The e#ects of dietary phytate and microbial phytase supplementation on amino acid availability in poultry
diets are of considerable practical importance. Published data on the influence of exogenous phytase
supplementation on the ileal digestibility of amino acids in poultry are reviewed in this paper, with emphasis on
the factors causing variability in amino acid responses. Several modes of action have been proposed by
researchers to explain the influence of phytate on protein digestion, but it is hypothesised that the de novo
formation of binary protein-phytate complexes in the gut under acidic conditions in the proventriculus may be
the main mechanism whereby phytate depresses the digestibility of dietary amino acids as bound protein is
refractory to pepsin digestion. It is also likely that phytate promotes the flow of endogenous amino acids. The
review demonstrates that the choice of inert marker used in the digestibility assays is a major factor responsible
for the variable phytase responses reported in the literature. The important diet-related causes for the variability
include di#erences among ingredients, dietary levels of Ca and nonphytate P and dietary electrolyte balance, and
clearly amino acid responses with added phytase may be enhanced by considering these issues in poultry feed
formulations. Phytase feed enzymes, however, do not degrade the majority of dietary phytate in poultry and
consequently the negative influence of phytate on protein digestibility is not completely removed by phytase
supplementation. It is proposed that the experimental use of ‘dephytinised’ feed ingredients may be useful to
define the actual extent to which amino acid digestibility is compromised by dietary phytate. The e$cacy of
current phytase feed enzymes may be further enhanced by the simultaneous use of other exogenous enzymes,
which complement their activity, increase substrate access and/or absorption of liberated nutrients.