Hi All,
This is a topic I've been studying in detail for some time. The progress in
understanding milk synthesis comes partly from dairy physiologists (who
have financial interest in knowing exactly how to make lots of milk) and
partly from those helping women. Here's a very quick summary of a 2-hour
talk I've presented on the subject:

1. Before the 1940's, everyone thought most of the milk was made during the
let-down reflex because it flows faster then. The emphasis was on avoiding
stress, which was thought to inhibit the let-down. (True for dairy people
as well as breastfeeding advocates).

2. In 1944, Peterson showed that secretion was continuous but let-down was
a different and separate process. Let-down (or MER - milk ejection reflex)
squeezes out milk that is ALREADY MADE and stored in the alveolar lumen. It
just FLOWS faster - it isn't made any faster during MER.

3. In the 1990's, Peter Hartmann's research in Australia with breastfeeding
women corroborated dairy research of Schanbacher and others. He found
(actually "is finding" - present tense- this research is ongoing) that the
rate of synthesis - how fast the secretory cells pump out milk - was
related to the degree of emptiness of the breast. This is autocrine
control. As the lumen fills, compounds in the retained milk itself
(peptides, fatty acids and possibly other components) signal the cell to
SLOW down making more of the stuff. The emptier the breast is, the faster
it tries to refill - similar to an automatic ice-maker. (A toilet tank
sorta works the same way.) Hartmann says the rate of milk synthesis ranges
from 11 to 58 ml/hour/breast, or about 1/3 of an ounce to 2 ounces per
breast per hour. Emptier breasts make milk faster than fuller ones.

4. If milk is regularly and thoroughly removed from the breast, milk
synthesis keeps chugging along unrestricted. Milk retained in the breast
downward-regulates total supply to about 15-20% more than the baby takes on
an average. The principle goes like this: if more than 80% of the milk is
removed, supply increases to maintain the 80-20 ratio. If less than 80% is
removed, supply decreases to maintain the 80-20 ratio. Of course this is an
oversimplification of a very complex process, but this principle has held
steady as new research emerges.

5. Mom's diet, her fluid intake, and other factors are relatively minor
players in this drama. (I make this statement based on my digging into the
literature and 27 years working with mothers.) If the "milk removal" piece
is in place, moms make plenty of good milk regardless of dietary practices.
If the "milk removal" part isn't there, nothing else can make up the
difference.

6. The significant inhibiting factors appear to be (1) breast surgery; (2)
retained placenta; (3) Sheehan's syndrome or pituitary shock; (4) hormonal
contraception. If none of those are factors, it's exceedingly rare than a
mom won't make plenty of milk. Rare situations DO exist, however.

There were several recent articles in the JHL on this topic by Daly and
Hartmann, and Hartmann's talk from ILCA 1990 is still available on tape.
Floyd Schanbacher's talk at ILCA 1996 (which was taped) explained this very
expertly.

My New Years' Wish/plea to all: look at the baby carefully. The most common
reason for low supply is usually a baby not at breast enough and/or not
removing milk effectively.  I don't hesitate to recommend pumps because I
see so many babies with poor suck responses at first. The poor suck leaves
milk in the breast which compromises milk supply, resulting in a hungry
disorganized baby and no milk. At least with a good pumping routine, we
have plenty of mom's milk to work with while we figure out how to help the
baby feed better.

Supply is usually the easiest part to fix. It's still supply and demand, or
"use it or lose it." Hope this helps.

Linda Smith, BSE, FACCE, IBCLC
Bright Future Lactation Resource Centre
Dayton, Ohio where it's cold and gray outside
http://www.bflrc.com