Friday, October 16, 2015

Why Do Girls and Boys Reach Puberty Younger Than They Used To?

Girls, and probably boys, are reaching puberty years younger than they did in our great-grandparents' generation.  Why?  There's no shortage of explanations, but the primary reason is probably quite simple.

In the 1980s, a Duke public health researcher named Marcia Herman-Giddens began to notice something strange.  While the textbooks said that most girls should begin showing signs of puberty after age 11, the majority of girls in the pediatric clinic where she worked were hitting this milestone before age 10 (1).  

This led her to conduct a study of more than 17,000 girls, confirming that the first signs of puberty were occurring before age 10 both in girls of European and African descent in the US (2).  Although the initial research was controversial, several other studies have bolstered the finding that puberty is occurring substantially younger in girls (and probably boys) than it used to.

The further back we look in history, the later puberty begins.  Among the Hadza, perhaps the last true hunter-gatherers left in the world, menarche (onset of menstruation) occurs at an average age of 16.5 years (Marlowe. The Hadza. 2010).  Historically, among the Ache, hunter-gatherers in Paraguay, the mean age of menarche was 15.3 years (Hill and Hurtado. Ache Life History. 1996).  Herman-Giddens found that in the modern US, menarche occurs at an average age of 12.2 years in girls of African descent, and 12.9 years in girls of European descent.  The limited data we have suggests that age of menarche in Europe and the US 150 years ago was similar to that of hunter-gatherers (3).


Explanations abound for the decline in the age of puberty in girls and boys.  Some contend it's due to the artificial hormones in conventionally produced milk.  Others attribute it to estrogen-like endocrine-disrupting chemicals in plastic and agricultural chemicals.  But there's a much simpler explanation that also happens to have some very convincing evidence behind it: we're bigger and fatter than we used to be.

As part of the research for my upcoming book The Hungry Brain, I recently interviewed Mark Wilson, a researcher at Emory who studies reproductive health (among other things) using rhesus monkeys.

Wilson pointed me to the research of Ei Terasawa and colleagues at the University of Wisconsin-Madison, who published a paper in 2012 reporting the effects of a high-calorie diet on puberty in female rhesus monkeys (3).

At the age of 12 months, monkeys were randomly assigned to receive either a standard low-fat/normal-calorie diet, or a higher-fat/high-calorie diet.  The animals were allowed to eat as much of each diet as they wanted.

As expected, the monkeys eating the high-calorie diet grew faster and fatter.  Yet the effects on sexual maturation were even more striking.  The high-calorie group reached menarche at 19.8 months, while the normal-calorie group didn't reach menarche until 25 months.


In this study, earlier menarche was associated with hormonal differences.  High-calorie-fed monkeys had higher levels of leptin, insulin-like growth factor-1 (IGF-1), follicle-stimulating hormone, and a trend toward higher luteinizing hormone.  These hormones are all involved in growth and sexual maturation.

In 2013, Mark Wilson and colleagues published a paper attempting to uncover the mechanism behind these hormonal effects (4).  To do this, they randomly assigned two groups of rhesus monkeys to receive either leptin injections, or injections not containing leptin, from ages 12 to 30 months.  All monkeys were on a standard, normal-calorie diet.  Remember that leptin is a signal that informs the brain of the amount of fat mass we carry.  Higher leptin makes the brain think there's more fat around.

Just as Terasawa had observed with a high-calorie diet, leptin caused the monkeys to mature faster.  "The girls that were getting leptin grew faster", explains Wilson.  "You had increased growth hormone particularly at night, you had increased IGF-1.  It all made sense."  And, most importantly, they reached menarche earlier.

As Wilson says, it all makes sense.  Energy is perhaps the most fundamental requirement for reproduction, and the reproductive system is therefore very sensitive to energy status.  As usual, the brain is at the helm of this regulatory system.

Since leptin is the main signal that communicates the amount of energy stored in the body to the brain, it plays a key role in the onset of puberty.  We actually know quite a bit about how this works.  Puberty results from a hormonal cascade originating in the hypothalamus of the brain and cascading down to the pituitary gland and the gonads.  The hypothalamus is also the primary part of the brain that detects and regulates body energy status, and neurons regulating body energy status and reproductive function are intimately connected.  So it makes perfect sense that leptin, and ultimately calorie intake and body fatness, regulates the hormonal cascade that determines when puberty happens*.

Simply stated, we're hitting puberty earlier because our food environment and lifestyle are driving us to eat excess calories.  This is consistent with our increase in body fatness over the last century, and particularly over the last 35 years.  My upcoming book The Hungry Brain will attempt to explain why we often eat too much, despite our best intentions.

* This also explains why women who are very lean or have high physical activity levels often stop menstruating.  Their leptin levels are too low, and the brain shuts down ovulation because it doesn't think there's enough energy in the body to support reproduction.


DrGecko said...

As music historians (such as myself) know, it's not just girls. Boys' voices used to break with they were 17 or 18 years old, so there were boy sopranos with a good ten years of experience under their belts. There's no way to duplicate that today.

tomR said...

