Quantitative Measurement of Lactose Absorption

The quantity of lactose not absorbed by 4 normal and 6 lactase-deficient subjects was determined by three indirect methods which involved: (1) measurement of pulmonary hydrogen (H₂) excretion, (2) pulmonary ¹⁴CO₂ excretion, and (3) stool ¹⁴C excretion, after ingestion of 12.5 g of 1-¹⁴O-lactose and 4 g of polyethylene glycol (PEG). Results were compared with absorption determined directly from the ¹⁴C:PEG ratio of multiple terminal ileal aspirates., The fraction of lactose not absorbed determined by ileal aspiration ranged from 0 to 8% in normals and 42 to 75% in milk-intolerant subjects. Whereas all three indirect methods were useful in qualitatively separating normal from deficient subjects, the quantity of lactose absorbed as determined by H₂ excretion correlated most closely with ileal measurements (r = 0.94). Pulmonary ¹⁴CO₂ excretion for 24 hr after ¹⁴C-lactose ingestion did not distinguish normal (17 ± 4% (SEM) of ingested ¹⁴C per 24 hr) from lactase-deficient subjects (21.1 ± 3%). Likewise, stool ¹⁴C:PEG ratios grossly underestimated malabsorption with less than one-quarter of the nonabsorbed ¹⁴C appearing in the stool. This study suggests that individual differences in susceptibility to diarrhea after milk ingestion by lactase-deficient subjects may be due to differences in the quantity of lactose not absorbed and/or differences in the rate of bacterial metabolism of lactose in the colon. Analysis of ileal fluid collected during passage of the lactose meal indicated that about two-thirds of the osmotic load entering the colon consists of endogenous electrolytes. Thus the water load delivered to the colon is about 3 times that calculated to be osmotically held by the nonabsorbed sugar.