Are Adult Cats Fed on Wet Maintenance Diets Less at Risk of Developing Chronic Kidney Disease Compared to Adult Cats Fed on Dry Maintenance Diets?

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Appraisal, application and reflection

The feeding of dry food to cats has been hypothesised by some authors to cause CKD due to a low water content within the food (see for example, Greene et al., 2014). From the three relevant papers found within this literature search, only Hughes et al. (2002) comprehensively examined the effect of nutritional parameters within food whereas the other two papers examined clinical records for descriptions of food given, as noted by the owners. Unfortunately, the small sample size and potential misclassification bias (by not abiding by current IRIS guidelines to diagnose CKD)  within Hughes et al. (2002) reduces its reliability. Ultimately, none of these three papers provided conclusive evidence to answer the PICO question.

For the general practitioner, it is encouraging to find evidence that certain nutritional parameters may be beneficial in delaying the onset of CKD diagnosis. Hughes et al. (2002) found that an increased fibre content in the diet and the ambiguous “factor-2” were both protective in delaying CKD development whereas ad-libitum feeding was detrimental. Whilst performing the literature search, a small number of additional papers were uncovered which appeared to suggest that diet could help to reduce the risk of CKD development (DiBartola et al., 1993; Frantz et al., 2007; Lane et al., 2012; Geddes et al., 2016), although these were all excluded from the current PICO question as they did not look at the difference between wet and dry food. Further research, including a broader literature search, is needed in this area.

There was a significant association between CKD diagnosis and a history of previous anaesthetics, periodontal disease, hyperthyroidism, hypertension, cystitis and diabetes mellitus. This is perhaps unsurprising since some cats may be more likely to present for more acute problems such as urinary tract infections than chronic conditions such as CKD. Moreover, a previous, acute illness such as cystitis, or chronic condition like hyperthyroidism, may facilitate more careful veterinary monitoring by both owners and vets. However, none of these studies analysed whether diet fed was a significant interaction term with the comorbidity. This is a frustrating omission given the frequency of veterinary advice recommending a diet change after for example, lower urinary tract conditions or periodontal disease. Another interesting Knowledge Summary could be developed to look at the usefulness of wet food on preventing further episodes of feline lower urinary tract disease.

This PICO question has an underlying assumption that all wet maintenance diets, and all dry maintenance diets, can be treated equally with regards to water content and nutritional value. To the author’s knowledge, there have been no studies performed which confirm this assumption. Arguably, the plethora of commercial diets available for cats makes this an unrealistic assumption. Furthermore, the proportion of cat owners amongst the general cat-owning population who change diets or feed a mixture of wet and dry food is unknown. Therefore, it seems unlikely that any retrospective study into this PICO question would yield sufficient power to answer this question with certainty. Furthermore, it should be acknowledged that these studies have looked at risk factors for CKD diagnosis, not the onset of kidney damage. It is possible that cats which obtain an increased water content, by for example, eating a wet diet, are diagnosed later in the disease process due to a relative compensation for kidney damage. Alternatively, it is theoretically possible that a risk factor may create such severe kidney disease that cats are never presented to veterinary practices. Therefore, the potential biases of using the veterinary-presenting population of cats should be considered when considering the risk factors for CKD. Ultimately, a prospective randomised control trial (RCT) would provide greatest evidence on whether the feeding of wet or dry diets can reduce the risk of kidney damage. Further research into this area is needed to ensure that vets aren’t inadvertently placing cats at an increased risk of CKD when suggesting a diet change after being diagnosed with periodontal disease, cystitis or several other comorbidities.

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