If you searched the archives, you might find this note I sent in on CCK, but
since it was posted under the subject heading "brain dead nursers," I offer
it again under a more apt title.
Chris

Many, many hormones are released during nursing, and among them is my
favorite, CCK, or cholecystokinin.  If you're not clear about CCK, look it up
in Kerstin Uvnas-Moberg's article "The gastrointestinal tract in growth and
reproduction," Scientific American 261, 78-83, 1989, a marvelous article and
easily accessible in many public libraries.

In a nutshell, the mother's GI tract releases CCK in response to vagal
stimulation triggered by the baby's suckling at her breasts.  In addition,
the baby's GI tract releases CCK in response to the vagal stimulation
triggered by his own sucking activity, and also in response to food,
especially fat, entering the stomach.  CCK induces sleepiness, or, to be
fancy, postprandial sedation.  Thus the baby "knows" when to stop sucking
because sucking and food make him sleepy.  And the mother gets to rest while
nursing, and if she falls asleep while nursing this leads to her staying
close to the baby, protecting him and keeping him warm, after he is done
feeding.

I love to explain this to parents, so they understand that there is a
PHYSICAL cause for the mother's sleepiness.  Some women say that they find
breastfeeding "draining," but in my opinion they just haven't understood that
they are SUPPOSED to feel sleepy, and they are SUPPOSED to use their nursing
time to rest.

The article also explains another bit of newborn behavior.  When the
researchers measured CCK levels in babies' blood, they found a peak at the
end of a nursing, then a DROP almost back to baseline about 10-15 minutes
after the end of the nursing, and finally a rise to even higher levels 30 and
60 minutes after the end of the feed.  Their interpretation of these findings
was that the first peak was a sucking-induced CCK peak, and the steady rise
later was a food-induced CCK rise.  So the baby nurses, gets sleepy from
sucking, lets go and naps a bit, then wakes again to nurse some more until
the food he is digesting induces the second and longer-lasting CCK rise.

This interpretation is also great for parents, because it explains why their
baby nurses, falls asleep, and then wakes up just after they lay her down.
Instead of thinking that there's "not enough milk" for the baby, I tell them
that this CCK drop gives the baby a little window of opportunity to eat some
more if she fell asleep before she was really "finished" eating.  It
especially makes sense to me that the mom might offer the same breast again,
to give the baby a higher-fat second course.
I include information about CCK in every "Breastfeeding 101" class that I
teach, since it seems to explain so much of baby and mother behavior.  I
consider spreading the word about CCK as one of my "missions" in life.

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