Page 919 - Small Animal Clinical Nutrition 5th Edition

Feline Lower Urinary Tract Diseases 951

Box 46-4. Determining the Effect of Food on Urinary pH.
VetBooks.ir Several important effects of food on acid-base balance can be Another study was conducted recently to determine if urinary pH

described by the anion-cation balance (ACB). Calculation of a food’s could be predicted using the nutrient components of feline foods.
ACB has been evaluated as a practical method for predicting the One-hundred-fifty foods (90 dry, 60 wet) were fed to groups of 10
effect of a food on urinary pH. In this method, the ACB is calculat- adult cats to determine urinary pH of cats fed each food. Each food
ed from the concentrations of alkaline and acid compounds in the was fed for seven days and pH was determined on freshly voided
food, expressed as mmol/kg dry matter, using the formula: urine on Days 5 to 7 of the study. Using stepwise regression, it was
ACB = 49.9 (Calcium) + 82.3 (Magnesium) + 43.5 (Sodium) + determined which cations, anions and sulfur-containing amino
25.6 (Potassium) – 64.6 (Phosphorus) – 13.4 (Methionine) – 16.6 acids were of importance for predicting urinary pH. Separate formu-
(Cysteine) – 28.2 (Chloride). Factors take into account atom- las had to be used for dry and wet foods to maintain accuracy.
ic/molecular weight and valence (2 for phosphorus). Although calculation of ACB may roughly estimate urinary pH and
This method was evaluated in a study involving 10 commercial formulas can be used to predict urinary pH based on nutrient con-
foods (moist and dry) and several additives. Feeding trials involved tent of food, the most accepted method of comparing foods is to
four to six cats per trial. Cats were fed the foods for two days and feed the food to a group of cats and compare urinary pH values.
urine was collected for at least five days. During the eight hours However, although most reputable cat food manufacturers provide
after feeding, urinary pH was measured immediately after urination urinary pH data for their products, no standard urinary pH testing
and urine excreted during the remainder of the day was tested the protocol has been developed. Consequently, it is important to know
following morning. A highly significant correlation was seen the protocol used to measure urinary pH before comparing results
between ACB of the food and the mean urinary pH. In the amounts from different companies or laboratories.
used in this study, the addition of calcium carbonate and calcium
lactate significantly increased urinary pH; dibasic calcium phos- The Bibliography for Box 46-4 can be found at
phate and ascorbic acid had no effect; and calcium chloride, www.markmorris.org.
ammonium chloride and phosphoric acid decreased urinary pH.

define “normal” urinary pH, it is necessary to consider the “nor-
Table 46-16. Urine acidifying and alkalinizing pet food
mal” or habitual diet. On a volume basis, the gastric content of
ingredients.
feral cats is approximately 90% small mammals (e.g., mice, rats)
(Coman and Brunner, 1972). In one study, the average urinary Protein sources that are acidifying ingredients
Poultry meal
pH was approximately 6.3 when cats were fed a diet of rat car-
Corn gluten meal
casses (Vondruska, 1987). In another study, mean urinary pH Other acidifying ingredients
was reported to be 5.97 for feral female cats and 6.37 for feral Ammonium chloride*
Calcium chloride
male cats eating a natural diet (Cottam et al, 2002).
Calcium sulfate
The kidneys provide long-term defense against acid and dl-methionine
alkali deviations; this process occurs continuously as endoge- Phosphoric acid
Alkalinizing ingredients
nous acids are generated.The kidneys must conserve bicarbon-
Calcium carbonate
ate in the glomerular filtrate and regenerate bicarbonate de- Potassium citrate
graded by the reaction with metabolic acids to maintain normal Magnesium oxide
*Not approved in the United States as a food additive.
plasma bicarbonate levels.The kidneys can increase the amount
of net acid excretion in urine and generate bicarbonate in
response to exogenous acid loads. Normally, the kidneys syn-
+
thesize urinary ammonia (NH ) and thus ammonium (NH ) Sulfuric acid is formed when sulfur-containing amino acid
3
4
almost exclusively.With chronic metabolic acidosis, more NH 3 (e.g., methionine and cysteine) residues of proteins are oxidized
is produced and urinary NH 4 + ion concentration is increased. to sulfate. In general, animal-source protein ingredients contain
+
The kidneys excrete hydrogen ions (H ) in the form of titrat- more sulfur-containing amino acid residues than do plant-
able acid (e.g., phosphoric acid) and NH 4 + ions. Reduction of source proteins. Phosphorus has strong effects on acid-base
urinary pH greatly increases the ratio of NH 4 + to NH . balance, depending on its chemical form. Inorganic phosphorus
3
Acidifying foods, therefore, increase urinary concentration of can be ingested as phosphoric acid, monobasic and dibasic or
NH 4 + ions, one component of struvite. Although decreasing anionic phosphate. Phosphoric acid is used in cat foods to
urinary pH theoretically increases urinary NH 4 + concentra- enhance palatability, either separately or as a component of top-
tion, the same change in urinary pH decreases anionic phos- ically applied animal digests. Phosphoric acid has a strong acid-
phate levels in urine. Thus, as urine becomes more acidic, pre- ifying effect. Monobasic phosphate also is acidifying, whereas
cipitation of struvite becomes less likely. dibasic phosphate has little effect on urinary pH (Kienzle et al,
The effect of a food on urinary pH is the net effect of its con- 1991). Anionic phosphate is alkalizing.
stituent nutrients (Box 46-4). Dietary acid is derived from sev- Mineral salts vary in their effect on urinary pH and thus are
eral nutrients (Table 46-16) (Halperin and Jungas, 1983). potential acid or base sources. Oxides and carbonates are alka-