematically collected from the same region and by 1 research group at the beginning and end of the recent 15-year period.
METHODS. Girls (N = 2095) aged 5.6 to 20.0 years were studied in 1991–1993 (1991 cohort; n = 1100) and 2006–2008 (2006 cohort; n = 995). All girls were evaluated by palpation of glandular breast, measurement of height and weight, and blood sampling (for estradiol, luteinizing hormone, and follicle-stimulating hormone). Age distribution at entering pubertal breast stages 2 through 5, pubic hair stages 2 through 5, and menarche was estimated for the 2 cohorts.
RESULTS. Onset of puberty, defined as mean estimated age at attainment of glandular breast tissue (Tanner breast stage 2+), occurred significantly earlier in the 2006 cohort (estimated mean age: 9.86 years) when compared with the 1991 cohort (estimated mean age: 10.88 years). The difference remained significant after adjustment for BMI. Estimated ages at menarche were 13.42 and 13.13 years in the 1991 and 2006 cohorts, respectively. Serum follicle-stimulating hormone and luteinizing hormone did not differ between the 2 cohorts at any age interval, whereas significantly lower estradiol levels were found in 8- to 10-year-old girls from the 2006 cohort compared with similarly aged girls from the 1991 cohort.
CONCLUSIONS. We found significantly earlier breast development among girls born more recently. Alterations in reproductive hormones and BMI did not explain these marked changes, which suggests that other factors yet to be identified may be involved.
Key Words: puberty • breast development • menarche • obesity • endocrine-disrupting chemicals • phytoestrogen
Abbreviations: Bn―breast stage number • PHn―pubic hair stage number • FSH―follicle-stimulating hormone • LH―luteinizing hormone • PI―prediction interval • CI―confidence interval • EDC―endocrine-disrupting chemicals • BPA―bisphenol A
Accepted Feb 10, 2009.
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