Interactions Between Phytochemicals and Minerals in Terminalia ferdinandiana and Implications for Mineral Bioavailability

Abstract

Oxalic and phytic acid are phytochemicals considered to be anti-nutritional factors as they are predominantly found as oxalates and phytates bound to minerals like calcium and potassium. Studies have associated excessive oxalate consumption with increased urinary excretion of oxalate (hyperoxaluria) and calcium oxalate kidney stone formation, and excessive phytate consumption with decreased bioaccessibility and bioavailability of certain minerals and reduced utilization of dietary protein. However, other studies suggest that dietary consumption of phytate may be beneficial and inhibit formation of calcium oxalate kidney stones. In light of these conflicting reports, dietary intake of oxalate and phytate enriched plants should be considered in relation to potential health outcomes following consumption. Terminalia ferdinandiana is one such plant and is investigated here with respect to oxalate, phytate, and mineral contents. Assessment of oxalate and phytate contents in T. ferdinandiana fruit, leaf, and seedcoat tissues through hydrolysis into acid forms revealed oxalic acid contents ranging from 327 to 1,420 mg/100 g on a dry weight (DW) basis whilst phytic acid contents ranged from 8.44 to 121.72 mg/100 g DW. Calcium content in the different tissues ranged from 131 to 1,343 mg/100 g. There was no correlation between oxalic acid and calcium, however a significant, positive correlation was observed between phytic acid and calcium (r = 0.9917; p < 0.001), indicating that tissues rich in phytic acid also contain higher levels of calcium. The high content of phytic acid in comparison to oxalic acid in T. ferdinandiana fruit found in this study and the dietary significance of this in terms of calcium bioavailability, needs to be investigated further.

Keywords: Kakadu plum; Terminalia ferdinandiana; ascorbic acid; mineral; oxalate; phytate.

Figure 1

Structures of ascorbic acid (L-AA), phytic acid, and oxalic acid.

Figure 2

Phytate content of T. ferdinandiana. Results are shown as the mean of triplicate experiments ± SD. Different lower case letters indicate significant differences (P ≤ 0.05).

Figure 3

Correlation between oxalate, phytate, and calcium of T. ferdinandiana. (A) Phytate vs. oxalate, (B) oxalate vs. calcium, and (C) phytate vs. calcium in fruits , leaves , seedcoats . No significant associations were found except with phytate vs. calcium (pearson r = 0.9917; p < 0.001).