Hard water, tap, bottled, sparkling, distilled, filtered water...what to choose? If you have a cat with FLUTD, you've inevitably been faced with this choice of what water you should be providing to your cats with a history of crystalluria or being fed a therapeutic diet. But does it actually matter?
First, what is hard water? Water that is high in dissolved minerals, particularly calcium and magnesium, is called 'hard' water. Hard water is not bad, toxic or contaminated in any way, and doesn't mean your pipes (real or figurative) will become clogged with scale.
The total hardness of water is typically expressed as the equivalent of concentration of calcium carbonate in water as mg CaCO3/L. For comparison, 1mg/L is roughly equal to 1ppm. In most states in Australia, the water is considered soft - though of course this will vary depending on the catchment area - with only Queensland and South Australia having 'moderately hard' water (see below maps).
Source: Lehr, J. et. al., 1980. Domestic Water Treatment, New York, NY: McGraw-Hill Book Company
So when we look at hard water and its relationship with health, we are quite limited to epidemiological studies that have been based on statistical relationships, not directly looking at urinary health and hard water. The National Research Council published an extensive report on hard water, and on the topic of urinary health, stated "The impact of water hardness of urinary stone formation remains unclear, despite a weak correlation between water hardness and urinary calcium, magnesium, and citrate excretion. Several studies have shown no association between water hardness and the incidence of urinary stone formation."
Despite this apparent lack of evidence, I continue to see many veterinary professionals telling pet owners to provide distilled, filtered or demineralised water to pets. But actually, is the mineral content of water really a problem for pets with a tendency toward obstructions?
Kirk et al, (1995) point out that the mineral content of individual water supplies are expressed as parts per million, whereas the mineral content of food is expressed as parts per hundred, which is a 10,000-fold difference. So even hard water (high in mineral content) would not contribute substantially to the amount of available minerals, compared with the minerals supplied by diet.
Similarly to the NRC's report, searches on water hardness and its relationship with FLUTD returned next to no results - many hits were unsubstantiated or unreferenced opinions with no solid evidence. One such result that has been referenced, is a Trupanion (a pet insurance company) study from 2016 that supposedly noticed more urinary claims coming from areas where water hardness is rated "extremely hard" by the EPA (greater than 180 mg/L) but unfortunately every single link to said "study" returns a blank page and errors, indicating the aforementioned post had been removed. At time of publication, Trupanion Chief Veterinary Officer, Dr Steve Weinrauch BVMS, MRCVS stated that "further study is warranted to identify a connection between hard water and urinary crystals in cats." He goes on to recommend pet owners make no changes without consulting their vet which indicates that the data was unconvincing. Considering I can't locate the data being quoted to analyse it and nothing seems to have been studied since, I'd find it hard to look at this a little more than a case of correlation, rather than causation.
What about products marketed in this way to help 'prevent' obstructions by feeding demineralised or reverse osmosis water? Let's take a look at their claims:
There are a few products that are marketed as special water for cats that are demineralised and "don't contain minerals associated with urinary problems in cats", despite the fact that we know mineral content in water can't possibly contribute to urinary problems due to the amounts of minerals being so negligible compared to dietary intake (Kirk et al, 1995).
Another claim is that they "have a perfectly balanced pH between 6.2 and 6.4 that suits cats' needs and helps to create a level of acidity in the urine to keep the urinary system of the cat healthy." Before we get into the problems with that statement, let's think about the pH of drinking water - can it actually affect the pH of urine?
Yıldırım, İ., & Koçan, H. (2023) studied rats being given drinking water at a number of different pH levels and found that "The variation in drinking water pH does not directly change urine pH." And went on to discuss that pH fluctuations were mainly based on time of day, rather than the water itself.
So on this second point, drinking a slightly acidic water isn't going to have any impact on reducing stone or crystal formation either - food and time of day is still more important in altering urinary pH.
So what should you do?
Save your money on fancy water, and invest instead in a veterinary prescribed therapeutic urinary diet that is appropriate for your cat's (or dog's) urinary condition. Uroliths and crystalluria are complex, and they don't all require the same treatment so always follow your vet's advice.
As for drinking water, whatever you drink is absolutely fine for your cat or dog - you don't need to buy anything special for them. As long as your pet is drinking, that's all that really matters.
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