Background Dietary calcium (Ca) concentrations might affect regulatory
pathways within the Ca and vitamin D metabolism and consequently excretory
mechanisms. Considering large variations in Ca concentrations of feline diets,
the physiological impact on Ca homeostasis has not been evaluated to date. In
the present study, diets with increasing concentrations of dicalcium phosphate
were offered to ten healthy adult cats (Ca/phosphorus (P): 6.23/6.02,
7.77/7.56, 15.0/12.7, 19.0/17.3, 22.2/19.9, 24.3/21.6 g/kg dry matter). Each
feeding period was divided into a 10-day adaptation and an 8-day sampling
period in order to collect urine and faeces. On the last day of each feeding
period, blood samples were taken. Results Urinary Ca concentrations remained
unaffected, but faecal Ca concentrations increased (P < 0.001) with increasing
dietary Ca levels. No effect on whole and intact parathyroid hormone levels,
fibroblast growth factor 23 and calcitriol concentrations in the blood of the
cats were observed. However, the calcitriol precursors 25(OH)D2 and 25(OH)D3,
which are considered the most useful indicators for the vitamin D status,
decreased with higher dietary Ca levels (P = 0.013 and P = 0.033). Increasing
dietary levels of dicalcium phosphate revealed an acidifying effect on urinary
fasting pH (6.02) and postprandial pH (6.01) (P < 0.001), possibly mediated by
an increase of urinary phosphorus (P) concentrations (P < 0.001). Conclusions
In conclusion, calcitriol precursors were linearly affected by increasing
dietary Ca concentrations. The increase in faecal Ca excretion indicates that
Ca homeostasis of cats is mainly regulated in the intestine and not by the
kidneys. Long-term studies should investigate the physiological relevance of
the acidifying effect observed when feeding diets high in Ca and P