Lactulose (Lac) and acetaminophen (APAP) are commonly used in research and clinical settings to measure gastrointestinal (GI) motility and absorption; similarly, Lac is used to measure GI Transit Time (GITT) and APAP to measure GI absorption as a function of GI motility. These measures are important as they influence absorption and bioavailability of orally administered drugs which in turn influence drug efficacy. These measures are also important because the effectiveness of orally administered drugs is affected by their rate of transit in the GI tract. GI motility has been shown to be slowed in flight and with certain orally administered drugs, astronauts report reduced efficacy and absence of common side effects elicited with the administered dose on the ground. Once validated, GITT and bioavailability measurements using Lac and APAP can serve as standard measures for estimating physiological and functional changes of the GI tract in microgravity and analog environments. These standard measures help understand and estimate risk of failure to treat due to ineffectiveness of medications.
Functional and pathologic changes in the GI tract, a site for absorption and extra-hepatic metabolism of drugs, were characterized by three tests: 1) the Lactulose/ Acetaminophen Test, 2) the Glucose Breath Test, and 3) the Urea Breath Test. The characterization of GI function and pharmacokinetic changes involved the collection of pre-dose biologic (breath, saliva, and urine) samples followed by the ingestion of oral acetaminophen as a pharmaceutical probe, and lactulose, a non-digestible sugar.
GBHT was implemented in three bed rest subjects. Results substantiate the variability in susceptibility of subjects to bacterial proliferation as a function of bed rest. A similar trend of inter-individual variability in bacterial proliferation after space flight was also noticed in three astronauts that performed GBHT pre- and post flight.