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Neutral P-galactosidase (lactase) located in the bush border of adults varied from 0.950-0.997; the ratio of the cumulative % dose the small intestinal enterocyte is the enzyme responsible for the [13Cj (cum % [13Cj) to the cumulative % dose [14cj (cum % [14cj) hydrolysis of lactose, which is the rate limiting step in lactose ranged from 0.89-1.15. Nineteen full-term infants (40.1 + 0.9 wk absorption (8, 11). Lactase, unlike sucrase and maltase, remains gestation) were subsequently studied. On day 1 baseline breath low in concentration during fetal development until approximately and stool collections were carried out with unlabeled substrate 36 wk of gestation and then increases continuously thereafter until (lactose-containing formula). In the initial ten infants [13Cj-1-L term (1, 10). Normal full-term infants, whose only natural dietary was added to the 7 a.m. or 8 a.m. formula feeding on day 2 and an carbohydrate is lactose, have always been considered "by defini&h breath test was done. A 24-h stool collection was also carried tion" to be lactose tolerant. Implicit in this belief have been the out. In nine subsequent infants a breath test and stool collection concepts that intestinal lactase is wholly responsible for lactose with [13Cj-l-glucose ([13CJ-1-G) were carried out on day 2 and the digestion and that the entire process of digestion and absorption studies with [13CJ-1-L were moved to day 3. Hz and % d [13Cj/h takes place in the small intestine. and cum % [13CJ in breath and [l3Cj recovered in stool were Auricchio and his collaborators (2) were among the first to determined. question these concepts. Based on in vitro studies with material All infants had detectable Hz in breath on the days of study. obtained at autopsy they concluded that the "large lactose load of Peak Hz excretion was <20 ppm in eleven infants. 220 ppm in the breast-fed (full-term) infant probably exceeds the ability of the eight. There was no relationship of HZ excretion to gestational small intestine to hydrolyze this sugar" (2). Both the low pH of age, birth weight, postnatal age or lactose intake. In all 19 infants stool and the fmding of significant (21%) amounts of reducing the % d [13cj in breath per h and the cum % [13Cj after [13C]-1-L substance in the stool of healthy term infants corroborate this idea were virtually identical to data in adults studies after consumption and suggest that carbohydrate normally passes beyond the ileoceof naturally [13C]-labeled maize glucose. In the infants studied cal valve. The excretion of hydrogen in breath in substantial with both substrates, there were no significant differences between amounts implicates the colonic microflora in anaerobic fermen-[l3Cj-1-~ and [l3CJ-1-G i, % d [13Cj at each time point. Cum % tation of this malabsorbed carbohydrate (3, 6). In addition to [13C] recovered from [13Cj-1-G become progressively greater than hydrogen gas, volatile fatty acids are produced. These compounds that from [13C]-1-L, significantly so at 6, 7, and 8 h (8 hour % d have been shown to be...

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