Inhibition of Calcium and Zinc Absorption by Phytate in Man Methodological Studies and Hydrothermal Processing of Cereals to Improve Absorption

Doctoral thesis, 2002

A mathematical model based on measurements in a whole-body counter described the retention of calcium in the human body. It was shown that the relative retention of intravenous and orally administered 47Ca in all subjects (n = 10 - 11) were well described by fitting a simple function of time with two free parameters to a few whole-body measurements after day 6, when the unabsorbed calcium had been excreted. Calculation from measurements three or more times in the whole-body counter results in greater accuracy and reduces the risk for methodological errors. Extrinsic labelling of leavened white wheat bread with radioisotopes 65Zn and 47Ca in three different ways by adding the isotopes to the bread 16 h before, shortly before serving or by adding the isotopes to the water used in dough making showed no significant difference between the measured absorption of zinc or calcium (expressed as calcium retention at day 7) (n = 8; p > 0.05). A study was done of single model meals, to which different doses of sodium phytate (0 - 250 mg P as InsP6) were added to white wheat bread. The rolls were labelled extrinsically with 65Zn and 47Ca, and whole-body retention was measured. The relationship between InsP6, as mg P and log zinc absorption was described by two linear regressions, one in the interval of 4 - 100 mg (n = 75) and one in the interval of 100 - 250 mg (n = 38). The relationship between InsP6 as mg P and log calcium absorption was described by a linear regression in the interval of 4 - 250 mg (n = 105). The absorption of zinc and calcium were significantly decreased in meals with added 50 mg P and 100 mg P, as InsP6, respectively (p = 0.001 and 0.03). Hydrothermal treatment of whole grains, comprising different wet and dry steeping steps, was shown to offer opportunities for varying the process (moisture content, temperature, pH and incubation time) in order primarily to degrade the phytate. Experiments conducted as central composite designs predicted optimal conditions for maximal degradation of phytate and increase in free myo-inositol. Optimal temperatures and lactic acid concentrations during the incubations were 48 - 50ºC and 0.8% (v/w) for barley and 55ºC and 1.3 - 1.5% (v/w) for wheat and rye. Phytate was reduced in barley, wheat, rye and rice by 94 - 99.8% and free myo-inositol in barley was improved five fold. Zinc absorption from hydrothermally treated barley porridge was 25% and absorption from control porridge, containing ten times the amount of phytate, was 11%, (n = 12; p < 0.001). Zinc absorption from malted barley with remaining phytase activity, served as breakfast cereals, also improved, compared with barley flakes with no phytase activity, 23 versus 15%, (n = 10; p < 0.05). Calcium absorption did not differ from the same meals (p > 0.05).