VITAMIN A STATUS AND DARK-GREEN LEAFY VEGETABLES
"There is little evidence to support the general assumption that dietary carotenoids [vitamin A precursors] can improve vitamin A status."* A twelve-week study in several villages in West Java, Indonesia investigated effects on vitamin A status of an additional daily portion of dark-green leafy vegetables in anemic women, who were breast feeding their children. "The women receiving vegetables were compared with others given a wafer enriched with beta-carotene, iron, vitamin C, and folic acid, [in order to] examine the effect of a similar amount of micronutrients in a simpler matrix with better bioavailability."
"Special care was taken to ensure consumption of the supplements and to avoid replacement of part of the usual diet by the supplement. Consumption of all supplements was observed and vegetable portions were given to subjects in the early morning when they would not replace vegetable dishes prepared by the participants." As well as closely matching the micronutrients in the supplemental vegetables and in the enriched wafers, other characteristics of the groups were carefully matched, including the per cent with various parasitic infections.
Vitamin A (retinol) improved significantly in the enriched-wafer group but remained virtually unchanged both in the blood serum and the breast milk of the participants in the other two groups. The authors describe a number of factors that may account for the low bioavailability of beta-carotene in dark-green leafy vegetables, including the fact that "physical inaccessibility of carotenoids in plant tissues may reduce their bioavailability. In green leaves, beta-carotene molecules are organized in pigment-protein complexes [within certain cell structures] and in fruits, beta-carotene is found in lipid droplets . . . . It may be difficult to free beta-carotene in dark-green leafy vegetables from its matrix. Perhaps beta-carotene in fruits is more bioavailable, as suggested by seasonal variation in vitamin A status in areas where mangoes are eaten. . . . Light cooking can increase bioavailability, but further cooking can produce isomers . . . with much lower provitamin A activity."
The authors conclude that "our findings do not support the long-standing assumption that vitamin A deficiency can be combated by increasing the intake of dark-green leafy vegetables. . . . Other food approaches to overcoming vitamin A deficiency, such as the use of foods naturally rich in retinol (eggs, whole fish, and liver) and fortified foods, should be developed further."
*de Pee, Saskia, Clive E. West, Muhilal, Darwin Karyadi, Joseph G. A. J. Hautvast, "Lack of improvement in vitamin A status with increased consumption of dark-green leafy vegetables," The Lancet, 346: 75-81, July 8, 1995.
Article from NOHA NEWS, Vol. XXI, No. 3, Summer 1996, pages 5-6.