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Plasma Retinol Is a Major Determinant of Vitamin A Utilization in Rats

Manuscript received 2 March 1998. Initial reviews completed 8 April 1998. Revision accepted 5 June 1998.

Sean K. Kelley and Michael H. Green

Nutrition Department and Graduate Physiology Program, The Pennsylvania State University, University Park, PA 16802

We studied relationships among vitamin A intake, liver levels of vitamin A, plasma retinol concentrations and the irreversible utilization of vitamin A. To supplement existing data, we first used model-based compartmental analysis to determine vitamin A utilization and other kinetic parameters in male Sprague-Dawley rats that had adequate liver vitamin A stores (~9000 nmol) and were fed a diet containing low levels of vitamin A. Plasma retinol kinetics were monitored for 43 d after administration of [3H]retinol-labeled plasma to rats consuming ~23 (Group 1, n = 6) or ~4.2 (Group 2, n = 6) nmol vitamin A/d. Data for plasma tracer vs. time and for tracer lost irreversibly by the end of the experiment, were fit to a three-compartment model in which plasma retinol exchanges with vitamin A in two kinetically distinct extravascular compartments. Irreversible utilization rates (~41 nmol/d) were similar to those for rats that are in vitamin A balance, suggesting that, when liver vitamin A stores are adequate, utilization rate is not decreased to compensate for a low vitamin A intake. Multiple linear regression analysis was then used to relate these and previously collected data (total, 62 rats) on vitamin A intake (4.2-49 nmol/d), plasma retinol concentration (1.4-2.5 µmol/L) and liver vitamin A level (1.2-11,000 nmol) to vitamin A utilization (disposal rate, 4.2-68 nmol/d). A significant relationship (R2(adj) = 0.93) was found for the equation [disposal rate (nmol/d) = -0.720 (nmol/d) + 0.844 (d-1)·(plasma retinol; nmol) + 0.00139 (d-1)·(liver vitamin A; nmol) + 0.220·(vitamin A intake; nmol/d)]. Plasma retinol accounted for 92% of the variability in disposal rate (vs. 5% for liver vitamin A and 3% for vitamin A intake). We conclude that plasma retinol is a main determinant of the irreversible utilization of vitamin A in rats with low to moderate vitamin A intake.

Key words: rats, vitamin A, disposal rate, simulation, analysis and modeling, nutrient utilization.

The Journal of Nutrition Vol. 128 No. 10 October 1998, pp. 1767-1773
Copyright ©1998 by the American Society for Nutritional Sciences




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