Comparisons of In Vitro and In Vivo Digestibility Assays for Phosphorus in Feline Diets and Associations with Dietary Nutrient Content

Phosphorus (P) is an essential nutrient; however, potential health impacts of high dietary levels of added soluble, highly bioavailable P salts especially are a concern. P sources with lower bioavailability are considered safer. Yet, speciation of different P sources to assess diets' risk to health is challenging. This investigation tested the value of in vitro water extraction and digestion assays to predict in vivo P apparent bioavailability/digestibility in feline diets. Thirty wet (n = 18) and dry (n = 12) format experimental and commercial cat foods were analyzed for nutrient content. Triplicate samples were subjected to in vitro water extraction, single-phase acidic (gastric; G) digestion, and dual-phase gastric and small intestinal (G-SI) digestion assays. Soluble and insoluble P were determined in the supernatant and pellet, respectively. A subset of the diets (seven wet, nine dry diets) was fed to healthy, adult cats (n = 7-24) to determine in vivo apparent P digestibility. Information on the soluble P salt sources and their contribution to total dietary P was available for some diets. Associations between data from the different in vitro assays and in vivo digestibility trials and the influence of different diet parameters were obtained using Pearson correlation and linear regression modeling. The % soluble P obtained from G-SI digestion assay correlated well with in vivo apparent P digestibility for wet (Pearson coefficient 0.926, p = 0.003), but not for dry diets (Pearson coefficient -0.074, p = 0.849). In contrast, the % soluble P determined by water extraction correlated well with the % soluble P salt contribution to total P for dry (Pearson coefficient 0.901, p < 0.001), but not for wet diets (Pearson coefficient -0.407, p = 0.365). Thus, 20 min water extraction can be used to predict soluble P salt content in dry diets; however, differing Ca:P ratios and water solubility of the P sources may affect the outcome and false-positive results can occur. The G-SI digestion assay employed can also be used to predict in vivo P digestibility. However, again, diet format, Ca:P ratios in diets, and possibly other factors can impact the results. Thus, data from in vitro assays to assess P sources and bioavailability need to be interpreted with care.