The latest theory that I remember hearing about how growth spurts work is
that the baby does not actually build up a larger milk supply.  The baby
nurses frequently for several days in response to some metabolic signal,
thus increasing his calories for those days.  When the growth spurt ends,
the baby goes back to his usual caloric intake that  existed before the
spurt.  I am tired and need to go to bed now, so don't have time to
research this, but maybe someone else can fill in the holes.

Sharon Knorr
Lactation Consultant Services
Rochester, New York
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