Evaluating phosphorus, calcium and magnesium content in commercial cat foods

Summers SC, Stockman J, Larsen JA, Zhang L, Rodriguez AS. Evaluation of phosphorus, calcium, and magnesium content in commercially available foods formulated for healthy cats. J Vet Intern Med. 2019;1–8. 

The authors’ work was supported by a grant from the Winn Feline Foundation (grant number MTW18-001: Evaluation of commercial feline diets for calcium, phosphorous and the calcium to phosphorous ratio in commercial cat foods).

Chronic kidney disease (CKD) is a common condition of geriatric cats. Recent studies have indicated that high dietary phosphorous (P) content may be a contributing factor in the development of CKD in adult cats. Diets containing P concentrations greater than 3.6 g/1000 kcal of metabolizable energy (ME) with a low calcium to phosphorous (Ca:P) ratio (< 1.0 ) have been associated with altered renal function, characterized by decreased glomerular filtration rate, increased serum creatinine concentration, renal echogenicity changes on ultrasonography, and nephrolithiasis. Dietary magnesium (Mg) intake influences P and Ca bioavailability in cats, however the metabolic mechanism is poorly understood.

Minimum dietary requirements for Ca, P and Mg have been established for commercial cat foods, however no dietary Ca and P maximums are currently cited in most recognized nutritional guidelines for cats (including the National Research Council [NRC], Association of American Feed Control Officials [AAFCO], and European Pet Food Industry Federation [FEDIAF]).

The objective of the present descriptive study was to quantify the P, Ca, and Mg concentrations in various commercially available cat foods. The authors aimed to compare the mineral concentrations among different food formats (dry, canned, and raw foods), as well as across protein concentration (low, moderate, and high protein), protein ingredient (poultry, fish, beef, and non-traditional), and food cost categories. As a secondary goal, the authors sought to compare analyzed P and Ca concentrations with the minimum amounts listed on the product labels.

Commercial adult cat food diets were randomly selected for analysis from local pet food stores. The minimum “as fed” concentrations of Ca and P, cost per ounce, and AAFCO statements were recorded for each diet. Each food sample underwent proximate analysis for crude protein, crude fat, crude fiber, moisture, and ash concentrations. Plasma-optical emission spectroscopy was utilized to quantify the Ca, P and Mg concentrations of each diet.

Overall, 82 diets were analyzed in the following formats: canned (n=30), dry (n=30), and raw (n=22). The raw group included freeze-dried, air-dried and frozen products. AAFCO nutritional adequacy statements were listed for 81/82 diets, for adult maintenance (n=28) or all life stages (n=40). Most foods were labeled as grain free (61/82, 74%). Upon review of the ingredient lists, 67% of dry foods, 83% of canned foods and 18% of raw foods contained a P additive or preservative.

No significant difference was identified between the analyzed mineral concentrations or Ca:P ratios among the primary protein ingredient categories, or between grain-free or grain-containing diets. Low protein foods had significantly lower analyzed P concentrations compared to moderate or high protein foods. Of 81 foods with AAFCO nutritional adequacy statements, 3 had analyzed P concentrations below the AAFCO Cat Food Nutrient Profile minimum values for adult cats. Twenty-seven (33%) foods contained P concentrations 3.6 g/1000 kcal ME.

In conclusion, the present study demonstrated that commercial adult cat foods contain a variable amount of total P and Ca, and therefore also have variable Ca:P ratios. The authors highlighted that 33% of the studied diets had P contents 3.6 g/1000 kcal ME, which is concerning as this concentration has been previously shown to cause renal dysfunction in healthy cats. The authors speculate that high P-containing diets may be involved in the etiology of CKD in cats, taking into account the significantly higher P and protein intakes of cats with CKD prior to diagnosis compared to age-matched controls without CKD as established in previous studies.

For both Ca and P, the analyzed concentrations were often greater than the minimum concentrations declared on labels. Based on these findings, the authors caution that minimum values on food packages cannot be used to accurately estimate “as fed” amounts of Ca and P, and that labels cannot reliably be used to identify diets low in these minerals. In light of the recent findings indicating excess P may be implicated in sustained renal damage and decreased renal function, the authors call for an evaluation of existing regulatory guidelines with regard to P maximums in diets formulated for cats.

There are several potential limitations and considerations for the present study. Less popular diets may have been overrepresented, because all diets were treated equally during randomization. Potential variability among lot numbers of foods was not evaluated, as only one lot number was analyzed for each diet. Bioavailability in cats was not evaluated for the analyzed nutrients, nor the source or form of P, the content of vitamin D, or the clinical consequences of the findings. Thus, it cannot be concluded with certainty that any of the diets would cause renal damage to healthy cats, even if fed for extended periods. (HD)

See also:

Dobenecker B, et al. Effect of a High Phosphorus Diet on Indicators of Renal Health in Cats. J Feline Med Surg. 2018 Apr;20(4):339-343.

Piyarungsri K, Pusoonthornthum R. Risk and Protective Factors for Cats with Naturally Occurring Chronic Kidney Disease. J Feline Vet Surg. 2017 Apr;19(4):358-363.