Puberty is defined as the time when a child’s body matures into an adult’s body and they become capable of reproducing. In girls, puberty usually occurs around the ages of 10 to 14 and some of the physical signs include breast development, the appearance of underarm and pubic hair, growth spurts, menstruation and acne. Biologically, puberty in girls is thought to be triggered by a surge in a hormone called GnRH (gonadotropin releasing hormone), which stimulates the ovaries to release a different hormone called oestrogen. Oestrogen is the primary female sex hormone and is crucial for breast development, as well as some of the other physiological changes seen during puberty.
The average age of puberty has fallen over the past 200 years. In the nineteenth century, girls typically entered puberty between the ages of 15 and 17. The reason for this change is thought to be multifactorial, but includes improved nutrition, higher BMIs and an increased number of endocrine disruptors in the 21st century environment. Whilst some researchers have suggested that the age of puberty is continuing to fall, this is controversial and other studies have found little to no change in the past 20 years.
There are, however, girls that enter premature puberty, also called precocious puberty, and this can occur as early as age 7 or 8. Thought to be due to a complex interaction of genetic, endocrine and environmental factors, girls who experience this are at increased risk of psychosocial and health issues as they grow. Not only does early puberty frequently lead to low self-esteem and ongoing body image issues, but the increased exposure to the so-called sex hormones can put these females at greater risk of developing breast and ovarian cancer.
One of the key factors thought to be involved in precocious puberty is increased environmental pollutants. These pollutants often contain chemicals which enter the body through water, air and food; they can also transfer from mother to baby via the placenta (pre-birth), and breast milk (post-birth). As such, they are impossible to avoid. When these chemicals exhibit hormone-like activities they are referred to as endocrine disruptors, examples include phytoestrogens (naturally occuring) and diethylsibestrol (synthetically produced). They normally exert their effects by binding to hormone receptors, meaning that the natural hormones have less chance of binding to their cognate receptors. This competitive binding upsets the delicate balance of hormone levels and causes wide-ranging effects. When endocrine disruptors bind to oestrogen receptors they not only cause increased sensitivity to oestrogen, but also disrupt the levels of GnRH. As stated above, GnRH is critical for commencement of puberty.
Endocrine disruptors have been identified in pesticides, cleaning products, cosmetics, dyes and plastics, thus exposure to them is inevitable. If this is the case, then why is it that only some girls experience precocious puberty?
Well the exact answer to this question remains elusive, however, many factors are known to affect the onset of puberty including genetics, racial background (eg. African Americans typically enter puberty earlier than white Americans), and BMI (the higher the BMI, the earlier puberty starts). In fact, this last factor might explain why there are discrepancies in the recent scientific studies. Those carried out in the USA, where BMIs are steadily increasing, have shown that age of puberty continues to fall; those carried out in other parts of the world, such as Northern Europe, where BMIs have remained more stable, have not seen this reduction.
One final point to make, which adds to the complexity of the topic, is that some endocrine disruptors cause the opposite effect and delay puberty. This is again due to the artificial moderation of hormone levels, and further highlights the difficulty in establishing a definitive answer when it comes to ‘normal’ age of puberty.
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