Spinach is a good source of carotenoids and their oxidised products, the xanthophylls. Carotenoids and xanthophylls act as antioxidants in plants, and in this role can protect the plant tissues from the oxidative stress that results from excessive sun exposure. When animals eat plants they absorb the carotenoids and xanthophylls to their own tissues, where they may play a similar antioxidant role. However, as with all phytonutrients the absorption and therefore the bioavailability of the carotenoids and xanthophylls from spinach may be limited by the presence of the cellulose cell wall. Cellulose is not digestible by human enzymes and as such the walls must be mechanically damaged in order for the contents to be available for absorption. In the case of spinach this means that the cells must be cooked or chewed in order to allow the release of the carotenoids and xanthophylls contained within. The cellulose fibre is possibly therefore a limiting factor in the bioavailability of spinach carotenoids and xanthophylls.
Researchers have investigated the bioavailability of spinach carotenoids and xanthophylls in healthy humans. For example, in one study1, researchers fed subjects either a control diet, a diet supplemented with carotenoids, or whole intact spinach leaves, minced spinach leaves, enzymatically liquefied spinach leaves, or liquefied spinach with added dietary fibre. Consumption of spinach products significantly increase blood levels of β-carotene, lutein, α-carotene and retinol, but decreased blood levels of lycopene. The relative bioavailability of lutein compared to the supplement was 45, 52, 55 and 54 % for the whole leaf, minced, liquefied and liquefied with fibre groups, respectively. The relative bioavailability of β-carotene compared to the supplement was 5.1, 6.4, 9.5 and 9.3 % for the whole leaf, minced, liquefied and liquefied with fibre groups, respectively. Therefore the bioavailability of lutein from spinach is higher than that of β-carotene, but disruption of the matrix has little effect on xanthophyll absorption, although it may increase the absorption of carotenoids.