Spinach as a source of carotenoids, folate and antioxidant activity

Abstract

Fruits and vegetables are generally considered important contributors to a healthy diet and an increased intake of fruits and vegetables is related to a decreased risk of cancers, cardiovascular disease, and other diseases. In this thesis two aspects of spinach, a dark-green, leafy vegetable, are examined. The first aspect is the bioavailability of the carotenoids and folate present in spinach. The second aspect is the antioxidant activity of spinach consumption in humans and the antioxidant capacity of spinach products. The literature on carotenoid bioavailability and bioconversion was reviewed for each of the SLAMENGHI factors. These factors include: Species of carotenoid; molecular Linkage; Amount of carotenoid consumed in a meal; Matrix in which the carotenoid is incorporated; Effectors of absorption; Nutrient status of the host; Genetic factors; Host related factors; and Interactions.A dietary intervention study with 70 healthy human subjects divided over six treatment groups was conducted to examine the effect of the food matrix on the bioavailability of carotenoids and folate and to evaluate the effect of spinach intake on biomarkers of antioxidant activity. Four groups received a basic diet plus a spinach product (whole-leaf, minced, enzymatically liquefied, and liquefied spinach plus added dietary fibre), one group received the basic diet plus a carotenoid supplement ofβ-carotene, lutein and a small amount of zeaxanthin dissolved in oil, and one group received the basic diet only. Consumption of spinach (20 g/MJ), containing the carotenoidsβ-carotene and lutein, increased serum concentrations ofβ-carotene, lutein,α-carotene and retinol and decreased the serum concentration of lycopene compared with the control group. Compared with the synthetic carotenoid supplement, the relative bioavailability ofβ-carotene was low, 5.1-9.5%, but much higher for lutein, 45-55%. Serumβ-carotene responses differed significantly between the whole-leaf and liquefied spinach groups and between the minced and liquefied spinach groups. The plasma folate response was significantly greater in the spinach groups compared with the control group. Intake of minced and liquefied spinach resulted in greater plasma folate responses than consumption of whole-leaf spinach.Thus, disruption of the food matrix (cell wall structure) and loss of cellular structure had an effect on the bioavailability ofβ-carotene and folate, whereas the bioavailability of lutein was not affected. Addition of dietary fibre to the liquefied spinach to compensate for the fibre that was broken down during liquefaction had no effect on serum carotenoid or plasma folate responses.Consumption of spinach or the carotenoid supplement resulted in an increased erythrocyte glutathione reductase activity, and decreased erythrocyte catalase activity and serumα-tocopherol concentration. These changes were related to serum lutein concentrations. The antioxidant capacity of the differently processed spinach products was determined in different oxidation systems. Corrected for the total phenolics content of the spinach, the whole-leaf spinach extract showed the strongest inhibition of hydroperoxide formation, followed by extracts from liquefied and minced spinach. The calculated amount of lutein in the spinach samples used (amounts were adjusted for the total phenolics content of spinach) was correlated with the inhibition of hydroperoxide formation. In a meat ball model at 100 and 200 g spinach/kg meat, whole-leaf and minced spinach were prooxidative (increased the formation of thiobarbituric acid-reactive substances); the liquefied spinach became antioxidative at 200 g/kg. Minced and liquefied spinach were antioxidatively active with respect to the mean inhibition of hexanal formation, whereas whole-leaf spinach showed prooxidant activity.In conclusion, the results of the studies described in this thesis demonstrated an effect of the food matrix onβ-carotene and folate, but not on lutein, bioavailability and support the finding thatβ-carotene is not a good antioxidant in man or in foods, but indicate that lutein may play a role as antioxidant.</p