Influence of aging on vitamin D absorption and unstirred water layer dimensions in the rat

Because of the high prevalence of metabolic bone disease in older persons, we investigated the possibility of impaired intestinal absorptive capacity for vitamin D3 in aging animals. Using a single-pass technique, we measured vitamin D absorption and mucosal accumulation in male rats 9 to 101 weeks of age. Intestinal length, water absorption, and vitamin D3 intestinal tissue concentration remained constant after 41 weeks of age. Vitamin D3 absorption increased from 1209 pmol/100 cm/hr at 9 weeks of age to 2114 pmol/100 cm/hr at 41 weeks of age and remained relatively constant thereafter. Because vitamin D3 absorption rate is partly regulated by the dimensions of the unstirred water layer, we assessed the dimensions of the UWL of our aging animals. As the animals aged, the surface area of the UWL increased from 197 to 316 cm2/100 cm, and its resistance decreased from 1.2 to 0.7 min/cm3/100 cm by 41 weeks of age and remained stable thereafter. Inasmuch as the UWL is a major regulatory step in the absorption of vitamin D, its constant dimensions after 41 weeks of age explain the normal absorption of vitamin D3 observed in our aging animals. If these findings are found to be true in humans as well, they would argue against the possibility of vitamin D3 malabsorption as a cause of metabolic bone disease seen in aging individuals.