Struvite stones: help prevent them through nutrition
Significance of struvite stones in FLUTD
The incidence of FLUTD among cats is 1.26%. FLUTD may be due to a several different diseases with similar clinical pictures such as haematuria, pollakiuria (increased urinary frequency) or periuria (urination in inappropriate locations). However, over 85% of cases are caused by idiopathic cystitis, urolithiasis or partial urinary obstructions. More specifically, urinary stones account for as much as 25% of cases.
Urinary stones may consist of crystals of struvite, oxalate or less frequently other minerals such as phosphate. Struvite stones are the most common type to form in the lower urinary tract (bladder and urethra) (60%), followed by oxalate stones (30%). These differences seemed to be levelling out over the last 5 years, but in the last 3 years struvite stones have once again increased in incidence.
Predisposing factors for struvite stone formation
- Castration: due to hormonal changes in cats and their tendency to be overweight, the risk of struvite stone formation is almost 3.5 times greater.
- Age: although struvite stones may appear at any age, cats aged 4 to 7 years present the greatest risk.
Urine parameters influencing struvite crystal formation
1. The concentration of free crystal-forming substances: If a crystalloid substance reaches saturation point, the compound will start to precipitate out and form struvite crystals. Saturation point depends on urine pH, ionic charge, the presence of struvite crystal promoters and temperature.
Classification of urine based on the concentration or saturation of each compound
2. Urine pH: struvite is soluble at a pH ≤ 6.6. If the struvite crystals have already precipitated, the pH must be less than 6.1 to dissolve them.
3. Urine volume and specific gravity: The greater the volume of urine produced, the lower its specific gravity will be. A lower specific gravity decreases the concentration of minerals in the urine, so they will not reach saturation point. To achieve a higher urine volume, the cat’s water intake must be increased.
Secondly, a greater urinary volume will increase the cat’s frequency, thus reducing the time available for the struvite to form crystals.
4. The concentration of crystallisation inhibitors: Some compounds inhibit the formation of urinary crystals in highly concentrated urine. Citrate is probably one of the most powerful inhibitors, impeding the formation of calcium-containing crystals. There are others such as phytate or glycosaminoglycans (GAGs).
The degree of water mineralisation is important in struvite stone formation
Based on these four predisposing factors for the formation of struvite stones, we can conclude that increasing water intake and modifying urinary pH or adding crystallisation inhibitors may reduce the extent of struvite stone formation.
We analysed 12 cats of both sexes given water with either low or high mineral levels for 14 days. Urinalysis was performed 11 days after starting the water regimens. Relative supersaturation of struvite and calcium oxalate in urine was measured. We obtained the following statistically significant results:
- Water consumption was greater with water with low mineral levels compared to that of higher levels.
- Water with high mineral levels produced urine with more Ca, Na, K and SO-2, which promote the struvite crystallisation.
- Cats that drank the high-mineral water had lower concentrations of crystallisation inhibitors (pyrophosphate).
- Cats that drank the high-mineral water also presented higher values of relative supersaturation of calcium oxalate in their urine compared to the low-mineral water group.
Preventing struvite stone formation through diet
The aim of collating these data from Affinity was to assess the impact of dietary treatment with the new Advance Veterinary Diets Feline Urinary formula on urine parameters and volume.
Castrated cats were fed the classic Advance Veterinary Diet or the new formula for 15 days. Urine samples were collected at the end of the regimen and analysed to determine struvite and calcium oxalate saturation.
We assessed the risk of struvite and oxalate precipitation based on the activity product ratio (APR), according to the method described by Lulich et al. (2004). We obtained the following results:
- Urine output increased by 25% with the new formula.
- Struvite saturation fell by 38% with the new formula.
- There was a 30% reduction in the risk of struvite precipitation compared to the old formula.
- Calcium oxalate saturation was 27% lower with the new formula.
- The risk of calcium oxalate precipitation decreased by 15% compared to the old formula.
