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Phytic acid has been shown to be detrimental to the energy status of a diet.

In the piglet, it has been shown that increasing phytic acid concentration from7.8 to 17.0 g kg^-1can reduced the AID of energy (Liao et al., 2005). In broilers, an increase from 10.4 to 13.6 g kg^-1dietary phytic acid resulted in a decrease in apparent metabolisable energy (AME) from 14.04 to 13.73 MJ kg^-1dry matter (Ravindran et al., 2006). Miles and Nelson (1974) created a chick diet with and without the inclusion of dephytinised soybean meal and showed the removal of phytic acid increased energy utilisation.

It can be concluded therefore, that phytic acid can decrease the energy available in the diet to an animal. However, energy itself is not a nutrient, but the result of the metabolism of protein, carbohydrate and lipids. Therefore any effect in energy utilisation will be due to a change in the ileal digestibility of amino acids, carbohydrates and lipids (Camden et al., 2001). To

understand the effect of phytic acid on energy the effects of phytic acid on the individual components of energy need to assesed .

As discussed above, multiple studies have shown that phytic acid can

reduce the digestibility of protein and multiple amino acids through a number of possible mechanisms. As well as being an important nutrient within its own right, protein can be used as an energy source and so a reduction in

protein digestibility will affect the AME of a diet. In a broiler diet, the inclusion of sodium phytate, increasing the phytic acid concentration from 8.5 to 14.5 g kg^-1, resulted in an increase in the endogenous loss of protein of 6125 mg kg^-1(Cowieson et al., 2008).This protein loss is equivalent to 96kJ kg^-1DM energy intake (Woyengo and Nyachoti, 2011). However, the phytic acid doesn’t have to increase endogenous losses, just secretions, to have an effect on energy. The digestion of feed has an inherent energy cost, increasing endogenous secretions will increase these costs even if the proteins are later recovered.

Although phytic acid can restrict the energy of a diet through manipulation of protein digestion, this is not the only mechanism. Low phytic acid feeds were made by adding 300g kg^-1phytase treated rapeseed meal to a basal diet.

This was compared to a diet with 300g kg^-1placebo treated rapeseed meal and it was found that the low phytic acid diets had an improved apparent metabolisable energy but not an improved ileal protein digestibility. This suggests the phytic acid can inhibit energy utilisation without impeding protein digestion. Therefore, phytic acid must also interfere with the

digestion of the other energy providing nutrients, starch and lipids (Newkirk and Classen, 2001).

In vitro starch digestibility was improved by 25% when navy beans were treated to remove all their phytic acid (15 g kg^-1); this improvement was reversed by the addition of 10 g kg^-1sodium phytate (Thompson et al., 1987). Yoon et al. (1983) showed similar results whereby 20 g kg^-1phytic acid addition to a human saliva in vitro digestion of wheat starch reduced starch digestibility by 50 %. This was alleviated with the addition of calcium, presumably through the creation of calcium phytate complexes. Lee et al.

(2006) showed that the addition of 10 g kg^-1phytic acid to a phytate free diet reduces glucose absorption through low blood glucose levels in mice.

Onyango et al. (2008) also showed that phytic acid reduced glucose absorption in the jejunum of broilers. Similarly a 6% blood glucose level reduction was seen in broilers when dietary phytic acid concentration was increased from 7.9 to 15.7 g kg^-1(Liu et al., 2008b). This study also reported that increasing dietary phytic acid reduced the activity of maltase, ɑ-amylase and sucrase by, 6, 8.3 and 11.4 % respectively. It appears that phytic acid

could therefore reduce the absorption of carbohydrates by inhibiting the activity of the digestive enzymes. This may explain why Yoon et al. (1983) found calcium alleviated their depressed starch digestion as calcium competed with protein to form complexes. Woyengo et al. (2013) also

suggested that phytic acid could restrict carbohydrate digestion by binding to proteins that are associated with starch. Finally, it is structurally possible that phytic acid may bind to starch through phosphate linkages and therefore reduce starch solubility in addition to digestibility, but there is little evidence for this (Thompson, 1986).

Studies investigating the effects of phytic acid on lipid digestion are sparse, but some work has been done suggesting there is a phytate effect and to explain possible mechanisms. Lee et al (2005) showed that increasing dietary phytic acid from 0 to 15 g kg^-1in mice diets reduced the hepatic total lipids and total cholesterol while also reducing the concentration of

cholesterol, and low density lipoproteins in the blood. This could reflect a decrease in lipid digestion, and in 2007 a similar study supported these findings while showing a reduction in the apparent absorption of total lipids and cholesterol (Lee et al., 2007). Camden et al. (2001)also found that a reduction in phytic acid level with the inclusion of exogenous phytase increased the ileal digestibility of fats.

The reduced digestion of lipids could be due to the formation of lipid-peptide-phytin complexes called lipolipid-peptide-phytins. It is possible that calcium-phytate

complexes may form metallic soaps in the gastrointestinal tract. Matyka et al. (1990) found that the addition of beef tallow reduced phytate hydrolysis in chicks and increased the excretion of soap fatty acids. Ravindran et al.

(2000b) showed that the energy effect of phytate was more pronounced when calcium was increased in the diet, suggesting calcium-phytate

complexes are important in energy utilisation. It is also very likely that phytic acid inhibits the digestion of lipids by interacting with the enzymes of fat digestion. Phytic acid has been shown to reduce the activity of pancreatic lipase in vitro (Knuckles et al., 1989) and in vivo (Liu et al., 2009). Finally, there is the possibity that phtyic acid can, using multivalent cation bridges, bind to bile acids and form mineral-bile-phytic acid complexes. This would

reduce the digestion of fat but this possible mechanism has not yet been tested experimentally.

1.3.5.4 Conclusion on the Anti-nutritional Effects of Phytic Acid Phytic acid is a polyanionic negatively charged molecule within the

gastrointestinal tract, which enables it to bind to a multitude of nutrients and endogenous secretions. This causes the formation of insoluble phytate complexes which are refractory to phytase activity. The phytate-P bound within the complex are less available to the bird, as are the other nutrients bound to phytate. This could be through a drop in the solubility of nutrients within the complex or through increasing endogenous secretion as enzymes activities are reduced, Figure 1.2. Whatever the mechanisms, it is clear that phytic acid can be considered both a potential nutrient and an important anti-nutrient. Some have tried to solve this problem by producing plants low in phytic acid. This will remove the anti-nutritional effect of phytic acid, but also removes an important source of phosphorus and inositol from the animal.

The use of exogenous phytase is a better solution to the phytic acid problem. The increased hydrolysis of phytic acid can remove the anti-nutritional effects while increasing the availability of the phytate-P.

Figure 1.2 Summary of the anti-nutritional effects of phytic acid, adapted from Woyengo et al. (2013)