Nutritional importance of phytate
Phosphorus is an imperative nutrient for numerous biochemical pathways,
physiological processes and skeletal integrity, but due to the partial availability
of phytate-P, diets are supplemented with P sources such as dicalcium
phosphate or, where permitted, meat-and-bone meal to meet P requirements.
However, it may be argued that P requirements have been neither consistently
nor accurately defi ned, and are presently further complicated by the dietary
inclusion of microbial phytases. The dependence on inorganic P supplements
is a challenge, because global reserves of rock phosphate are not renewable
(Abelson, 1999) and the price of phosphates has escalated in recent years.
However, the nutritional importance of phytate is not limited to P availability.
Notionally, the polyanionic phytate molecule may carry 12 negative charges
that confer a tremendous capacity for IP6 to chelate divalent cations, including
Ca2+, Zn2+, Fe2+ and Cu2+, and the availability of these complexed minerals is
reduced. The formation of insoluble Ca–phytate complexes in the small
intestine, probably as Ca5-K2-phytate (Evans and Pierce, 1981), reduces the
availability of both Ca and P and, for this reason, phytate is considered to be an
aetiological factor in ‘rickets’ or osteomalacia (Mellanby, 1949). Similarly,
because of its particular affi nity for Zn, phytate limits Zn availability and phytate
is considered to be a causative factor of parakeratosis, which is a manifestation
of Zn defi ciency in swine (Oberleas et al., 1962). Indeed, much of our knowledge
of phytate stems from the development of procedures to extract phytate from
soy protein concentrates because of concerns about phytate reducing the
availability of Zn in diets for humans (Sandstead, 1992; Wise, 1995).
Phytate extraction from protein concentrates has revealed that phytate
has the capacity to bind protein either as binary or ternary protein–phytate
complexes (Cosgrove, 1966). Binary complexes are more important because
they have the potential to bind more protein than ternary protein–phytate
complexes (Champagne et al., 1990). Given the capacity of phytate to bind
directly with protein, it follows that phytate may depress amino acid digestibility
(Offi cer and Batterham, 1992a,b). In all likelihood this is the case, but outcomes
of phytase ileal amino acid digestibility assays in pigs, in particular, and poultry
do not consistently support this proposal. The reasons for these inconsistencies
and the mechanisms whereby phytate may depress amino acid digestibility are
discussed later, but the likelihood is that de novo formation of binary protein–
phytate complexes at acidic pH in the gut are fundamental to the ‘protein
effect’ of phytate and phytase.
Also, on the basis of responses to dietary inclusions of phytase, phytate
appears to depress energy utilization, which is more evident in broilers than in
pigs. Again, the ‘energy effect’ of phytate and phytase is discussed later.
However, graded inclusion levels of A. niger phytase in diets based on wheatsorghum
blends have illustrated the negative effects of phytate on protein and
energy utilization in broilers. As shown in Fig. 7.2, Ravindran et al. (2001)
reported that increasing phytase inclusion levels improved both average ileal
amino acid digestibilities and dietary available metabolizable energy (AME)
values in an essentially linear manner. The peak responses recorded were at
1000 FTU kg–1, where phytase increased average apparent ileal digestibility
(AID) coeffi cients of 15 amino acids by 5.7%, from 0.775 to 0.819, and at
750 FTU kg–1, where phytase increased AME by 0.50 MJ, or 3.5%, from
14.22 to 14.72 MJ kg–1.
Dephytinization
The pre-feeding elimination of phytate from a feed ingredient, or dephytinization,
is an interesting approach in overcoming the anti-nutritive properties of phytate
that may have application in practice, particularly in aquaculture. Also,theoretically, dephytinization should be a means of defi ning the anti-nutritive
properties of phytate. Canola meal contains relatively high phytate levels; in
one survey, canola meal contained averages of 8.76 g total P kg–1 and 6.69
g phytate-P kg–1, or 76.4% of total P (Selle et al., 2003b). Newkirk and Classen
(1998, 2001) dephytinized canola meal and incorporated untreated, shamtreated
and dephytinized canola meal into maize–soy broiler diets at 300 g
kg–1. In comparison with the sham-treatment, dephytinization of canola meal
with a purifi ed phytase increased average AID coeffi cients of 17 amino acids
by 16.0%, from 0.648 to 0.752. Individual increases in amino acid digestibility
ranged from 39.8% (proline) to 0.2% (methionine), and the majority of
responses were statistically signifi cant. Given that approximately half the
dietary protein was derived from canola meal, the implication is that
dephytinization substantially increased the digestibility of amino acids in canola
meal. This raises the question as to the actual extent by which phytate depresses
protein/amino acid digestibility. Therefore, the anti-nutritive properties of
phytate may be potent but they are not fully declared by phytase supplementation,
because degradation of phytate is incomplete (Selle and Ravindran, 2007).
However, though dephytinization via industrial processes is possible, it may
lead to inimical changes other than the removal of phytate, e.g. Maillard
complexing of lysine following heat treatment, and so a cost–benefi t analysis is
warranted.Determination of phytate concentrations
A fundamental issue, as emphasized by Lasztity and Lasztity (1990), is that the
determination of phytate concentrations is not a straightforward procedure. In
the majority of cases, phytate-P concentrations are determined by methods
based on the ferric chloride-precipitation principle of Heubner and Stadler
(1914). Phytate is precipitated by ferric chloride (Fe3Cl2) at acidic pH, and
concentrations of P or Fe are determined in the supernatant or the precipitate
from which the phytate-P concentration is calculated. Given complete phytate
extractions, these methods are satisfactory for phytate determinations in
individual feedstuffs, but they do not differentiate between the various myoinositol
phosphate esters present. However, in more complex samples (e.g.
complete diets, ileal digesta), ferric chloride-precipitation methods are not
satisfactory due to co-precipitation of P from other sources.
An important limitation is that basic methods of determining phytate-P do
not have the capacity to identify the various myo-inositol phosphate esters of
phytate. However, it is possible to differentiate phytate esters with high performance
liquid chromatography, anion exchange chromatography and
nuclear magnetic resonance spectroscopy (Phillippy and Johnston, 1985;
Rounds and Nielsen, 1993; Skoglund et al., 1998; Kemme et al., 1999).
However, these advanced methods require sophisticated equipment and may
be expensive and time consuming (Kwanyuen and Burton, 2005; Gao et al.,
2007). Arguably, the current diffi culties associated with accurate phytate
analysis have been an important constraint on scientifi c progress in this area.