Use of a plasma dual isotope ratio method to measure vitamin A absorption

We evaluated the possibility of modifying a method for cholesterol absorption (ABSN) (Zilversmit, D.B., Proc. Soc. Exp. Biol. Med. 140:862, 1972) to measure ABSN of vitamin A (VIT A). Adult male Sprague-Dawley rats (n = 14; liver VIT A ∼2200 nmol) received an IV dose of [¹⁴C]vitamin A- labeled lymph to label body pools; simultaneously [³H]vitamin A in soy oil was given orally. Blood was sampled on d 1, 2, 3, 6, 9 and 12. VIT A ABSN was estimated as 100 * the plasma ratio of [³H]/[¹⁴C] (normalized to doses administered). Apparent ABSN ranged from 132-330% on d 1 (perhaps because the VIT A load in the IV dose was high [77 nmol]). However, by d 6, ratios were stabilizing and ABSN was 74 ± 5% (mean ± SD) on d 12. For comparison, VIT A ABSN measured directly in lymph duct-cannulated rats was 76 ± 16% (n = 11) (Allen, LE. et al., Canc. Res. 54:3319, 1994). We conclude that the plasma dual isotope ratio method may be a simple and accurate new method for measuring VIT A ABSN. Plasma must be sampled after the ratio has stabilized (predicted to be 14 d for these data), and VIT A mass in the doses should be as small as possible. A modified method, using stable isotopes or non-radioactive analogues, may be useful for estimating VIT A ABSN in humans based on a single blood sample. (Supported by USDA CRG 9500618).