Camille,

I think your question is a good one.  The sentence you quote from Woolridge
and Fisher is confusing.  [Lancet, August 13, 1988, "The protein and lactose
concentrations in human milk remain fairly constant during a feed, whereas the
fat concentration of breast milk rises consistently throughout (though
individual feeds may not conform entirely.)"]

Here's how I picture it.  Lactose is about 7% of human milk, and it's in
solution in the watery part of the milk.  Protein is about 0.9%, and it is 60%
whey proteins, in the watery part of the milk.  (The 40% of protein that is
casein is someplace in the milk, probably floating around in the whey, or
maybe mixed in with the fat AND the whey---but I'm too lazy to go look it up
just now...To tell the truth, I'm not sure what role the proteins play in this
situation, so I'd rather focus just on the lactose and fat.)  Fat percentage
changes during the feed, so the proportion of "skim" milk, with its lactose
and whey proteins, is different in fore milk and hind milk.

A baby who's getting 1% fat in the fore milk is getting 99% non-fat milk.  A
baby who's getting 10% fat in the hind milk is getting 90% non-fat milk.  The
difference in the amount of lactose the baby gets from each type of milk is
not that great.  [7% of 99 is 6.93.  7% of 90 is 6.3.  So the fore milk would
actually contain about 6.9% lactose and the hind milk 6.3% lactose.]  BUT the
proportion of fat that the baby gets to balance that lactose is different when
you compare the fore milk and hind milk.

If you look at figure 2 from the article, you can see that throughout the
middle of the feed, the baby's volume intake runs ahead of his fat intake, but
near the end of the feed, the two lines get close together again.  Assuming
that this baby is choosing a feed length that lets him balance his fat and
volume intake optimally for his own needs, then it's clear that the grownups
shouldn't interrupt the feed in the middle before the fat intake has caught
up.

The fat will slow the transit time of the feed through the baby's gut and give
him time him to absorb more nutrients from every drop of milk.  So even a
relatively small feed, if it is a complete feed with a good proportion of hind
milk, will be more thoroughly digested to give the baby what he needs.  A
shorter and bigger feed, from a mother and baby who don't have the fore
milk/hind milk balance worked out well yet, might give the baby the same
amount of fat but packaged with a bigger load of lactose, so the gut transit
time is faster, fewer nutrients get absorbed, and the baby poops out a lot of
what he ate  ("The milk goes right through him!").

Does this explanation help?  Anyone else want to wade in?  Coach Smith?

Chris Mulford
BA, RN, BSN, IBCLC
leafy Swarthmore, south of Philadelphia