The effect of phytate and phytase on the bioavailability of phosphorus has been appreciated for many years and phytase has been exploited commercially since the late-1990s to considerable global success. Although a number of authors have reported improved digestibility of amino acids, energy and other minerals (most notably calcium) by the addition of phytase to poultry diets, these so-called ‘extra-phosphoric’ effects of phytase are less well understood. Recent work has shed light on the underlying mechanisms by which phytase influences the apparent digestibility of amino acids, energy and minerals and much of the improvements are thought to be related to enhanced absorption of exogenous and re-absorption of endogenous amino acids. The synthesis and loss of endogenous protein is nutritionally expensive for animals and is negatively correlated with metabolisable energy, net energy, and the digestibility of nitrogen and amino acids. It has been estimated that as much as 50% of the total nitrogen present in the small intestine during digestion is of endogenous origin, resulting in negative digestibility coefficients in the proximal small intestine. Clearly the successful resorption of the secreted protein in the small intestine is critical to maintain favourable nitrogen balance and most endogenous proteins will eventually be digested and absorbed. However, a considerable proportion of endogenous secretions leave the terminal ileum and represent a net cost to the animal, the environment, and, importantly, to the poultry producer. Thus, dietary anti-nutrients that induce secretion and/or impair absorption e.g. tannins, trypsin inhibitors, or phytate can have substantial adverse consequences for productivity. The measurement and nutritional significance of endogenous losses in poultry, specifically with regard to the effect of phytate and phytase, are examined in the current review, in an attempt to explain the extra-phosphoric nutrient matrices assigned to commercial phytase products.
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