Protein digestibility evaluations of meat and fish substrates using laboratory, avian, and ileally cannulated dog assays

Meat and fish are important protein resource in pets’ diet, but we don’t fully know their different digestibility. The purpose of this study is to evaluate the protein digestibility and bioavailability of the AAs in different protein sources through 3 different methods in vivo and vitro.

In recent years we are observing a growing interest in the quality of pet food among pet owners; the humanization process of the animals and the greater interest in their health have contributed to shifting the owners’ demand more and more towards good products and whose origin is easy to understand.
There are some works about the composition of different protein sources of animal origin in bibliography, while documents relative to animal protein digestibility are still insufficient. At the time when the study was conducted, there were no comparative studies on the digestibility of animal protein sources of mammalian, avian and fish origin.
The aim of this study was to evaluate the differences in the composition and protein digestibility of three sources of meat (beef loin, pork loin, chicken breast) and two sources of fish (pollock fillet, salmon fillet). The nutritional composition and digestibility of protein sources were assessed in detail through the use of three methods:

1. IDEA kit (enzymatic kit)
2. use of cecectomized roosters
3. use of ileally cannulated dog

Good quality cuts with relatively low fat content of meats and skinned pollock and pink salmon fillets were chosen. After collection, all the substrates were frozen without using preservative additives, and subsequently they were all dried by a dryer at low temperatures (6 hours at 71 °C), refrigerated and sent to the University of Illinois (Urban), where they underwent a grinding to obtain 2 mm particles.
All substrates were analyzed to obtain their value in dry matter, organic matter and ashes using AOAC approved methods (2006). For each substrate, crude proteins, total lipid content, total fiber, gross energy, concentrations of biogenic amines, long chain fatty acids, AAs, collagen were evaluated.

IDEA assay: each substrate was hydrolyzed by a mixture of enzymes present in the commercial IDEA enzyme kit. The protein content released the alpha-AAs, then they were mixed and linked to a marker (orthophthaldeyaldehyde – OPA). OPA absorbance was assessed through a spectrophotometer and it was used to indirectly obtain the quantity of AA released by enzymatic hydrolysis.

Cecectomized roosters assay: the protocol was performed as described by Sibbald (1979). 20 roosters underwent cecectomy at the age of 25 weeks, after a few weeks of healing, roosters underwent 24 hours of fasting and then after goiter intubation, they received 30 gr of each protein substrates (4 roosters were used for each of the 5 substrates). Roosters were subsequently fasted for 48 hours, and during this time the manure produced by them was collected on a plastic tray placed under each individual cage; the samples thus obtained were freeze-dried, weighed, ground to 0.25 mm and for each of them the AA content was quantified.

Ileally cannulated dog assay: for this study 5 hunting bitches, previously operated according to the procedure of Walker et al. (1994), were used. Each of them had individual accommodation with the same controlled and equal environmental conditions for all.
5 extruded diets with a typical formulation of a good quality feed (30% protein, 20% fat, mineral and vitamin supplements) were formulated, each containing as primary protein source only one of the 5 protein sources under study. At each dog was given a diet in the following way: 150 gr of feed twice a day, chromic oxide (0.2%) was added as a marker of digestibility, water at will.
Following a regular collection schedule for each dog, samples of ileal content and faecal samples were taken 3 times a day for 4 days. These samples were then stored and processed with methods specified in the study, in order to obtain information on the digestibility of the protein content, on the presence of biogenic amines, and other characteristics detected by the previous methods.

The results of the analysis of this work indicate that:

1. Concentrations of the individual AAs were similar in all the protein substrates under analysis, however the lysine was 26% higher in the pollock fillet compared to pork loin and beef loin;
2. Concentrations of essential AAs and non-essential AAs in the pollock fillet were respectively 15% and 22% higher than those in pork loin;
3. Concentration of hydrolyzed fat was higher in the beef and pork loin;
4. Concentrations of polyunsaturated fatty acids and n-3 fatty acids were higher in the pollock and salmon fillets;
5. Total dietary fiber, a measure of concentration of connective tissue, was low for all substrates, indicating the good quality of the meat and fish cuts used;
6. Digestibility values of the AA of the substrates obtained with the IDEA kit and those obtained with digestion in cecectomized roosters were consistent with each other, in general the digestibility was similar among all the substrates, but the pollock fillet showed the greatest digestibility;
7. In ileally cannulated dogs the digestibility of the raw protein showed high values for all substrates, probably because the cuts of meat and fish were all of good quality, minimizing the effect of the difference in content of connective tissue between substrates, a factor that normally negatively affects digestibility.

In conclusion results of this work show that the substrates in analysis were actually different from each other in composition, but their digestibility was rather similar. In particular ileal digestibility of each protein sources in dog assay was between 88,9 % of chicken breast at 90,5 % of pollock fillet and pork loin without significant differences. Total digestibility values in dogs also showed no difference between different protein sources, with values ranging from 94.4% to 94.8%.