Liver vitamin A levels in rats are predicted by a modified isotope dilution technique

We tested the hypothesis that the fraction of an injected dose (FD(p)) of [³H]retinol remaining in plasma could be used to predict liver vitamin A mass. Rats (n = 36) with liver vitamin A ranging from 1.4 to 23,000 nmol received an intravenous dose of [³H]retinol-labeled plasma. Plasma samples obtained at 4, 4.4, 5, 5.4, 6, 7 and 8 d were analyzed for radioactivity; liver vitamin A levels were determined at 8 d. Using a regression equation developed previously [liver vitamin A (nmol) = 7493·exp(-148·FD(p))], there was a log linear relationship between FD(p) and liver vitamin A mass from 1.4 to 5200 nmol retinol; the equation did not predict levels ≥7500 nmol. By expanding the regression equation [liver vitamin A (nmol) = 88,928·exp(- 1347·FD(p)) + 5606·exp(-120·FD(p))] and using the FD(p) at 4.4 d, liver vitamin A levels ranging from <4 to 23,000 nmol were well predicted. Satisfactory equations were also developed for data obtained at 4, 5 and 5.4 d. We conclude that this modified isotope dilution technique is a sensitive and reliable method for assessing vitamin A status in rats over a wide range of vitamin A nutriture. It is likely that this method could be modified for use in humans.