To Eliza Hirsh
Thanks for giving me a chance to review the physiology of dissacharidase
concentration in the gut of newborns.
In humans lactase appears in the small intestine of the fetus at around 3
months gestation.  The levels rise to higher level at the time of birth at
which time the levels are  2 to  4 times that of a 2-11 month old child.
There is a further rapid rise within the 1st week after birth in preterm
infants so that the level reaches that of a term baby in about  1 week.
Often there is incomplete lactose absorption in the small intestine and
there is a process of colonic salvaging by bacteria that allows for
absorption of the lactose not processed in the small bowel.  Remember that
babies have relatively to their size a shorter gut that adults do.  So
this(the lack of complete digestion of lactose) will happen especially in
breastfed babies where the stools are generally more acidic (because of
the fermentation of the lactose) which is a natural protection of the gut
against the bad bacterias (such as E Coli,...)
The following references will illustrate these points

1.  Lactase levels in newborns
        - Auricchio et al. Pediatrics 1965, 35:944-954
        - Antonovicz et al.  Gastroenterology 1974, 67:51-58
        - Antonovicz et al.  Gastroenterology 1977, 72:1299-1303
        - Dahlqvist et al.  Biol Neonate 1965, 9:24-32
        - Jirsova et al.  Biol Neonate 1968, 13:143-146

2.  Increase in Lactase in first week of life in prematures
        - Maine et al Lancet 1983: ii:622-623
        - Aurichio et al Pediatrics 1965, 35:944-954

3.  Colonic salvage by colonic flora
        - McLean et al J Pediatr 1980, 97:383-388
        - McLean et al Pediatr Research 1983, 17:629-633
        - Lifshitz et al J Pedatr Gastro & Nutr 1983, 2:478-482
        - Chiles et al Pediatr Res 1979, 13:365
        - Bond et al  Gastroenterol 1980 78:444-447
        - Kein et al Clin Nutr 1987 46:456-460

Hope these help
Maryelle