Laura,
There are several types of secretion which are important in the breast.
Merocrine secretion is when things are released out of the cell with no loss
of cell substance (membrane or cytoplasm).  Some proteins and other water
soluble milk components are secreted this way, and thus are being released
into the ducts from the alveoli at some level relatively constantly.  When
prolactin levels rise, these components are manufactured more rapidly, and
secreted as soon as they make their way to the apex of the alveolar cells.
        Lactose and other proteins are secreted by exocytosis, in a membrane
packet (remember the Golgi transport vessicles, these are them) which fuses
with the cell membrane and then passes out, taking a tiny bit of membrane
along as a "cap".  This is also a relatively fast, easy process, and occurs as
quickly as these components are manufactured and make their way to the cell
apex.  Prolactin fosters alpha-lactalbumin (protein) production, which
catalizes lactose production.  When prolactin rises, both this major protein
and the lactose production increase.
        Apocrine secretion is when the apex of the cell is budded off along
with the product being secreted.  Lipids (fats) are usually secreted this way,
as the cell membrane is a lipid bilayer and conveniently pinches off around
the fat globules.  Cytoplasm is lost in apocrine secretion.  The MER gives
apocrine secretion a boost by mechanically squeezing the alveolar cells,
causing them to release the fat globules more rapidly into the ducts.
        Prolactin levels increase with each MER, increasing the rate of fat
synthesis by increasing the uptake of fats from the blood by the mammary
gland.  Oxytocin increases fat release.
        The distinction between foremilk and hindmilk is a somewhat artificial
one (oversimplification).  What is true is that fat content of the milk
increases with the duration of suckling within a feed (due to the mechanics of
the different modes of secretion); the frequency of feeds (due to prolactin
levels); and the degree of emptying (due to autocrine control or negative
feedback.)  As others have stated, there is research that the storage capacity
of the breast feeds into the equation...complications upon complications.
        So, can you see that it is true that babies get all milk components
during a feed, but that the ratio of the different components changes within
the feed due to each MER and the subsequent increase in apocrine secretion of
fats?
Catherine Watson Genna, IBCLC  who loves mechanisms!  [log in to unmask]