This (high level of calories) might also mean shorter lifespans.

Tom A said...

You don’t think soy has anything to do with it? Soy-based formula is linked to all sorts of problems, and “soy milk” can cause “persistent sexual arousal” in women -- among other things that would seem to be hormone related. Soy oil is in everything these days. Consumption should be zero in my opinion.

Colin Macdonald said...

Thanks for this, you've made me realize that I'd settled on the environmental estrogen hypothesis without a sound basis of evidence. This is an intriguing alternative.

Aaron Blaisdell said...

Fascinating! This is consistent with the integral role of the timing of developmental processes in the development of phenotypic traits. There is resurgence in evolutionary biology (including within the neurosciences--which I discuss in my recent JevoHealth paper on play and human brain development) in recognizing the role of heterochrony (changes in timing of developmental processes in a descendant relative to its ancestors) in the evolution of new or altered traits. Evolution acts most powerfully through that which is easiest to change--which often turns out to be the timing of developmentally-programmed traits. If the modern world keeps up its leptinogenic environment, we may find the trait of early sexual maturity become canalized in our species. I wonder what the long-term consequences of this would be for human society and civilization. Sounds like a fascinating topic for a sci fi story, because it will take too many generations for the human species to lose its phenotypic plasticity in development of sexual maturational processes for us to witness it ourselves. (That is, imagine 500 or 1000 years from now, after following the current Western industrial diet the whole time that folks begin to "go paleo". They might find that many of the problematic traits that were attributed to poor diet no longer respond to a "good" diet. Of course, adaptation to the industrial diet might change our definition of a "poor" versus "good" diet!)

David L said...

As a skier, I recently saw a ski movie shot at a resort in the late 60s. I was so shocked to see how slender everyone was! I mean, these were athletic people and yet they were so skinny, even in their ski outfits.

When I look at the ski slopes these days, there's definitely a lot more body mass out there. However, I don't feel that all the calories have gone to fat. I don't have the scientific data in front of me, but I am convinced that, in addition to packing more pounds of fat, they also have a greater muscle mass, too. Or has all the additional calories gone to fat -- don't think so.

natronekemp said...

A question crossed my mind as I read this blogpost. Why will some women experience peri-menopause earlier than others? Can what is mentioned in this article be somewhat applicable? Will love to hear you thoughts.

kateR said...

Could it not be simply more nutrient-dense diets, rather than diets that fatten? I am thinking of my cohort (Canadian, born 1960) and how 11- 12 years was the age most of the girls began menstruating. It was unusual for children to be overweight - probably fewer than one child in every classroom (of 30 or so). But we were all middle-class with no food shortages. And children did often grow to be taller than their parents. Most foods were prepared in the home (no school lunches). My (lean)sons are taller than both of their tall parents -- I can't imagine that their children will outgrow them!!

tomR said...

People are also different now than then. In the past generally almost every person who had been able to have children (I mean both biological fertilit, as well as having a spouse/partner) had them. Then birth control become popular, people colud excuse themselves from procreation, or just minimize it. So you can expect more children in a new generation to come from parents with more hormones: who feel reproductive needs, rather than low-hormone parents - who don't. Also people would naturally come from ones that have hormones earlier at larger percent than from those who have delayed outset. Demographics don't seem to be good here, just one example: long lived people have worse fertility, than normal-lived ones:
(don't worry about this longevity/fertility being a rule of biology, aging strategies can vary:
That's not all: in the past people married by the geographic proximity; now it's more about assortative mating. That produces more on the extremes: greater great results, but also worse bad results.

Alexandra Preston said...

There is research suggesting that a more plant-based diet is associated with later menarche, while a diet higher in meat is associated with earlier puberty. It was discussed in class so I am sorry that I don't have the link.

Cryptandra said...

I wonder if the early onset of puberty and/or increased child calorie intake is linked to the increased incidence of certain childhood cancers.

Tony Bednarowski said...

This is a great article as well as super fascinating. This totally makes sense as Stephan makes a great comparative between current fat mass as a pre-determinant to early puberty vs. lean athletic women losing the ability to ovulate. The one thing I have a hard time believing is in the “calories hypothesis” and the importance of energy in vs. energy out. Although it may be true that all calories have the same amount of energy (one food calorie contains 4,184 Joules of energy), in that respect, a calorie is a calorie, but things aren’t quite that simple when it comes to our body, and there a many factors that go into the biological process of different foodstuff, namely the thermic effect, the satiating value, and the glycemic response of different food calories.

Sarah Rowe said...

This is an interesting theory, but I don't know that it holds up to empirical observation. In my school, almost every girl had reached menstruation by 12-13. This was true amongst at least every school in the area, and is considerably earlier than the ~15.5 years of the Hadza. Nearly 0% of the students in this school were overweight or obese, so clearly other factors are in play. I do think calorie excess could be a major contributor to the ~8-year-old onset of menstruation seen too often today, in combination with other issues such as environmental estrogens. Calorie excess alone seems like an insufficient explanation.