Processing of vegetable-borne carotenoids in the human stomach and duodenum

Carotenoids are thought to diminish the incidence of certain degenerative diseases, but the mechanisms involved in their intestinal absorption are poorly understood. Our aim was to obtain basic data on the fate of carotenoids in the human stomach and duodenum. Ten healthy men were intragastrically fed three liquid test meals differing only in the vegetable added 3 wk apart and in a random order. They contained 40 g sunflower oil and mashed vegetables as the sole source of carotenoids. Tomato purée provided 10 mg lycopene as the main carotenoid, chopped spinach (10 mg lutein), and carrot purée (10 mg beta-carotene). Samples of stomach and duodenal contents and blood samples were collected at regular time intervals after meal intake. all-trans and cis carotenoids were assayed in stomach and duodenal contents, in the fat and aqueous phases of those contents, and in chylomicrons. The cis-trans beta-carotene and lycopene ratios did not significantly vary in the stomach during digestion. Carotenoids were recovered in the fat phase present in the stomach during digestion. The proportion of all-trans carotenoids found in the micellar phase of the duodenum was as follows (means +/- SE): lutein (5.6 +/- 0.4%), beta-carotene (4.7 +/- 0.3%), lycopene (2.0 +/- 0.2%). The proportion of 13-cis beta-carotene in the micellar phase was significantly higher (14.8 +/- 1.6%) than that of the all-trans isomer (4.7 +/- 0.3%). There was no significant variation in chylomicron lycopene after the tomato meal, whereas there was significant increase in chylomicron beta-carotene and lutein after the carrot and the spinach meals, respectively. There is no significant cis-trans isomerization of beta-carotene and lycopene in the human stomach. The stomach initiates the transfer of carotenoids from the vegetable matrix to the fat phase of the meal. Lycopene is less efficiently transferred to micelles than beta-carotene and lutein. The very small transfer of carotenoids from their vegetable matrices to micelles explains the poor bioavailability of these phytomicroconstituents.