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

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Abstract
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, possible changes in the UWL dimensions were investigated in the aging rat in vivo. Thickness (d) of the UWL was measured using rapid changes in the luminal Na concentration to induce changes in the transmucosal potential differences. Surface area (Sw) and resistance (d/SwD) of the UWL were assessed at varying flow rates by using increasingly lipophilic, medium-chain, saturated alcohols as probes. At high UWL resistance, d decreased from 318-268 .mu.m between 1 and 29 mo. of age. As the animals aged, Sw changed from 114-106 cm2/100 cm and from 262-214 cm2/100 cm at low and high flow rates, respectively, using dodecanol as a probe. UWL d/SwD remained relatively stable at all ages studied. 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. Previous studies 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.

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