Vitamin A supplementation redirects the flow of retinyl esters from peripheral to central organs of neonatal rats raised under vitamin A-marginal conditions

Abstract

Background: Vitamin A (VA; retinol) supplementation is used to reduce child mortality in countries with high rates of malnutrition. Existing research suggests that neonates (<1 mo old) may have a limited capacity to store VA in organs other than the liver; however, knowledge about VA distribution and kinetics in individual, nonhepatic organs is limited.Objective: We examined retinol uptake and turnover in nonhepatic organs, including skin, brain, and adipose tissue, in neonatal rats without and after VA supplementation.Design: Sprague-Dawley neonatal rats (n = 104) were nursed by mothers fed a VA-marginal diet (0.35 mg retinol/kg diet) and treated on postnatal day 4 with an oral dose of either VA (6 μg retinyl palmitate/g body weight) or canola oil (control), both containing 1.8 μCi of [³H]retinol. Subsequently, pups (n = 4 · group^-1 · time^-1) were killed at 13 different times from 30 min to 24 d after dosing. The fractional and absolute transfer of chylomicron retinyl esters (CM-REs), retinol bound to retinol-binding protein (RBP-ROH), and total retinol were estimated in WinSAAM software.Results: VA supplementation redirected the flow of CM-REs from peripheral to central organs and accumulated mainly in the liver. The RBP-ROH released from the liver was acquired mainly by the peripheral tissues but not retained efficiently, causing repeated recycling of retinol between plasma and tissues (541 compared with 5 times in the supplemented group and control group, respectively) and its rapid turnover in all organs, except the brain and white adipose tissue. Retinol stores in the liver lasted for ∼2 wk before being gradually transferred to other organs.Conclusions: VA supplementation administered in a single high dose during the first month after birth is readily acquired but not retained efficiently in peripheral tissues of neonatal rats, suggesting that a more frequent, lower-dose supplementation may be necessary to maintain steady VA concentrations in rapidly developing neonatal tissues.

Keywords: chylomicron; extrahepatic; growth and development; mathematical modeling; neonate; retinyl esters; vitamin A deficiency; vitamin A metabolism; vitamin A supplementation.

FIGURE 1

Plasma model of VA kinetics in neonatal rats dosed orally with [³H]retinol on postnatal day 4. Circles represent compartments, squares represent delay components, and arrows represent their interconnections. CM-RE, chylomicron retinyl ester; DT, delay component; IC, initial condition; L, fraction; RBP-ROH, retinol bound to retinol-binding protein; VA, vitamin A.

FIGURE 2

Liver (A) and stomach (B) models of VA kinetics in neonatal rats dosed orally with [³H]retinol on postnatal day 4 and a simplified liver model (C) developed to calculate retinol turnover rate [R(I,J)]. Circles represent compartments, squares represent components defined by the plasma forcing function, and arrows represent their interconnections. L(I,J)s represent the fractional transfer of chylomicron-derived, RBP-derived, or total retinol into or out of the organ. CM-RE, chylomicron retinyl ester; IC, initial condition; L, fraction; RBP, retinol-binding protein; RBP-ROH, retinol bound to retinol-binding protein; VA, vitamin A.

FIGURE 3

Mean organ fraction of the ingested [³H]retinol dose between days 0 and 24 after dosing in control and VA-supplemented rats dosed on postnatal day 4. The inset shows the nonhepatic tissues. Bars represent the means ± SEMs, n = 52 rats. *P < 0.05 (Student’s t test). BAT, brown adipose tissue; VA, vitamin A; WAT, white adipose tissue.

FIGURE 4

Stomach (A) and intestine (B) fraction of ingested [³H]retinol dose in control and VA-supplemented rats from 0 to 24 d after dosing on postnatal day 4. Insets show the first 24 h after dosing. Each symbol represents the mean of 4 rats. There were no significant differences between groups. VA, vitamin A.

FIGURE 5

Plasma fraction of ingested [³H]retinol dose (A) and fraction of ingested dose as CM-REs, RBP-ROH, and total ROH in control (B) and VA-supplemented (C) rats from 0 to 24 d after dosing on postnatal day 4. The inset shows the first 48 h after dosing. Tracer profiles for CM-REs and RBP-ROH were generated by the model based on the assumption that total ROH is the sum of CM- and RBP-ROH. Each symbol represents the mean of 4 rats. *P < 0.05 (Student’s t test). CM-RE, chylomicron retinyl ester; RBP-ROH, retinol bound to retinol-binding protein; ROH, retinol; VA, vitamin A.

FIGURE 6

Liver (A–C), lung (D–F), and kidney (G–I) fraction of ingested [³H]retinol dose and fraction of ingested dose as CM-derived, RBP-derived, and total ROH in control and VA-supplemented rats from 0 to 24 d after dosing on postnatal day 4. Insets show the first 48 h after dosing. Tracer profiles for CM- and RBP-derived ROH were generated by the model based on the assumption that total ROH is the sum of CM- and RBP-ROH. Each symbol represents the mean of 4 rats. *P < 0.05 (Student’s t test). CM, chylomicron; RBP, retinol-binding protein; RBP-ROH, retinol bound to retinol-binding protein; ROH, retinol; VA, vitamin A.

FIGURE 7

Brain (A–C), BAT (D–F), skin (G–I), and carcass (J–L) fraction of ingested [³H]retinol dose and fraction of ingested dose as CM-derived, RBP-derived, and total ROH in control and VA-supplemented rats from 0 to 24 d after dosing on postnatal day 4. Insets show the first 48 h after dosing. Tracer profiles for CM- and RBP-derived ROH were generated by the model based on the assumption that total ROH is the sum of CM- and RBP-ROH. The carcass contained bones, muscles, and connective tissue remaining after dissection of other organs. Each symbol represents the mean of 4 rats. *P < 0.05 (Student’s t test). BAT, brown adipose tissue; CM, chylomicron; RBP, retinol-binding protein; ROH, retinol; VA, vitamin A.

FIGURE 8

WAT fraction of ingested [³H]retinol dose in control and VA-supplemented rats from 0 to 24 d after dosing on postnatal day 4. No dissectible WAT was found before postnatal day 12. Each symbol represents the mean of 4 rats. There were no significant differences between groups. VA, vitamin A; WAT, white adipose tissue.