Responsiveness of dark-adaptation threshold to vitamin A and β-carotene supplementation in pregnant and lactating women in Nepal

Background: Impaired dark adaptation occurs commonly in vitamin A deficiency. Objective: We sought to examine the responsiveness of dark-adaptation threshold to vitamin A and β-carotene supplementation in Nepali women. Design: The dark-adapted pupillary response was tested in 298 pregnant women aged 15-45 y in a placebo-controlled trial of vitamin A and β-carotene; 131 of these women were also tested at 3 mo postpartum. Results were compared with those for 100 nonpregnant US women of similar age. The amount of light required for pupillary constriction was recorded after bleaching and dark adaptation. Results: Pregnant women receiving vitamin A had better dark-adaptation thresholds (-1.24 log cd/m²) than did those receiving placebo (-1.11 log cd/m²; P = 0.03) or β-carotene (-1.13 log cd/m²; P = 0.05) (t tests with Bonferroni correction). Dark-adaptation threshold was associated with serum retinol concentration in pregnant women receiving placebo (P = 0.001) and in those receiving β-carotene (P = 0.003) but not in those receiving vitamin A. Among women receiving placebo, mean dark-adaptation thresholds were better during the first trimester (-1.23 log cd/m²) than during the second and third trimesters (-1.03 log cd/m²; P = 0.02, t test). The mean threshold of nonpregnant US women (-1.35 log cd/m²) was better than that of all 3 Nepali groups (P < 0.001, t test, for all 3 groups). Conclusions: During pregnancy, pupillary dark adaptation was strongly associated with serum retinol concentration and improved significantly in response to vitamin A supplementation. This noninvasive testing technique is a valid indicator of population vitamin A status in women of reproductive age.