Joanne add to that #3 possibility being the use of novel proteins .
Exotic ingredients have different nutritional profiles and different digestibility than typical ingredients and have the potential to affect the metabolism of other nutrients. For example, the bioavailability of taurine is different when included in a lamb-based diet, compared with a chicken-based diet, and can be affected by the amount and types of fiber in the diet.“
The increased use of previously uncommon animal protein sources such as rabbit, venison, bison, lamb, and wild boar, especially in the category of diets marketed as grain-free, warrants characterization of their typical amino acid profiles including the degree and nature of any variability. In fact, certain meats are unexpectedly low in taurine (such as rabbit) or low in sulfur amino acid precursors (such as lamb meal). In addition, the bioavailability of taurine and its precursors in many animal protein sources is not known. Many of the baseline diets in this study contained animal protein sources that until recently were not commonly found in canine diets.”
