Jeremias JT et al. 2013
One of the most frequent pathologies in cats is urolithiasis, the formation of stones from oxalate or struvite in the lower urinary tract. A lot of studies are oriented to the description of the prevention and treatment of this pathology and almost all of them are addressed to formulation of specific foods that prevent or dissolve stones.
In one of these studies it was shown that the composition of micro-elements of a diet, in particular the relationship between cations and anions, is related to the pH of the urine of cats (Markwell et al., 1998; Wagner et al., 2006). These changes in the pH of the urine are in fact due to the impact of the cationic and anionic balance on the acid-base state of the animals (Cohen and Hurwitz, 1974; Block, 1984; Pizzorno et al., 2009). The excretion of hydrogen or bicarbonate by the kidneys is changed in an attempt to maintain constant the pH of the blood, with consequent changes in the pH of the urine (Riond, 2001)..
Considering the strong influence of urine pH on the formation of certain types of uroliths, there has been a strong interest in the development of food formula equations to estimate the urine pH induced, based on the composition of macroelement, micro-element of amino acids from diets (Kienzle et al., 1991; Allen and Kruger, 2000; Zentek and Schulz, 2004). A practical tool to predict the effect of food on the urine pH is the calculation of the excess of BE. The amount of BE ( mEq / kg) present in food can be calculated from starting from micro-elements (Ca, P, S, Cl, K, Na and Mg) or using sulfur amino acids (Ca, P, Cl, K, Na, Mg, methionine and cysteine) or with total sulfur. Food that has a relative predominance of cations has a positive BE value and promotes the formation of alkaline urine, while the opposite occurs in food with a relative predominance of anions (Kienzle and Wilms-Eilers, 1994). However, there are some controversies about the accuracy and validity of these formulas (Yamka et al., 2006).
These calculations, if reliable, could reduce animal testing and the cost of developing specific foods, allowing for the optimization of urine pH through diet. Such methods could also help determine the optimal pH value for the prevention of oxalate and struvite stones . Therefore, a very interesting result is the use of sulfur or methionine and cysteine for the calculation of food BEs ,as well as the influence of BE alimentar and on blood gas analysis and pH of urine in cats.
A study was conducted that compared the use of sulfur or methionine and cysteine for the calculation of the food BE (experiment 1) and studied the influence of the food BE on the blood analysis and pH of the urine of cats, is has offers a predictive formula to estimate the urine pH of cats fed with dry diet (croquettes) with known BE (experiments 2 and 3). In experiment 1 of this study, nine healthy adult cats were fed commercial cat diets. The cats were housed in metabolic cages for seven days in adaptation and urine was collected over the next three days. All urine produced within 24 hours was grouped by cat and diet. The acid-base status of cats was assessed through blood gas analysis after 10 days of dieting. An average difference of -115 mEq / kg between BE calculated from sulfur ( BEs ) and BE calculated from sulfur amino acids ( BEaa ) was observed and could be explained justified by a higher concentration of sulfur in the whole diet compared to methionine and cysteine. Experiment 2 included 30diets in croquettes dry food each diet was tested in six cats. A significant correlation was found between the measured urine pH and the food BE s (urinary pH = 6.269 + [0.0036 x BEs) + [0.000003 x BEs2); R2 • 0.91; P <0.001). In experiment 3, eight dry diets were tested to validate the equation proposed in experiment 2 and compare the results obtained with the previously published formulas.
In conclusion, the composition of macronutrients diet has a strong influence on the acid-base balance of cats and the calculation of BE from sulfur is a useful tool for formulating and balancing dry diets for felines.
The present study confirmed that the food cation and the anionic balance have a strong correlation with the urinary pH of felines (Kienzle and Wilms-Eilers, 1994; Markwell et al., 1998; Wagner et al., 2006; Yamka et al., 2006). The acid-base balance in cats is also closely related to the balance of cations and food anions. In experiment 1, the commercial food has stimulated several acid-base metabolic responses, but is different diets don’t induced promoted a adequate urine pH ideal for imitate the onset of rainfall of calcium oxalate or struvite (interval pH described as ideal: 6.2 – 6.8). A urine pH range between 6.2 and 6.8 has been proposed desirable for the felines to avoid an increased risk of calcium oxalate or struvite precipitation (Allen and Kruger, 2000).
Hawthorne and Markwell (2004) discussed the correlations between sodium intake, urine volume and feline lower urinary tract disease prevention. High sodium intake is effective in increasing urine volume, as also observed in the present study. The quantity of sodium per kg of diet, based on dry matter, which seems to be safe for cats is of 11 g (Xu et al. 2009). However, the real benefit of the increased urine volume induced by the dietary addition of Na for the prevention of feline lower urinary tract disease is unclear. You need a best comparison between the effectiveness of sodium increase and other methods of increasing urine volume, such as the use of wet diets (Artichokes et al., 2005), including long-term studies.
The BE calculation feed on food has proven to be an adequate and valuable tool to evaluate and balance i macronutrients the including trace elements in dry diets for cats. The average difference of -115 mEq / kg observed between BSs food and BEaa can be attributed to the differences in the sulfur content of the diets (average of 3.1 g / kg for the nine diets) and in the sulfur content in the form of methionine and cysteine (average of 1.95 g / kg for the nine diets). Others compounds ingredients dietary present in cats diet, such as sodium bisulfate, chondroitin sulphate of chondroitin, biotin, thiamine and taurine, could possibly contribute to the quantity remaining of sulfur, thus justifying the differences in the two procedures for the calculation of BE foods. Considering that the addition of sulfur-containing substances other than methionine and cysteine may vary consistently between the formulations, the BE calculation procedure using total sulfur seems to be better than using methionine and cysteine alone.
In conclusion, the composition of macroelements diet has a strong influence on the acid-base balance of cats and the calculation of BE from sulfur is a useful tool to formulate and balance dry diets for cats with urolithiasis. The results of this study demonstrate the important influence of the food cation and the anionic balance on the intermediate metabolism of cats. Ingestion of macronutrients and microelements alters the animal’s metabolism, determining the metabolic adaptation of the buffer systems. The change in urinary pH is only one of the consequences and may not be the most important. Therefore, food BEs must be calculated and maintained within an appropriate range. To maintain a cat’s urine pH in the range of 6.4 to 6.6, the BES in the feed, must be calculated e maintained between 30 and 80 mEq / kg of food. Subsequently to calculation of BES, the pH cat urinary can be estimated using the following formula: pH urinary = 6.269 + [0.0036 x BES) + [0.000003 x BEs2), which has proven to possess a high level correlation (0.942) and high accuracy (0.979) compared to the real values found in the urine of cats.