Following a review of the literature, Cheryan (1980) surmised that phytate can chelate almost all divalent cations and that the stability of the complexes, in order of strongest to weakest, is as follows: Zn2+, Cu2+, Ni2+, Co2+, Mn2+, Ca2+ and Fe2+.
Several others have also found that phytate forms stronger complexes with Zn2+ and
Cu2+ than with other minerals, and there is general agreement that the strength of the phytate-mineral interaction increases as the relative atomic mass of the mineral
increases (Pontoppidan et al., 2007a). Bound cations may interact with one phosphate from the phytate molecule, or form a cationic bridge between two phosphates from either the same or different molecules (Erdman, 1979). These mineral chelates may exist as an insoluble complex that is resistant to hydrolysis, or alternatively an available soluble chelate (Cheryan, 1980).
The extent of phytate-mineral complex formation that occurs within the GIT, and the solubility of these complexes, is driven by the relative proportions of
mineral:phytate and the ambient pH (Weaver and Kannan, 2002). Typically at pH values of 6 - 7 or above, such as those experienced in the distal intestines, mineral complexes begin to precipitate from solution. This is particularly true when the mineral:phytate ratio is high. In general, as the mineral:phytate ratio increases, the pH necessary for complex precipitation decreases (Grynspan and Cheryan, 1983; Pontoppidan et al., 2007a). Conversely, at low pH, there is greater protonation of the phytate molecule, and thus the potential for the formation of insoluble
complexes is minimised (Maenz, 2001). Using a potentiometric technique, Persson et al. (1998) found that stability of the phytate-mineral chelates decreased as the level of phosphorylation on the phytate molecule decreased. It was concluded that degradation of phytate to at least InsP3 is necessary to minimise insoluble mineral
complex formation at higher pH levels and thus improve both mineral and phytate bioavailability.
Oberleas et al. (1962)were among the first to demonstrate the inimical effects of phytate on mineral availability in pigs. Here, weaned pigs were fed a purified casein control diet with or without supplementary phytate (0.7%) or zinc (Zn; 100 mg/kg) for 6 weeks. Pigs fed the diet containing supplementary phytate had a depressed rate of growth and displayed signs of Zn deficiency, such as parakeratosis. These effects were not observed in pigs offered the same diet with supplementary Zn. The authors concluded that phytate was inhibiting Zn absorption.
Numerous in vivo studies have since confirmed an inverse relationship between phytate concentration and cationic mineral digestibility in a range of monogastric species. Woyengo et al. (2009a) fed piglets a casein-maize based diet supplemented with 0, 5, 10 or 20 g/kg sodium phytate and reported quadratic reductions in the AID of Na, K and P, and linear reductions in the AID of Ca and Mg in response to
Mg and Na were negative in pigs receiving the 20 g/kg of sodium phytate treatment (-0.03% and 0.18% respectively), suggesting that phytate is stimulating the
endogenous secretions of these minerals. It is not yet fully understood how phytate stimulates endogenous mineral secretions; however, it has been proposed the presence of insoluble phytate-protein complexes trigger endogenous secretions of Na (as sodium bicarbonate) into the intestines (discussed below in Section 1.5.3; Cowieson et al., 2004). Alternatively, it is possible that through chelating with enzymatic cofactors, phytate is stimulating compensatory mineral outputs via negative feedback mechanisms (Woyengo et al., 2009).
Ravindran et al. (2006)reported a similar effect of phytate on mineral absorption in broilers, as they observed reductions in the AID coefficients of Ca, P, Fe, and Na in response to a 3.2 g/kg increase in dietary phytate concentration. Similarly, in fish, Gatlin and Phillips (1989) found that a 1% increase in dietary phytate reduced Zn availability in Channel Catfish. Many have also studied this inhibitory effect in humans, most notably for Fe, Zn and Ca as deficiencies of these minerals are of global concern (Hallberg et al., 1989; Hurrell et al., 2003; Hambidge et al., 2004; Hambidge et al., 2005).
1.5.1.1 Phytate-calcium interactions
Although phytate has a stronger affinity for minerals such as Cu and Zn, the formation of de novo phytate-Ca complexes are considered more important (Angel et al., 2002). This is because Ca is typically present in pig diets at levels of 50 to 100-fold more than other cations. In a recent review of Ca-phytate interactions in pigs and poultry, Selle et al. (2009a) concluded that up to one third of ingested Ca may be tied up in a phytase resistant Ca-phytate complex. The general consensus is that Ca-phytate complexation occurs in the small intestine, in a 4.93:1 (Ca:phytate) molar ratio (Marini et al., 1985). Precipitation of the Ca-phytate complex begins to take place at pH 4 to 4.5, and increases with increasing pH (Wise and Gilburt, 1981; Grynspan and Cheryan, 1983; Oberleas and Chan, 1997). As the level of
phosphorylation on the phytate molecule decreases, the affinity of the molecule to chelate Ca diminishes in a disproportionate fashion. For example, InsP3 has 10% of
the binding potential of InsP6 (Luttrell, 1993).
A common misconception in phytate nutrition is that monogastric species lack the enzymes necessary to hydrolyse dietary phytate. However, this is not the case, and
several studies have in fact demonstrated that monogastric species can effectively digest phytate, so long as it remains soluble in the intestines. For example, in a study by Nahapetian and Young (1980),rats were fed a high Ca (30.6 mM/ 100 g) or a low Ca (2.9 mM/ 100 g) diet and were provided with an oral dose of C14 labelled phytate. The authors reported a 54% reduction in phytate excretion and a significant increase in C14 recovery from body tissuesin rats fed the low Ca diet. Similarly, Tamim et al. (2004) reported a 63% reduction in the AID coefficient of phytate in broilers fed a corn-SBM diet following an increase in dietary Ca from 2 to 7 g/kg. These studies show that phytate can be hydrolysed by the animal provided the dietary Ca concentration is low. The negative effect of Ca on phytate availability can be attributed to the formation of insoluble Ca-phytate complexes that render both constituents unavailable for absorption (Wise, 1983). Consequently intestinal Ca levels are considered the primary determinant of phytate availability.