Tuesday, November 5, 2013

Age of Puberty: Why This Isn't About Fat (or Facts)

The Huffington Post threw up another scare-mongering article about how fat people are ruining the world yesterday, with a classic "think of the children" twist.  The reporter breaks the startling news that onset of puberty (both menarche and spermarche) are occurring at a younger age than in previous generations.  He draws the conclusion that obesity is to blame.  Of course, since fat people are to blame for everything from global warming to the decline of the mitten industry in the U.S., I'm starting to experience a little fatigue as to what people are willing to hang on my body in order to avoid facing reality. 

But let's analyze.

#1:  Correlation is not causation.  Let's say it again: CORRELATION IS NOT CAUSATION.  You cannot conclusively say that X causes Y without a well-constructed, replicable, controlled study.  This obviously cannot be done, because you would have to take identical children, make one group of them fat, and observe onset of puberty.  One of the reasons why this wouldn't work is that no one has found a way to make a body sustain large weight changes in either direction.  Another reason is that it would violate a whole lot of ethical standards to attempt it.  So instead, the studies listed in this article seem to take the false claim that we are getting fatter by the minute and the real fact that we are experiencing early puberty and make the correlation seem more ominous than it actually is.

My favorite correlation to illustrate this kind of correlation fail is an oldie but goodie. Both murder and consumption of ice cream have a strong positive correlation.  Is rocky road driving us to kill?  Not so much.  Could it be that hot weather increases both temper and temptation of frozen goodness?  Maaayyybee.....

#2:  We are experiencing earlier puberty....but this isn't news.  In fact, we have been doing so every generation since the 1840s.  Until the early 18th century women didn't experience puberty until age 17. About that time, we saw the beginning of a secular trend where each generation was a few centimeters taller and experienced puberty a few weeks earlier than the previous generation. The current theory is that steady advancement in health and nutrition is the cause. Less developed countries have not experienced the same rate of change, possibly due to poor health care and malnutrition. Even today people in less developed countries hit puberty later than people of the same ethnicity in developed countries (Eveleth & Tanner, 1990; McDowell et al., 2007; Susman & Rogol, 2004).

So the alarming news that children today are experiencing earlier puberty than 15 or 30 years ago by a few months is neither alarming, nor news.   It is like shouting frantically from the rooftops that people are taller than they were a hundred years ago. 

Here's a fun fact though. Stress can cause early puberty (Warshofsky, 1999; Ellis & Garber, 2000, Romans et al., 2003) including stress from family and society. You could easily make the case that the constant pressure to conform to unrealistic beauty and other standards in our culture, including infants placed on weight-loss diets, are contributing to early onset of puberty.  It makes as much sense as blaming adipose tissue. 

2 comments:

Katie said...

Classic! Loving this blog. I recently posted one on how journalists use obesity stats to lend credibility and weight (pun intended...) to otherwise totally useless articles (http://bit.ly/19luUc5) . It's painful and dangerous. Thanks for this :) Are you on Twitter?
- Katie

JoGeek said...

@Katie: I'm not on twitter, sorry. I'm something of a social networking Luddite. I didn't even get a Facebook account until this year :-)