Pre-hydrolysis improves utilisation of dietary protein in the larval teleost Atlantic halibut (Hippoglossus hippoglossus L.)

A protein preparation labelled by incorporation of U]¹⁴C-AA was hydrolysed to various degrees and administered to a teleost fish larva (Atlantic halibut, Hippoglossus hippoglossus L.) by tube-feeding, and its post-administration utilisation was studied. Three treatments were prepared: IntP—intact protein, PHP—pepsin-hydrolysed protein, and HHP—highly hydrolysed protein (using pepsin, trypsin, endoproteinase Glu-C, Asp-N, and Pro-C). At small doses (11.4±1.5 μg larvae⁻¹), the intact protein (IntP) was digested and absorbed to 36±5.5%. However, the relative absorption efficiency of the intact protein was reduced as the dose increased. Absorption efficiency was higher when the protein was hydrolysed prior to feeding the larvae and was constant at 63% (R²=98) independent of degree of proteolysis and dose (ranging from 3.5 to 35 μg larvae⁻¹). The initial absorption rate increased with the degree of hydrolysis. Calculations based on data collected during the first 30-120 min show that the absorption of PHP and HHP into extra-intestinal body tissues was 2.2 and 3 times as fast, respectively, as that of intact protein. However, the rates of absorption did not influence the distribution of absorbed AA into either catabolism or anabolism, as all larvae, independent of which protein solution they were given, catabolised 42±7% of the absorbed AA, and accumulated 49±6% into the body tissue, during the 20 h post-feeding incubation period.Larval age and size did not influence the absorption and utilisation of the hydrolysed protein preparations. This was different from the intact protein, as significantly higher fractions of the intact protein were absorbed by the larvae at 31 days past first feeding (dpff) than by larvae at 25 dpff. Analysis of the faecal evacuation suggested that the poor protein utilisation in the younger larvae was due to enhanced faecal evacuation, which in some larvae was more than 50% only 4 h post diet administration, at a time when the process of digestion and absorption was far from complete. This indicated that faecal evacuation is a critical factor in the utilisation of slowly digested and absorbed feed components, such as intact proteins, by fish larvae.