Feline struvite urolithiasis: fasting reduced the effectiveness of a urinary acidifier (ammonium chloride) and increased the intake of a low magnesium diet
Tarttelin, M.F.
Veterinary Record 121(11): 245-248
1987
ISSN/ISBN: 0042-4900 PMID: 3686783 Document Number: 297894
In three separate experiments nine male cats were fed either a canned complete diet or a commercially available dry pelleted diet or the same dry diet containing 1.6% ammonium chloride for seven days and then fasted for 20 hours. Then ad libitum feeding was continued and urine samples were taken at four-hour intervals for 12 hours and a final sample 12 hours later. Urine pH and the presence of struvite crystals in urine sediment were evaluated. The food and water intake of four of the nine cats was measured at the time of urine collection. After the fast, urine pH was raised, even after feeding the dry diet supplemented with ammonium chloride. A post-prandial rise in urine pH was also seen on all three diets. After feeding the dry diet the postprandial peak pH was 7.97 and struvite occurred spontaneously. Urine pH after feeding the dry diet supplemented with ammonium chloride peaked at 7.75 then fell to 6.1, 12 hours after the start of feeding. Struvite occurred spontaneously at all times until the pH reached 6.1, but when the pH of urine was raised to 7.0 the struvite crystallised. Urine pH on the canned complete diet peaked at 6.8 then fell to 5.8; struvite did not occur spontaneously, but when urine pH was raised to 7.0 struvite crystallised except at the 8th and 12th hour sampling. These results show that fasting initiates a post-prandial rise in urine pH and struvite crystalluria, even when a normally effective urinary acidifier is used. Intake data also indicated that the level of magnesium in the food may be overemphasised as a factor in the aetiology of struvite crystalluria because after a fast, the intake of the canned diet (which contained only 0.025% magnesium) was greater than either of the dry diets on an energy intake basis, and the intakes of magnesium were similar throughout the experimental period. The dietary intake of magnesium rather than its dietary concentration should be emphasised. It is concluded that factors affecting urine pH are more important than magnesium levels in the diet in the aetiology of struvite crystalluria and that irregular meal patterns may reduce the protection afforded by urinary acidifiers.