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Q. I have two older cats, each with chronic kidney disease. I feed them prescription kidney diets, both wet and dry, and they have good appetites and act as if they are very healthy.
Living here in the desert, we rely on groundwater for our domestic supply, and it has a high total dissolved solids (TDS) content—that is, it’s “hard” water. I heard that giving them distilled water may help their kidney disease, but I also read that providing only distilled water can be harmful.
So, I buy distilled water and cut it with tap water at 2/3 distilled, 1/3 tap. At a recent exam, my 15-year old’s blood test showed a lowering of his excessive calcium levels. My vet couldn’t say if the distilled water had any benefits. Are there any studies along these lines?
A. Thank you for getting in touch about this controversial issue, and I hope things are going well with your two feline friends. Perhaps a brief description of what distilled water is would be a good place to start answering your question.
Tap water (and other unpurified waters) contains dissolved electrolytes (i.e., sodium, chloride, potassium) and minerals (i.e., calcium, iron, magnesium).
The “hardness” of water is a measure of the amount of minerals found in water. Unpurified water can contain metals like arsenic, lead, and aluminum, and in some cases, even biological contaminants such as bacteria and viruses.
Distilled water is produced by boiling water and collecting the condensation that forms from the steam produced by boiling. The boiling process leaves behind the electrolytes, minerals, metals, and other contaminants, which leaves pure water (or as close to pure as can be realistically expected).
A literature search reveals that there are no peer-reviewed scientific publications on the risks and benefits of giving cats only distilled water. In addition, there are few studies addressing this issue in people.
There are, however, numerous anecdotal accounts of these risks and benefits that can be found online.
One condition that may benefit from a cat drinking solely distilled water is the formation of certain types of stones in the urinary tract (uroliths). Since most of these stones are composed of various minerals (calcium, magnesium), it makes intuitive sense that eliminating these minerals from drinking water may help prevent their formation, and there is some evidence in the literature that this may be the case. It is important to realize, though, that cats (and people) obtain most of their minerals from the food they eat rather than from the water they drink, so we aren’t concerned about that.
Regarding the potential risks and benefits of providing only distilled water to cats with chronic kidney disease (CKD, as opposed to urolith formation), while there have been concerns raised about distilled water depleting electrolytes due not only to its lack of electrolytes, but also to the effects of the pH of distilled water on electrolyte homeostasis (the kidneys are vital for normal regulation of electrolytes), I am not aware of any scientific proof that this is the case. I am also not aware of any proven benefits of providing only distilled water to cats with chronic kidney disease.
In summary, more research needs to be done to clarify the potential risks and benefits of giving cats only distilled water, whether they are ill or well, and I think that in the interim, your idea of mixing distilled and tap water for your cats should be fine.
