Aging: its influence on the intestinal unstirred water layer thickness, surface area, and resistance in the...

The intestinal absorption of some nutrients changes with aging. As the unstirred water layer (UWL) is an important rate limiting step in the absorption of nutrients in general and of lipid soluble nutrients in particular, we investigated possible changes in the UWL dimensions in the aging rat in vivo. We measured the thickness (d) of the UWL using rapid changes in the luminal sodium concentration to induce changes in the transmucosal potential differences. We assessed the surface area (Sw) and resistance (d/SwD) of the UWL at varying flow rates by using increasingly lipophilic medium chain saturated alcohols as probes. At high UWL resistance, d decreased from 318 to 268 microns between 1 and 29 months of age. As the animals aged, Sw changed from 114 to 106 cm2/100 cm and from 262 to 214 cm2/100 cm at low and high flow rates, respectively, using dodecanol as a probe. The resistance of the UWL (d/SwD) remained relatively stable at all ages studied. These experiments demonstrate that age-related changes in absorption are dependent on the aqueous diffusion coefficient and degree of lipid solubility of the specific nutrients. At low UWL resistance, absorption of compounds with higher diffusion coefficients and greater aqueous solubility is decreased with aging. In contrast, previous studies have demonstrated that the absorption of nutrients with low diffusion coefficients and high lipid solubility increases with aging especially when the resistance of the UWL is high.