Cheryl posted some excerpts from the archives:

<Michael Woolridge reports -- the closer the intervals of breastfeeding,
the HIGHER the fat content of the milk. Fat is what makes
babies feel satiated more than volume. . . .  Nursing frequency and fat
content are positively correlated -- the more
frequent the nursing, the higher the fat content of the milk.  ref:
Michael Woolridge, "Baby-Controlled Breastfeeding: Biocultural
Implications" in the book, "Breastfeeding: Biocultural
Perspectives",edited by Patricia Stuart-Macadam and Katherine A.
Dettwyler,
published by Aldine de Gruyter, 1995.  Here is a piece on his sponge
analogy from this reference, p. 238, footnote number 6.
"Hytten (1954) proposed and tested a physical model of milk storage and
delivery.  He immersed a natural sponge in whole milk and
left it for some period, then gradually compressed milk from the sponge;
the first milk draining off was low in fat, similar to foremilk,
and with progressive milk removal the fat concentration rose, finally
approximating to hindmilk. The explanation he proposed was
that fat droplets, being adherent, became adsorbed onto the internal
luminal walls of the sponge, and that sustained milk removal
was necessary to dislodge the adherent fat.  His physical model of
storage and delivery duplicated the changes in fat concentration
during milk removal sufficiently closely that it has not been necessary
to invoke further models..."

In previous discussions on LN, I gave an additional explanation for this
phenomenon which provoked a controversy. In the meantime, I have found
the reference for my statement at that time.

To preface, I want to review the meaning of 4 terms.
1) A lobule is the name for a cluster of alveoli arranged around a space
where the milk components collect temporarily after secretion across the
alveolar membrane.
2) A lobe refers to the entire group of lobules that all end up
contributing to the same main duct.
3) A ductule is the type of tubing with the smallest lumen in the ductal
system. The milk collecting in the lobule drains outward through the
ductule. In addition to the small bore of the ductule, it is also
different from the other ducts because there are no myoepithelial cells
on its outer surface.
4) Ducts are the tubings with larger lumens that receive milk via the
ductules and convey it forward towards the nipple. While alveoli have
myoepithilial cells arranged in a basketlike pattern, ducts have
myoepithelial cells arranged in a spiraling longitudinal pattern. (and
ductules, as above, have no myoepithelial cells themselves.)

 To quote from Mepham, TB, Physiology of Lactation, Open University
Press, Philadelphia, 1987, p. 60,

"Although the composition of milk removed from glands is essentially
constant, the possibility exists that there are local variations in the
concentrations of specific components in different regions of the duct
system. This is known to be the case for milk fat, which, probably
because the large fat globules are impeded in their passage through the
fine-bore ducts, is present at higher concentration in milk removed at
the end of a milking than in that removed at the beginning."

Therefore, it is not only the adsorption of fat particles to the walls of
the ducts, (different from absorption), but also the force of the MER
periodically propelling the fat globules through the ductules which
contributes to a rise in the fat content.(The fat-free portion of the
milk probably seeps unimpeded through the ductules at a faster pace and
more constant rate after secretion.)

The oftener a mother nurses, the oftener she stimulates the MER, and
therefore, the oftener the milk fat is moved through the ductules to mix
with the milk already stored in the ducts. This contributes to more of a
balance of milk components (and higher fat content) that the baby gets
when feeding frequently.

And still to be taken into consideration is the difference in specific
gravity which causes cream to tend to separate and rise upward away from
gravity during longer periods where it is not disturbed (by MER's, outer
pressure, etc.) Anyone raised near dairy animals or in years before the
invention of homogenization is familiar with the fact that cream
separates and rises to the top of the milk storage container. I believe
this is a third factor partly reponsible for the 'foremilk/hindmilk'
phenomenon that may sometimes happen in the mother with a large storage
capacity with infrequent removal of milk. I see this also having some
bearing on the explanation for 'cluster feeds'.

Jean
********
K. Jean Cotterman RNC, IBCLC
Dayton, OH, USA

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