Absorbed dose from 14 C xylose and 14 C mannose

Tissue distribution and excretion studies have been performed in rats and mice for up to 1 week after oral administration of 14 C xylose and 14 C mannose. The effective dose-equivalent is calculated to be 15 /iSv/MBq for xylose and 120 /iSv/MBq for mannose. Since there was no clearance of mannose during the period of the study, the effective dose-equivalent for mannose is almost entirely dependent upon the assumptions made about expected lifetime exposure. Bacterial colonisation of the small intestine can be detected either directly, by intubation of the jejunum and culture of the aspirate, or indirectly by measurement of gas released during bacterial metabolism of an administered substrate. Direct sampling by intubation is difficult, time consuming and carries a risk of giving false positives as a result of contamination of the small bowel during intubation. Measurement of the concentration of hydrogen in the breath following an oral glucose load (Metz et al, 1976) is a reliable means of detecting colonisation by anaerobic organisms, but in clinical practice carries a 20-30% false negative rate (King & Toskes, 1983). 14 C xylose is a pentose sugar of plant origin which has been advocated as an alternative breath test substrate (Toskes et al, 1978; King et al, 1979), but suffers from the disadvantage that it undergoes metabolism in the liver after absorption from the small bowel, with the release of 14 CO2 which cannot be distinguished from that produced by intraluminal bacteria. Mannose is a C6 epimer of glucose which is absorbed from the small intestine much more slowly than xylose, and is not reabsorbed from the renal tubule (Wood & Cahill, 1963). These properties suggest it warrants investigation as a potential diagnostic agent for detecting small-bowel colonisation. It is in principle possible that 14 C in any administered sugar could be incorporated into fat or other substances with a long biological half-life. Accurate dosimetry data are therefore essential. There are no published estimates of the absorbed dose likely to result from the use of 14 C mannose, and published data on xylose dosimetry (King et al, 1980) are unsatisfactory because they are incomplete. We have performed tissue distribution studies of these two sugars in rats and mice in order to estimate the absorbed dose and calculate the effective