The restriction of phosphate (P) intake is crucial in patients with chronic renal insufficiency (CRI), especially in case of hyperphosphatemia. Major sources of P are proteins, bones and cartilages as well as inorganic supplements for nutritional and technical purposes. To date, the total amount of P in a diet is used to assess the daily load of P for the patient. Besides, P excess is suspected to play a role also in the pathogenesis of CRI.^1,2,3 The aim of this study was to test the effects of different P sources on the body based on a different availability of the mineral.

In 8 adult Beagles the aD of P and calcium (Ca) was determined after feeding a control diet (0.5% P/DM) for 18 d (13 d adaptation, 5 d balance). This was repeated aiming at 2.2% P/DM by adding different phosphates (CaHPO₄, NaH₂PO₄, poultry meal, Na₅P₃O₁₀, Ca(H₂PO₄)₂, bone meal, KH₂PO₄, K₄P₂O₇) while adjusting the Ca/P ratio to ∼1.3/1 using CaCO₃ (exceptions diet poultry meal: Ca/P 1.7/1; diet Na₅P₃O₁₀:1.5% P/DM due to low acceptance) with wash-out periods of ≥10 d. Serum P, Ca and PTH was determined at d 18 pre- and 2 h postprandially. Pre- and postprandial urine was analysed for creatinine and P. Statistics: ANOVA (Bonferroni) or Kruskal Wallis (Tukey) according to data distribution.

Compared to control aD P was reduced in diet CaHPO₄, pentaphosphate, poultry and bone meal. In postprandial urine P/creatinine increased significantly in all diets but control, poultry and bone meal. The same was true for postprandial serum P concentrations with levels even above reference range in NaH₂PO₄, Na₅P₃O₁₀, Ca(H₂PO₄)₂, KH₂PO₄ and K₄P₂O₇. Postprandial PTH levels increased up to threefold in Na and K compounds (NaH₂PO₄, Na₅P₃O₁₀, KH₂PO₄, K₄P₂O₇) and Ca(H₂PO₄)₂ causing mean concentrations near or above reference range.

For the first time this study demonstrated in dogs systematic differences in P digestibility and metabolism between different P sources often used in petfood. Because of the relevance of P in renal diets but also because of the potentially harmful effects of elevated serum P and PTH levels on skeleton, cardio-vascular system and kidneys in healthy animals, the intake of highly soluble P compounds such as Na and K phosphates have to be assessed differently. The knowledge of the total amount of P in a diet does not suffice to decide about adequacy or potential harm.

Disclosures

No disclosures to report.

References

1.  Calvo, Uribarri. 2013.

2.  Dobenecker et al. 2013.

3.  Pastoor. 1994.