tinal tract, (3) uptake of carotenoids by intestinal mucosal cells, and (4) transport of carotenoids and their metabolic products to the lymph or portal circulation.
Of the factors that affect carotenoid bioavailability, the food matrix effects on carotenoid absorption are generally the most critical. The absorption of β-carotene supplements that are solubilized with emulsifiers and protected by antioxidants can be 70 percent or more. In contrast, less than 5 percent bioavailability of carotenes has been reported from raw foods such as carrots (Rodriguez and Irwin, 1972). Recently, van her Hof et al. (1999) reported substantial differences between the relative bioavailabilities of β-carotene (14 percent) compared to lutein (67 percent) when feeding a high-vegetable diet (490 g of vegetables without supplements) and comparing it to a low-vegetable diet (130 g of vegetables) supplemented with β-carotene (6 mg/day) or lutein (9 mg/day), both of which were assumed to be 100 percent bioavailable. These differences were based on changes in plasma concentration of β-carotene or lutein.
Daily supplementation of dark-green leafy vegetables rich in carotenoids to lactating Indonesian women with low vitamin A status did not increase vitamin A status, whereas a similar amount of β-carotene given in a wafer supplement led to a significant increase in plasma retinol (de Pee et al., 1995). More recently, the same group (de Pee et al., 1998) studied anemic school children in Indonesia and calculated the relative vitamin A equivalency of β-carotene from different food sources. The calculated equivalencies were as follows: 26 µg of β-carotene from leafy vegetables and carrots corresponded to 12 µg of β-carotene from fruit, and equaled 1 µg of preformed vitamin A in vitamin A-rich foods. In contrast, Mahapatra and Manorama (1997), in a small study with vitamin A-deficient school children in India, concluded that β-carotene from red palm oil was as bioavailable as preformed vitamin A.
β-Carotene in the form of supplements has a much higher bioavailablity than β-carotene from foods. Micozzi et al. (1992) demonstrated that 30 mg/day of supplemental all-trans β-carotene produced more than a fivefold increase in plasma β-carotene compared to 29 mg/day of β-carotene from carrots. The relatively low bioavailability of plant carotenoids may be due to the fact that they can be bound in carotenoproteins and are often associated with the plant matrix. Typically in green leafy vegetables, carotenoids are found bound in chloroplasts where they play roles in photosynthesis. In