Understanding measurements of intestinal permeability in healthy humans with urine lactulose and mannitol excretion

Abstract

Our aim was to understand the information from differential two-sugar excretion (2-SE) in measuring intestinal permeability. In a crossover study in 12 healthy volunteers, we compared urinary excretion ratios of lactulose (L) to mannitol [(M) LMR] after ingestion in liquid formulation (LF) or in delayed-release, methacrylate-coated capsules (CAP). Both formulations were radiolabelled. Urine was collected every 2 h from 0 to 8 h, and from 8 to 24 h. Two hours after LF, gastric residual was 15.9 +/- 6.2% (SEM), and the percentage in colon was 49.6 +/- 7.8%; in 11/12 participants, liquid had entered colon within 2 h. Average CAP arrival time in colon was 5.16 +/- 0.46 h (mode 6 h). After LF, mannitol was extensively absorbed in the first 8 h; lactulose absorption was low throughout the 24 h. After the LF, the LMR (geometric mean, 95% CI per h) in the 0-2 h urine was [0.08 (0.05, 0.11)], which was lower than in 8-24 h urine [0.32 (0.16, 0.46); P < 0.05]. Urine LMRs at 8-24 h were similar after LF or CAP. We concluded that, after LF, sugar excretion in 0-2 h urine may reflect both SI and colon permeability. Colonic permeability is reflected by urine sugar excretion between 6 and 24 h. CAP delivery reduces mannitol excreted at 0-6 h, compared with LF. The 0-5 or 6 h 2-SE urine likely reflects both SI and colon permeability; the higher LMR in the 8-24 h urine relative to 0-2 h urine should be interpreted with caution and does not mean that colon is more permeable than SI.

Figure 1

Accuracy of LS/MS-MS measurements of urinary lactulose and mannitol in comparison to the (commercial) enzyme-based assay. Note most of the data fall close to the line of identity. Correlation coefficient for mannitol and lactulose are respectively 0.977 and 0.662 (both p<0.001)

Figure 2

Bland Altman plots demonstrating the delta enzyme assay minus average of 2 estimates LC/MS-MS assay on the Y axis and the average of the enzyme assay and the mean of 2 estimates by LC/MS-MS on the x axis.

Figure 3

Summary of mass of mannitol and lactulose excreted per hour after capsule and liquid formulation; note that, after capsule formulation, the mannitol excretion lags significantly behind excretion after liquid formula, consistent with the delivery of the sugars to the absorbing small intestine or colon. While the data from liquid formulation suggest greater absorption earlier, this may simply reflect the relative amount of mannitol available for absorption from the liquid formulation compared to the capsule.

Figure 4

Proportionate excretion of lactulose and mannitol after administration via capsule (upper panel) or liquid formulation (lower panel). The mass of mannitol absorbed and excreted is greater for mannitol even though there was 5 times as much lactulose ingested. The cumulative average excretion of mannitol reaches less than 300 mg, suggesting that at least 700 mg of mannitol is available for absorption between 8 and 24 hours after liquid formulation.

Figure 5

Urine excretion of mannitol and lactulose in aliquots from the total urine output obtained every 2 hours from 0–8 hours and from 8–24 hours. All data are provided for the administration of sugars in aqueous solution. Amounts of sugars excreted after capsule formulation are summarized from 6–8 hours and from 8–24 hours.

Figure 6

Comparison of the lactulose to mannitol ratio at different times using the two formulations. Data show the geometric means and 95% confidence intervals. After liquid formulation, the ratio is higher at 8–24 hours than with all other urine samples. Note also that the ratio is higher for the urine samples after liquid and capsule formulations at 8–24 hours compared with 0–2 hours with the liquid formulation. These data suggest colonic (6–8 hours or 8–24 hours) permeability is higher than small bowel permeability (0–2 hours). There were no significant differences in ratios at 6–8 hours and 8–24 hours with either formulation, and there were no differences between the different formulations at these times.