The absence of gravity might reduce gastrointestinal motility (the smooth muscle action that mixes the food and propels it through the gastrointestinal tract), which could decrease appetite and absorption of fluids, nutrients, and medications. The objective of this study was to evaluate the effect of space flight on the gastric emptying rate and intestinal transit time.
Two astronauts were studied at 60, 45, and 30 days before launch, on the fifth day of flight and nine days after landing. Their gastric emptying rates were determined by following the concentrations of acetaminophen in their saliva, and of acetaminophen and its metabolites in their urine, after an oral dose of acetaminophen. The absorption rate of this drug after an oral dose is directly proportional to the gastric emptying rate. The concentrations of the drug and its metabolites were measured by high-performance liquid chromatography and analyzed with a pharmacokinetic simulation computer program. Intestinal transit time was measured indirectly with a lactulose-hydrogen breath test. Lactulose is a sugar that passes undigested through the small intestine, but is fermented by bacteria in the colon, producing hydrogen that is then exhaled. The period from the ingestion of lactulose to the peak in breath hydrogen represented the transit time from the mouth to the cecum (the first part of the large intestine). Performing the acetaminophen absorption test and the lactulose-hydrogen breath test at the same time in each subject allowed the intestinal transit time to be determined by subtracting the gastric emptying time from the mouth-to-cecum transit time. These methods had been validated in six subjects in an earlier 10-day head-down bed-rest study.
Results from the subjects varied. Gastric emptying seemed to be prompt, as salivary acetaminophen levels peaked within 15 minutes of a dose. Intestinal transit time, however, seemed to increase during the flight. Too few astronauts were studied for conclusions to be drawn. It was recommended for future studies that hydrogen concentrations be measured simultaneously in the breath and in the air inside the spacecraft, that the astronauts provide several baseline breath samples before ingesting lactulose, and that breath samples be collected for longer than four hours after lactulose ingestion.