carrot root, α- and β-carotene are largely in crystal forms. In both cases, the carotenoids are not easily solubilized out of these tissues by the digestive process.
The hypothesis that cooking may improve the bioavailability of carotenoids has been tested. The bioavailability of lycopene from tomato juice is vastly improved by heat treatment in the presence of oil (Gartner et al., 1997; Stahl and Sies, 1992). When subjects consumed tomato juice (equivalent to a single lycopene dose of 2.5 µmol/kg body weight) that had been heated at 100°C for 1 hour with oil, they experienced a serum lycopene peak at 24 to 48 hours. In contrast, equivalent doses that were not heat treated did not result in an increase in serum lycopene. Steaming has also been shown to increase the amount of extractable carotenoids in spinach and carrots (Dietz et al., 1988). In contrast to steaming, more prolonged exposure to high temperatures (boiling) can reduce the carotenoid availability of vegetables by increasing the production of isomers or oxidation products. For example, canned carrots contain 73 percent all-trans β-carotene, 19 percent 13-cis-β-carotene, and 8 percent 9-cis-β-carotene, while fresh carrots contain 100 percent of the β-carotene in the all-trans configuration (Chandler and Schwartz, 1987). The relative vitamin A values of cis isomers of β-carotene compared to all-trans β-carotene is an active area of research.
Many research groups have shown that to optimize carotenoid absorption, dietary fat must be consumed during the same eating period as the carotenoid. Roels et al. (1958) demonstrated that in boys with vitamin A deficiency in an African village, supplementation of their carotene-sufficient but low-fat diets with 18 g/day of olive oil improved carotene absorption from 5 to 25 percent. More recently, Jalal et al. (1998) studied the roles of β-carotene-rich meals (mostly red sweet potatoes), extra dietary fat (15 g/day), and deworming on serum retinol concentrations of children in Sumatra. Prior to the intervention, these children all had intestinal infestations and were consuming diets with about 7 percent of calories from fat. A 3-week intervention of β-carotene-rich meals alone improved vitamin A status without added fat or deworming, but the combination of all three measures—β-carotene meals, added fat, and deworming—provided the greatest increase in serum retinol.