The purpose of the present study was to investigate the effect of supplementary phytic acid (PA), phytase, calcium and citric acid (CTA) on the in vitro- and in vivo-availability of minerals (Ca, P, Mg), trace elements (Mn, Zn) and potential toxic heavy metals (Cd, Pb).
Phytate rich diets (0.69 %) based on maize, soybean meal, corn starch and soybean oil (containing 5.0 g Ca, 1.2 g Mg, 22 mg Zn, 10 mg Pb and 5 mg Cd per kg diet) were supplemented with (a) 0.3, 0.6, 0.9 and 1.2 % PA as sodium phytate, (b) 250, 500, 1000 and 2000 phytase units (PU) from Aspergillus niger, (c) 0.2, 0.4, 0.6 and 0.8 % Ca as CaCO3 and (d) 0, 1, 2, 3 and 4 % of CTA. All diets were digested in a multi-enzyme in vitro system followed by equilibrium dialysis. Under the conditions investigated, dietary addition of PA resulted only in a marginal reduction of the in vitro-availability of Ca and Mg whereas dialysability of P was slightly enhanced. Mn, Zn and Cd were not affected. No Pb dialysability could be detected. These results can be explained by the low affinity of Ca, Mg and Mn towards PA as well as by an extremely stable and almost complete complexation of Zn, Pb and Cd by the PA molecule.
Phytase supplements resulted in a significant and dose-dependend increase of Ca, P, Mg, Mn, Zn and Cd dialysability. These findings demonstrate the efficiency of microbial phytase to hydrolyze PA-mineral-complexes. Under given conditions 2000 PU per kg diet not result in a further improvement of element availability. Therefore supplementation of 1000 PU/kg can be considered as an optimum improving dialysability of the investigated elements.
Calcium supplements resulted in a significant increase in Ca, Mg as well as Mn dialysability and a tendencial increase in Zn availability. These effects might be due to a displacement of elements from the PA-complex owing to excess Ca. P and Cd however were not influenced dose-dependently by Ca supplements. These results can be explained by the high affinity of these elements towards PA as well as a calcium induced stabilisation of PA.
CTA enhanced dialysability of calcium, phosphorus, magnesium, manganese and zinc. Furthermore, a dose-dependent increase in lead and cadmium dialysability due to CTA supplementations was evident. This effect might be due to ligand competition between CTA and PA.
Thirty-five (5 groups with 7 animals each) male albino rats (initial average weight = 44 g) were fed phytate-rich diets (analysed phytic acid concentration = 6.9 g/kg) based on maize and soy bean meal (5 g Ca, 3 g P, 1.2 g Mg, 23 mg Zn, 10 mg Pb, 5 mg Cd/kg diet) for 28 days. Experimental diets were supplemented with 0, 2, 4, 6 and 8 g calcium from CaCO3 per kg. The supplementation of increasing amounts of calcium resulted in a dose-dependent decrease in the apparent absorption of phosphorus. Furthermore, apparent zinc absorption and femur zinc concentration were moderately decreased due to the calcium supplementation. Kidney Cd concentration was significantly lower in rats fed the high calcium diets as compared to the control animals. Femur lead concentration and hepatic d-aminolevulinic acid dehydratase known as sensitive parameters of lead accumulation remained unchanged by the different dietary treatments. Magnesium absorption as well as liver and plasma zinc concentration and activity of plasma alkaline phosphatase were also unaffected. Although calcium supplementation may lead to a decrease in the accumulation of certain heavy metals such as cadmium, the carry-over of lead was not affected under the given experimental conditions. Furthermore, calcium-phytate-zinc interactions may adversely affect zinc bioavailability in growing rats.
Thirty-five (5 groups with 7 animals each) male albino rats (initial average weight = 51 g) were fed phytate-rich diets (analysed phytic acid concentration = 6.9 g/kg) based on maize and soy bean meal (5 g Ca, 3 g P, 1.2 g Mg, 23 mg Zn, 10 mg Pb, 5 mg Cd/kg diet). Experimental diets were supplemented with 0, 1, 2, 3 and 4 % citric acid (CTA) per kg. Increasing dietary amounts of CTA did not improve bioavailability of essential elements. However, CTA supplements resulted in a dose-dependend and partially significant increase in liver and kidney Cd accumulation.
The given results demonstrate, that in vitro-results do not always correspond well with in vivo findings. In vitro systems never can simulate all relevant factors influencing element availability in vivo. Therefore, in vitro results should always be considered as relative rather than absolute indicators of bioavailability.