Measures of amino acid (AA) digestibility are used widely to estimate bioavailability of AA in feed and food ingredients for monogastric animals. In principle, the digestibility assay is simpler than in vivo assessments of AA bioavailability and allows for simultaneous estimation of the bioavailability of all AA in an experimental diet. It is generally assumed that absorption of intact AA in the hindgut of monogastrics is minimal, even though colonocytes do contain AA transporters and have been shown to absorb AA. This assumption is supported by the observation that infusion of AA into the hindgut does not improve nitrogen balance in monogastrics. In addition, growth performance of monogastrics is more highly correlated with ileal than faecal AA digestibility. Therefore, ileal digestibility coefficients provide better estimates of AA bioavailability than faecal digestibility coefficients. Measures of apparent ileal digestibility (AID) of AA are confounded with endogenous gut AA losses (EAAL). The curvilinear increase in AID of AA with increasing dietary AA level has been attributed to the relatively large contribution of EAAL to total ileal AA flows at low dietary AA levels. Subtracting basal EAAL from total ileal AA flows yields standardized ileal digestibility (SID) coefficients that appear to be more additive than AID coefficients in mixtures of feed ingredients. An implicit assumption when using SID AA coefficients in diet formulation is that the post-absorptive utilization of AA is not influenced by the dietary protein source. This assumption appears inappropriate when using feed or food ingredients that have been over-heated, induce large amounts of EAAL, or contain substantial amounts of fermentable fibre. Improved understanding of processes that contribute to the discrepancy between bioavailability and ileal digestibility will allow a more effective use of AA digestibility coefficients in diet formulation.

Phytases have been used commercially since the early 1990s and have been the focus of considerable and sustained research for many decades. Despite this heroic effort there are still areas of persistent uncertainty such as the obscurity surrounding total compared with digestible calcium, appropriate modification to dietary sodium (and other electrolyte) concentrations, the usefulness of the amino acid and energy digestibility improvements and ultimately the effect of phytase on nutrient requirement. One further area which has attracted some attention recently is the effect of unconventionally high doses of phytase (i.e. >2,500 FTU/kg from Aspergillus niger or Escherichia coli) in an attempt to ostensibly ‘de-phytinise’ the diet. The effects of such ‘super’ doses of phytase can be considerable, and often beyond that which may be reasonably expected based on improvement in P digestibility per se. This review article addresses these effects and suggests mechanisms by which they may be explained.