Nevertheless, we observed some differences in AA frequencies that, even though they were not significant, may affect digestibility. For example, we observed a lower frequency of Lys in cat than dog (see Figure 1). The greatest change in frequency in Lys and Arg among milk proteins was observed for the caseins (significant for both residues; see Figure 2b), suggesting a greater susceptibility of these proteins to pepsin digestion in mammals. Here, it is hard to distinguish between cause and effect. Although variation in AA composition might be driving differences in potential digestibility (especially because other AA were also significantly variable), it is possible that the reverse might be true.