Effects of gravity on gastric emptying, intestinal transit, and drug absorption.

The effects of microgravity on the physiologic response of the human body, the physical properties of gastrointestinal contents, and the influence these responses have on drug absorption are becoming more and more critical as the duration of humans in the hostile space environment dramatically increases. In this environment, some conventional oral dosage forms may be severely limited as an effective drug regimen. To understand the effects of microgravity, one must first understand the basic forces acting on a particle moving through a walled-tube such as the small intestine: gravity (FG), buoyancy (FB), and drag (FD). These forces can be combined and rearranged into a dimensionless ratio of gravitational forces to viscous forces. This is the most important dimensionless group influencing the motion of a particle relative to the fluid. Gastric emptying is highly influenced by several factors: volume, calories, exercise, size, density, temperature, viscosity, osmolality as well as those factors associated with physiologic responses: splanchnic blood flow, body position, and electrolyte balance. This array of factors can lead to variability in drug plasma levels. In the absence of gravity, the factors of size and density would appear to be most directly altered due to their dependence on the force of gravity. Intestinal transit rate in a gravity environment is highly dependent on the motility state of the GI tract either fasted or fed partly due to the higher viscosities of chyme in the fed state. In space, the absence of gravity may tend to increase the transit rate along the small intestine by decreasing the dimensionless ratio of gravitational forces to viscous forces. In zero gravity, therefore, these alterations in GI emptying and intestinal transit rate could lead to erratic plasma levels and inefficient absorption.