Small quantities of carotenoid-rich tropical green leafy vegetables indigenous to Africa maintain vitamin A status in Mongolian gerbils ( Meriones unguiculatus)

Abstract

Leafy vegetables are important sources of provitamin A carotenoids. Information on their ability to provide vitamin A is often misleading because of the methodology used to assess bioefficacy. Mongolian gerbils were used to evaluate the bioefficacy of provitamin A carotenoids in tropical leafy vegetables (i.e. Solanum nigrum, Moringa oleifera, Vernonia calvoana and Hibiscus cannabinus) that are indigenous to Africa. Gerbils (n 67) were vitamin A-depleted for 5 weeks. After a baseline kill (n 7), the gerbils were weight-matched and assigned to six treatment groups (n 10; four vegetable groups; negative and positive controls). For 4 weeks, the treatments included 35 nmol vitamin A (theoretical concentrations based on 100 % bioefficacy) in the form of vegetables or retinyl acetate. In addition to their diets, the control and vegetable groups received daily doses of oil, while the vitamin A group received retinyl acetate in oil matched to prior day intake. Serum and livers were analysed for vitamin A using HPLC. Serum retinol concentrations did not differ among groups, but total liver vitamin A of the vitamin A and vegetable groups were higher than that of the negative control group (P < 0.0001). Liver beta-carotene 15,15'-monooxygenase-1 expression levels were determined for two vegetable groups and were similar to the positive and negative controls. Conversion factors for the different leafy vegetables were between 1.9 and 2.3 microg beta-carotene equivalents to 1 microg retinol. Small quantities of these vegetables maintained vitamin A status in gerbils through efficient bioconversion of beta-carotene to retinol.

Fig. 1

Liver retinol reserves in μmol/liver of gerbils fed Solanium nigrum (SN) Moringa oleifera (MO), Vernonia calvoana (VC), or Hibiscus cannabinus (HX). A baseline (Base) group was killed before treatment started. A control (Con) group received cottonseed oil only and the vitamin A (VA) group received a dose of retinyl acetate that was matched to one-half (in mol) of the β-carotene equivalents (1 mol β-carotene + 1 mol cis-β-carotenes + ½ mol α-carotene) that the vegetable groups consumed the day before. Groups with a common letter are not significantly different.

Fig. 2

Expression of β-carotene 15,15′-monooxygenase-1 (BCMO1) in the livers of gerbils fed a vitamin A-free basal diet and given daily oral doses of cottonseed oil only, vitamin A (35 nmol), or fed a customized diet containing powdered S. Nigrum or V. calvoana containing 35 nmol theoretical vitamin A (A). α-Tubulin was used as a house keeping protein. The BCMO1 bands were densitometrically analysed (B) with normalisation to α-Tubulin to measure expression level differences among treatment groups (P = 0·12).