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Community and International Nutrition

The Concentration of Free Holo-Retinol Binding Protein Is Higher in Vitamin A–Sufficient than in Deficient Nepalese Women in Late Pregnancy^1,2

Sandhya Sankaranarayanan^*, Monica Suárez, Douglas Taren^**, Denise Genaro-Wolf^**, Burris Duncan^**, Kamal Shrestha, Narayani Shrestha and Francisco J. Rosales^,3

^* The Huck Institutes of the Life Sciences, Department of Nutritional Sciences, The Pennsylvania State University, University Park, PA; Boston College, Boston, MA; ^** Mel and Enid Zuckerman Arizona College of Public Health, University of Arizona, Tucson, AZ; Tribhuvan University, Kathmandu, Nepal; and National Eye Hospital, Kathmandu, Nepal

³To whom correspondence should be addressed. E-mail: fxr5{at}psu.edu.

ABSTRACT

Free holo-retinol binding protein (RBP) [i.e., unbound to transthyretin (TTR)] plays a role in transporting vitamin A across the placenta during pregnancy. In a cross-sectional study of clinically healthy urban women, we assessed the association among clinical and biochemical factors on estimated concentrations of free holo-RBP during the last trimester of pregnancy. Serum samples obtained from a subsample of women (n = 259), who had participated in the Night Vision Threshold Test study in Nepal, were analyzed for determinations of retinol by HPLC, and RBP, TTR, and -1 acid glycoprotein by radial immunodiffusion. Free holo-RBP concentrations were calculated using dissociation constants for free holo- and apo-RBP. Among these women, 30% were vitamin A deficient based on either the RBP:TTR index 0.36 or serum retinol < 1.05 µmol/L. Using stepwise regression analyses, the RBP:TTR index explained 75% of the variance in free holo-RBP concentrations, whereas retinol explained only 14%. Women were classified as vitamin A sufficient (n = 185) or deficient (n = 74) using the RBP:TTR index and were stratified into 3 gestational groups (I: 24–28 wk, II: 29–33 wk, III: >33 wk). Concentrations of free holo-RBP were higher in vitamin A–sufficient women than in vitamin A–deficient women (mean ± SEM, 48.1 ± 1.2 vs. 27.6 ± 0.8 nmol/L; P < 0.001), and in a 3 x 2 factorial analysis, the interaction between gestational group and vitamin A status was significant. These results demonstrate that the RBP:TTR index is a useful proxy for free holo-RBP concentration and that vitamin A status affects its distribution.

KEY WORDS: • retinol • retinol binding protein • RBP:TTR index • transthyretin • vitamin A