ABSTRACT

Provitamin A carotenoids, tocopherols, and phylloquinone, as the major fat-soluble micronutrients in salad vegetables, play essential roles in maintaining various physiological processes, such as cell differentiation and proliferation, normal organogenesis, and blood clotting. The non-provitamin A carotenoids, particularly lutein, zeaxanthin, and lycopene are important for maintaining ocular health and preventing chronic diseases. The intestinal uptake of fat-soluble micronutrients involves both simple diffusion and receptor-mediated transport. Various exogenous factors are able to affect the uptake process, e.g. food matrix and processing, dietary fat and fiber, and nutrient species and stereoisomers. Transferring fat-soluble micronutrients into the blood circulation depends on the normal synthesis of apolipoproteinB (apoB) and apoB-containing lipoprotein, which is promoted by the presence of sufficient lipids and suppressed by insulin. Overall, dietary fat is an elementary factor regulating the absorption of fat-soluble micronutrients.

Lecithin may influence the bioavailability of fat-soluble micronutrients. The role of the large very-low-density lipoprotein (VLDLA) plasma fraction in postprandial fat-soluble micronutrient transport is not clearly defined. Therefore, we conducted a human study to investigate the effects of the lecithin/oil ratio in salad dressing on the absorption of: 1) carotenoids, phylloquinone, and tocopherols from salad vegetables; 2) retinyl palmitate formed in the intestine from the provitamin A carotenoids. An additional objective was to investigate the origin of plasma chylomicrons and VLDLA and their roles in the transport of the absorbed fat-soluble micronutrients. Healthy women (n = 12) each consumed three salads with salad dressings containing: 1) 4 g soybean oil and 0 g hydroxylated soy lecithin (Solec® 8120, Solae, St. Louis, MO); 2) 3.8 g soybean oil and 0.2 g Solec® 8120; or 3) 3.2 g of soybean oil and 0.8 g Solec® 8120. The order in which the salads were consumed was randomly assigned according to a Williams Latin square design; salads were separated by ≥ 2 weeks. Blood was collected at baseline and 2, 3.5, 5, 7, and 9.5 h postprandially. Chylomicrons and VLDLA fractions were analyzed by HPLC with coulometric array electrochemical detection. ApoB-48 and apoB-100 contents in chylomicrons and VLDLA were determined by ELISA. There were no significant differences among the salad dressings in the resulting AUC values of the fat-soluble micronutrients in the chylomicron and VLDLA fractions. The exception was a decrease in the AUC value of phylloquinone in the chylomicron fraction when the salad dressing containing 0.8 g was compared with that containing 0 g Solec® 8120 (P < 0.02). The AUC values for α-carotene, β-carotene, lycopene, retinyl palmitate, and phylloquinone were substantially higher in the VLDLA than in the chylomicron fraction (P < 0.05). Both chylomicron and VLDLA fractions contained apoB-48 and apoB-100. ApoB-48 and apoB-100 were predominantly found in the VLDLA fraction (P < 0.0001). The AUC value of apoB-100 in VLDLA was significantly higher than that of apoB-48 in chylomicrons (P < 0.0001). We concluded that the hydroxylated soy lecithin in the amounts added to the salad dressing did not enhance the absorption of carotenoids, retinyl palmitate, phylloquinone, or tocopherols. The majority of the newly absorbed carotenoids, retinyl palmitate, and phylloquinone, as well as enterogenous (apoB-48) and hepatogenous (apoB-100) lipoprotein particles, were contained within large VLDL. Thus this lipoprotein subfraction has a major role in the transport of newly absorbed carotenoids and fat-soluble vitamins.