>Susan Smallwood writes:

>I was talking with
>my mother about all the great things I am learning in my breastfeeding
>classes and the topic of girls starting menstruation earlier now than
>they did even 2 generations ago came up.  She seemed to think it has to
>do with all the hormones they are giving cows now.  I thought this may
>hold some water so I thought I would run it by you gals and see if there
>is any information out there on this topic.

First of all, girls are NOT starting their periods earlier than they
did two generations ago.  Average age of menses is 12.8 years in the
U.S., and that figure has not changed significantly over the past
half century.

Instead, what seems to be happening is that average onset of
THELARCHE is going down; that is, the age at which pubic hair growth
and breast development begins is becoming younger and younger.  Thus,
compared to two generations ago, there is now a longer window of time
between the beginning of puberty and the beginning of menstruation.

No one knows what factors are responsible for this.  There are at
least three competing hypotheses, none mututally exclusive.  One
attributes the downward shift to increased rates of childhood
obesity.  (Body fat produces a certain amount of estrogen.)  Another
attributes it to milk consumption.  Another blames
endocrine-disrupting chemical contaminants in the environment, such
as pesticides, dioxins, etc., which can sometimes mimic estrogen or
potentiate its effects.

What we need are studies that attempt to tease these factors apart.
Is accelerated puberty seen in lean girls as well as fat ones?  In
milk drinkers as well as those who are lactose intolerant?  In those
with higher body burdens of certain synthetic toxics?  etc.

I'm personally skeptical of the milk hypothesis--but willing to
consider it--if only because milk consumption among children is
lower, not higher, than it was a half century ago. Consumption of
soft drinks has replaced milk in a lot of childhood diets in the U.S.

The toxicologists here at New York's Program on Breast Cancer and
Environmental Risk Factors are looking at this phenomenon closely
because it may hold clues to breast cancer risk.  If interested,
search our web site for publications on early puberty:
http://www.cfe.cornell.edu/bcerf/

hope this is helpful.

Sandra


--
--

Sandra Steingraber, Ph.D.
Visiting Assistant Professor
Program on Breast Cancer and Environmental Risk Factors
110 Rice Hall
Cornell University
Ithaca, NY  14853
[log in to unmask]
www.steingraber.com

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