Predicting the digestibility of pet food

Variations in digestibility

Nutrient composition and digestibility are of crucial importance for the qualitative evaluation of pet food. The pet food industry traditionally uses a wide range of protein sources, both of animal and vegetable origin, including meat, bone and soybean meals, which vary considerably in terms of their digestibility. Obviously, variations in the digestibility of ingredients directly affect the availability of nutrients. Unfortunately, while the quantity of main nutrients must be stated on the label, information about the digestibility of pet food is rarely included, as it is not requested by current legislation.

Measuring digestibility

In the past, the digestibility of dog food has been evaluated using kennel dogs or, in some cases, fistulated animals. Nowadays, for obvious ethical reasons, the use of fistulated animals is no longer acceptable. However, digestibility trials with kennel dogs are expensive and time-consuming, even if they do not involve any invasive procedure or suffering and stress for the animals. For these reasons, the pet food industry would benefit from the availability of a reliable in vitro method to estimate the digestibility of pet foods.

A new method

At the Department of Veterinary Medical Sciences in Bologna, Italy, we have developed a relatively simple procedure for predicting the digestibility of commercial diets for dogs. The method consists of two main phases that simulate the digestion processes taking place in the stomach and small intestine of dogs. Briefly, the pet food sample is dried at 65°C and then incubated in a shaking water bath at 39°C for 2 hours in the presence of hydrochloric acid, pepsin and gastric lipase to simulate gastric digestion. Then, in order to simulate the digestion process that occurs in the small intestine, a solution containing pancreatic enzymes and bile salts is added to the sample which is incubated for another 4 hours at 39°C. Finally, the undigested fraction of the food sample is collected by centrifugation, dried and weighed to determine dry matter digestibility. Moreover, both the pet food and its undigested residue are analysed to determine crude protein, ether extract, ash and starch in order to estimate the digestibility of these nutrients. The whole in vitro procedure has been accurately described in Biagi et al. (2016, Italian Journal of Animal Science).

Validation

In order to validate the new in vitro method, the digestibility of sixteen dry extruded commercial diets for adult dogs was evaluated both in vitro and in vivo with kennel dogs. The results obtained showed a very good correlation between in vitro and in vivo digestibility of dry matter and the method proved to be very reliable in predicting the digestibility of fat and starch. Correlation between in vivo and in vitro digestibility of protein was not very accurate but this result was expected because faeces contain not only protein of dietary origin, but also bacteria and other endogenous protein sources (mainly sloughed-off intestinal cells and digestive enzymes), leading to an underestimation of protein digestibility from trials with dogs.

Simple and quick procedure

In conclusion, the in vitro method that has been developed proved to be a simple and quick procedure for predicting the digestibility of commercial diets for dogs. Moreover, despite the fact that validation of the method was conducted only with dry extruded commercial diets for dogs, the present method may also be used to estimate the digestibility of wet food for dogs, cat food, and raw materials. The utilization of such a method may significantly reduce the need for in vivo digestion trials with dogs and cats.