Towards establishing no observed adverse effect levels (NOAEL) for different sources of dietary phosphorus in feline adult diets: results from a 7-month feeding study

Abstract

High dietary phosphorus (P), particularly soluble salts, may contribute to chronic kidney disease development in cats. The aim of this study was to assess the safety of P supplied at 1g/1000kcal from a highly soluble P salt in P-rich dry format feline diets. Seventy-five healthy adult cats (n=25/group) were fed either a low P control (1.4g/1000 kcal; calcium:phosphorus ratio, Ca:P 0.97) or one of two test diets with 4g/1000 kcal (4184kJ); Ca:P 1.04 or 5g/1000kcal; Ca:P 1.27, both incorporating 1g/1000kcal (4184kJ) sodium tripolyphosphate (STPP) ‒ for a period of 30 weeks in a randomised parallel-group study. Health markers in blood and urine, glomerular filtration rate, renal ultrasound and bone density were assessed at baseline and at regular time points. At the end of the test period, responses following transition to a commercial diet (total P – 2.34g/1000kcal, Ca:P 1.3) for a 4-week washout period were also assessed. No adverse effects on general, kidney or bone (skeletal) function and health were observed. P and Ca balance, some serum biochemistry parameters and regulatory hormones were increased in cats fed test diets from week 2 onwards (p≤0.05). Data from the washout period suggest that increased serum creatinine and urea values observed in the two test diet groups were influenced by dietary differences during the test period, and not indicative of changes in renal function. The present data suggest NOAELs for feline diets containing 1g P/1000kcal (4184kJ) from STPP and total phosphorus level of up to 5g/1000kcal (4184kJ) when fed for 30 weeks.

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

“…Positive relationships and significant correlations and linear models were also observed between data from in vitro assays and total P and ash contents of dry diets, but again not for wet diets, with the latter showing consistently inverse relationships. However, more cohesive effects of dietary Ca:P ratio were observed across diet formats and here the inverse relationships were in line with the effect of high calcium content in diets reducing or slowing phosphate absorption in vivo, , apparently due to formation of indigestible complexes between Ca, P, and Mg in the intestine. , Whether the diet format differences detected were influenced by the higher Ca:P ratios or variables including larger, less digestible food particle sizes in wet diets compared to dry cannot be directly established in the current study. However, data from the dry format diets indicated that higher Ca:P ratios did lower % soluble P from in vitro assays for diet D02, which had the highest Ca:P (>2) among dry diets; for D05 compared to D04, which only differed in Ca:P ratio (Table ); and from Ca spiking of G digestion assays, which also reduced % soluble P obtained, but not for the G-SI digestion assay (Figure ).…”

“…The time lapse from feeding the diets to the appearance of clinical signs and/or morphological changes in the kidneys was dose-dependent, with the higher SDHP level in diets causing changes faster and adversely affecting the health of more cats than the lower level. A more recent study indicated that sodium tripolyphosphate (STPP) at an inclusion level of 1.5% in an extruded feline diet was safe following 7 months of feeding to healthy, adult cats . Thus, adding soluble P salts to diets may be safe, but an improved understanding of the safe upper limits for different soluble P sources in diets for different species needs to be established.…”

Clinical progression of cats with early‐stage chronic kidney disease fed diets with varying protein and phosphorus contents and calcium to phosphorus ratios

“…Mean PTH (7.69, 95% CI 5.67, 11.4 pg/mL) and FGF23 (116; 95% CI 34.8203 pg/mL) were within levels reported in healthy cats. 7 , 29 …”

The Phosphate Additives Phosphoric Acid and Sodium Phosphate Lead to Hyperphosphatemia as well as Increased FGF23 and Renal Phosphate Excretion in Healthy Cats

“…This is further proof that both the total amount of ingested phosphate and its source are responsible for the effects on calcium and phosphorus metabolism. Previously, a SUL for inorganic phosphate intake of 1 g/Mcal in cats has been suggested [95,96]. This recommendation was based on results from medium-term studies in cats using primarily sodium tripolyphosphate (STPP).…”