Lymphatic absorption of retinol in young, mature, and old rats: influence of dietary restriction

Earlier studies have shown that hepatic vitamin A stores increase with age and that rats subjected to life-long dietary restriction (DR) have greater hepatic retinoid concentrations than ad libitum-fed rats. It was suggested that these changes may be attributed to altered intestinal absorption, and so we measured retinol absorption in 3, 12, and 22 month-old female Sprague-Dawley rats fed ad libitum (AL) or a restricted diet (40%) enriched (DR+) or not (DR) with vitamins and minerals. An in vivo model was used and lymph collected from the mesenteric duct while rats were infused a micellar solution containing 3Hretinol into the proximal duodenum. Aging significantly decreased lymph flow, and at all ages DR and DR+ rats exhibited a higher flow than age-matched AL rats. Absorption of retinol was not significantly modified by age in AL and DR+ rats. However, in the DR group, there was a marked decrease in absorption between 3 and 12 months of age. Retinol absorption was significantly higher in 12 and 22 month-old DR+ rats than in age-matched AL rats, a finding that may be explained by both higher lymph flow rate and higher long-term vitamin A intake per body weight. In conclusion, the increased content of vitamin A in the livers of older AL rats is not associated with changes in intestinal absorption. In DR rats, however, retinoid accumulation in the liver may be explained by increased intestinal absorption of the vitamin, though other mechanisms may be involved. A possible explanation may be a decreased output of retinol from the liver as a result of lower metabolic needs. The lower serum vitamin A concentrations observed in DR rats would tend to support this hypothesis.