Bioavailability of calcium and its absorption inhibitors in raw and cooked green leafy vegetables commonly consumed in India – An in vitro study

The objectives of this research were to assess the bioavailability of calcium using equilibrium dialysis after simulated gastric digestion method in 20 commonly consumed green leafy vegetables (GLVs) from the typical Indian diet, provide data on the content of calcium absorption inhibitors, like oxalate, phytate, tannin and dietary fibres, and evaluate the inhibitory effect of these compounds on calcium bioavailability in raw and cooked GLVs. Cooking did not affect significantly calcium bioavailability in any GLVs. Sesbania grandiflora had a very high content of total oxalates, tannins and dietary fibers, which reduced calcium bioavailability. Calcium content was determined by atomic absorption spectroscopy, oxalate by titrimetry, phytate and tannin by colorimetric and dietary fibres by an enzymatic gravimetric method. Chenopodium album, Alternanthera philoxeroides and Centella asiatica, with lower total calcium content, had nearly twice as much bioavailable calcium than other GLVs, because of low fibres, oxalate, phytate and tannin content.