Is the ileal apparent digestibility of amino acids in rats a predictor of heating effects on fishmeal

The aim of the study was to determine the ileal apparent digestibility of aminoacids (AA) through a rat bioassay, as a predictor of the effect of processing (pelletized, extrusion) of fishmeal (FM1 and FM2). An animal model of 24 male Sprague-Dawley rats (190 g bodyweight) was used. Rats were alloted to two groups and housed individually in stainless steel wire-bottomed cages in a room maintained at 25oC with a 12 h light/dark cycle and were fed diets containing fishmeal (FM1 or FM2) as the sole protein source. Chromic oxide (0.4%) was included in the diet as an indigestible marker. The rats were trained to consume the diets during a single 3-hour period (8:30 - 11:30 h) for 7 d. Then for another 7 d the experimental diets were offered. In the last three experimental days feeding times were staggered to allow killing the individual rats 4 h after the initiation of feeding. The ileal AA digestibilities (%) for FM1 and FM2 were respectively: lysine 89, 87; methionine 86, 85; histidine 82, 85; phenylalanine 82, 86; tyrosine 84, 80; threonine 78, 72; leucine 87, 87; isoleucine 83, 84; valine 87, 89; aspartic ac. 71, 73; serine 76, 73; glutamic ac. 85, 83; glycine 75, 72; alanine 86, 84; arginine 85, 86. The high ileal digestibility values reflect good quality in both fishmeals. There were no significant differences in ileal AA digestibility values between the two fishmeals analyzed, despite the higher temperature generated during the extrusion process. The ileal AA apparent digestibility is not a good predictor for the adequacity of thermal processing, since AA suffering thermal damage may be capable of being absorbed but are inefficiently utilized. It is recommended that studies related to true AA availability, especially that of lysine be conducted.