Objective: In the present study, Lactobacillus johnsonii (La1) doubly labelled with 15 N and 13 C (dlLa1) was used to follow the metabolic fate of orally administered dlLa1 in humans. Design: Experimental study. Setting: Research Laboratory, Children's Hospital, University of Rostock. Subjects: A total of 10 healthy adults aged 23-36 y. Intervention: The subjects received 87 mg/kg body weight viable dlLa1 and 10 g raffinose together with breakfast. Expired air samples were taken over 14 h, whereas urine and faeces were collected over 2 days. A blood sample was taken after 2 h. 13 C-and 15 N-enrichments were measured by isotope ratio mass spectrometry (SerCon, UK) and H 2 -concentrations were measured by electrochemical detection (Stimotron, Germany). Results: The orocaecal transit time (OCTT) was reached after 3.7 h. The 13 CO 2 -exhalation amounted to 8.6% of ingested dose. The urinary excretion of 13 C and 15 N was 1.3 and 12.4% of ingested dose, respectively, whereas the faecal excretion was 39.9 and 37.6% of ingested dose, respectively. After 2 h, 13 C-and 15 N-enrichment of fibrinogen amounted to 70 and 90 ppm excess, respectively. Conclusions: In comparison to OCTT of 3.7 h, both stable isotopes appear after 30 min in breath and urine, indicating that dlLa1 is rapidly digested in the small bowel before reaching the caecum. This is confirmed by 13 C-and 15 N-enrichments of blood plasma fractions. The ingestion of dlLa1 led to an excretion of 50% of ingested dose of both stable isotopes.
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