Influence of aging on vitamin A transport into the lymphatic circulation

Vitamin A is a lipid soluble essential dietary micronutrient. Because aging individuals have elevated serum levels of vitamin A, we tested the intestinal absorptive capacity of the vitamin in aging rats by measuring the appearance rate of vitamin A in the lymphatic circulation as a measure of intestinal absorption. We infused the vitamin in a physiological concentration of 350 nM into the proximal jejunum of groups of Sprague-Dawley rats 2 to 23 months of age. Lymph was collected for 6 h and vitamin A radioactivity was measured. Lymph flow rate over 24 h ranged from 16 +/- 3.1 ml in the oldest to 21.5 +/- 2.4 ml in the youngest animals. Vitamin A transport into lymph was highest at 23 months of age, reaching 7869 +/- 154 pmol/6 h as compared to 6732 +/- 106 pmol/h at 2 months (p less than .01). The increase in vitamin A absorption with aging represented an increase from 35.6% of the infused vitamin at 2 months to 41.6% at 23 months of age. Because of the cumulative storage of vitamin A in the liver this increase in absorption and lymphatic appearance is of nutritional and metabolic significance. Increased lymphatic appearance of the vitamin could explain its higher absorption and serum levels in aging individuals.