SummaryTo investigate the relationship between faecal calcium (Ca) and phosphorus (P) excretion in different mammalian species, a meta-analysis on digestibility data derived from the literature was conducted. Seventy-three studies on carnivores, omnivores, large and small hindgut fermenters, ruminants and hippos (a total of 21 mammalian species, precondition for inclusion dietary Ca/P ratio 1.5/1 -3.0/1) were analysed for Ca and P digestibility. Dietary Ca/P ratios were lower than faecal Ca/P ratios in carnivores, omnivores, ruminants and hippos. In hindgut fermenters, dietary Ca/P ratios were higher than faecal Ca/P ratios, indicating higher intestinal Ca absorption in these species. In all species investigated, there was a significant positive relationship between Ca intake and faecal Ca excretion and between P intake and faecal P excretion. In the biologically relevant range, these equations predicted lower faecal Ca losses in hindgut fermenters than ruminants, for faecal P vice versa. In all species, faecal Ca and P excretion correlated significantly. In carnivores, this highly linear correlation was exceptionally strong (R² = .92). Yet, the linearity of the correlation was questionable in omnivores and ruminants. Possibly, the strong linear correlation of faecal Ca and P excretion in carnivores is due to the formation of insoluble Ca/P complexes in their relatively short and simple gastrointestinal tract. Another hypothesis is that in carnivores, Ca homeostasis relies on modifying bone turnover to a higher degree than on changes in intestinal Ca absorption. For the formation of bone matrix, a constant ratio of Ca and P absorption is of advantage.
K E Y W O R D Sbone turnover, calcium, comparative, digestibility, phosphorus
| INTRODUCTIONIn a meta-analysis on calcium (Ca) and phosphorus (P) intake vs. faecal excretion in dogs and cats, Mack, Alexander, Morris, Dobenecker, and Kienzle (2015) observed a close linear relationship between faecal Ca and P excretion (R² = .94 in dogs), regardless of the Ca/P ratio and the source of Ca and P in the food. A possible explanation could be the formation of insoluble Ca/P complexes. Whether similar close relationships can be observed in mammalian species other than domestic carnivores has, to our knowledge, not been investigated. A comparative investigation of the relationship between faecal Ca and P excretion may help to understand differences and similarities between species. One hypothesis is that the regression equation of faecal Ca and P excretion reflects the intestinal formation of insoluble Ca/P complexes. In chickens, a constant ratio of intestinal Ca/P has been found (Hurwitz & Bar, 1971) and interpreted as the formation of insoluble Ca 3 (PO 4 ) 2 (Humer, Schwarz, & Schedle, 2015). The atomic weight ratio of Ca to P in this complex is 2 to 1, which should be represented in the regression equation of faecal Ca and P excretion if the complex actually is a major determinant of mineral excretion.Another assumption is that microbial activity in the gastrointestina...