Vitamin A Intake and Status Influence Retinol Balance, Utilization and Dynamics in Rats ,

Abstract

To study effects of vitamin A status on retinol dynamics, male rats were fed purified diets varying in vitamin A concentration. Group 1 rats had marginal liver vitamin A levels (approximately 500 nmol) and were in a slight positive vitamin A balance; Group 2 had similar liver levels but were in a slight negative balance; Group 3 had lower liver levels (approximately 370 nmol) and were in a slight negative balance; Group 4 had depleted liver reserves (<10 nmol) and were in vitamin A balance. [3H]Retinol-labeled plasma was injected intravenously, and serial plasma samples were collected for 41 d while rats (six per group) consumed approximately 50 nmol retinol/d (Group 1) or -25 nmol/d (Groups 2-4). Plasma retinol was normal in Groups 1-3 (1.9-2.0 micromol/L) and lower in Group 4 (0.96 micromol/L). Plasma tracer data were fit to a three-compartment model. The central plasma retinol compartment (transit time, 1.5-1.7 h) exchanged with a fast turning-over extravascular vitamin A pool (transit time, 3-4.5 h; -40 nmol) and with a larger, slow turning-over extravascular pool (transit time, 5.5-10 d) that was the site of irreversible utilization of vitamin A. Irreversible utilization was 36 nmol/d (Group 1), 29 nmol/d (Groups 2 and 3) and 20 nmol/d (Group 4). The data indicate that in rats with low or marginal vitamin A status, vitamin A intake, vitamin A reserves and plasma retinol concentration all influence vitamin A utilization and other aspects of retinol dynamics.

Evaluation of the “Olson Equation”, an Isotope Dilution Method for Estimating Vitamin A Stores

“…This important observation deserves emphasis: it makes sense that using population means for factors F and S might give a good estimate of stores in a given population, but there is no reason to expect good prediction of a specifi c individual's TBS since F and S vary among individuals. Large inter-individual variation has also been found in kinetic parameters when vitamin A kinetic data from studies in both rats [8] and humans [13] were analyzed by model-based compartmental analysis. The issue of defi ning TBS for individuals will require further refi nement of the technique if this is deemed a needed measurement.…”

“…Indeed, when stores are high, the liver is undoubtedly the major (albeit not the only) site of vitamin A storage. In contrast, in rats with low liver vitamin A stores, more than 90 % of total body vitamin A is actually present in extrahepatic tissues [8]. This distinction is important because many of the human populations of interest in fi eld studies have low liver vitamin A stores.…”

“…These forms include stable isotopes such as deuterium (for examples, see [2 -6]) and 13 C (e. g., [9]), and radiolabeled forms (e. g., see [7,8]). Because of the use of deuterium in the original Furr et al work in Olson's lab [2] and by many subsequent users of the isotope dilution method (e. g., [3 -6]), the method is sometimes called the deuterated retinol dilution (DRD) technique or the Olson equation.…”

Research Recommendations for Applying Vitamin A-Labelled Isotope Dilution Techniques to Improve Human Vitamin A Nutrition

“…It remains to be investigated if the equilibration time and the fractional catabolic rate are infl uenced by vitamin A status in the higher range of liver reserves in these populations. Studies in rats demonstrated a higher catabolic rate with higher vitamin A intake, reserves, and serum concentrations [28]. Kinetic experiments are needed in various populations like schoolchildren and pregnant women to defi ne life-stage infl uences on equilibration time and fractional catabolic rate.…”

Effects of ethanol intake on retinol concentration in the milk of lactating rats

“…Breast milk is the only source of vitamin A (retinol) during the neonatal period for exclusively breast-fed infants. On the other hand, there are two determining factors for the neonates to achieve their vitamin A requirements: the concentration of this vitamin in the milk and the milk volume consumed, both influenced by maternal vitamin A status and dietary intake (1,2).…”