Calcium content, in vitro digestibility, and bioaccessibility in leaves of spinach (Spinacia oleracea), sweet potato (Ipomea batatas), and drumstick tree (Moringa oleifera)

Abstract

Low calcium intake, poor calcium absorption, excessive calcium losses, or some combination of these factors contribute to calcium deficiency diseases. Calcium insufficiency is associated with osteoporosis, hypertension and colon cancer among other diseases. For individuals who do not have access to milk and dairy products, do not tolerate them, or prefer not to consume them, plants may be  alternative sources of calcium. However, calcium bioavailability may be low in plant foods because calcium forms complexes with oxalates, phytate and other competing minerals. The objective of this study was to compare the calcium content, digestibility and bioaccessibility in the leaves of spinach (Spinacia oleracea), sweet potato (Ipomea batatas), and drumstick tree (Moringa oleifera). Calcium content was analyzed in dry leaf powder by atomic absorption, followed by a two-stage in vitro digestion and dialysis against a mock serum solution to determine calcium digestibility and bioaccessibility. Moringa oleifera had higher calcium content than spinach and sweet potato leaves: 1.54±0.11% of dry matter for three Moringa samples (two African and one from India), and 0.99±0.001 and 1.06±0.001, respectively, in spinach and sweet potato leaves. The mean in vitro calcium digestibility was 1.62±0.08% in spinach, 3.4±0.68% in sweet potato leaves and 33.7±9.6% for Moringa. A dialysis system was designed to model bioaccessibility of calcium, revealing that bioaccessible calcium in sweet potato leaves was a non-significant 1.4 times higher and in Moringa was 9.2 to 19.4 times higher than in spinach. Therefore, the calcium contained in Moringa leaves does not appear to be associated with poorly bioavailable complexes such as oxalate. We confirmed previous reports that bioaccessibility of the calcium is low in spinach. These findings imply that increased utilization of Ipomea batatas and Moringa oleifera leaves might increase calcium intake in people in tropical and warm temperate regions where these plants grow, or these plants might become a valuable export crop.