Numerous factors affect the intestinal absorption of carotenoids. Digestion of food in the stomach increases accessibility of carotenoids for absorption by maceration in HCl and digestive enzymes. The acidic environment of the stomach helps to disrupt cell walls and other cellular ultrastructure of raw fruits and vegetables and causes further breakdown of cooked foods to release carotenoids from food matrices in which they are contained or bound.
Carotenoids in green leafy vegetables are found in chloroplasts; those in fruit are located in chromo-plasts. Absorption studies comparing plasma levels of 0-carotene and retinol after consuming fruit vs. green leafy vegetables showed that 0-carotene is more efficiently absorbed from fruit, indicating that chloroplasts (or the bonds linking chloroplast proteins and carotenoids) are more resistant to disruption in the digestive tract than chromoplasts. Thus, the location of a carotenoid in the cell affects its accessibility.
Carotenoid isomerization can occur in the acidic gastric milieu. Lycopene present in fruits and vegetables occurs almost exclusively as the all-trans isomer, but is converted to cis isomers, which seem to be more bioavailable. Plasma and tissue profiles show that cis isomers make up more than 50% of the total lycopene present. On the other hand, studies show that no trans/cis isomerization of 0-caro-tene occurs in the stomach. In fact, evidence has been found for transfer of a significant portion of both 0- and a-carotene to the fat phase of the meal in the stomach, which would increase bioavailability of these carotenoids for absorption. No studies are available relating isomerization to bioavailability of other carotenoids.
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