Does wet food *actually* make your pet more hydrated? What about adding water or liquid to dry food?
I’ve talked about water intake at length in the context of FLUTD and dry food for cats, but often people extrapolate things they’ve heard about feline urinary or renal disease to be a blanket rule for healthy animals.
Claims like wet food helps ‘flush’ the kidneys better and cats that consume wet food are “more hydrated” than those that don’t, seem to be parrotted and thanks to the reiteration effect – believed as fact, despite seriously lacking evidence.
So let’s unpack some of these claims with evidence – but first, let’s discuss hydration status in animals:
Hydration status in animals refers to the amount of water contained within their body. I really want to emphasise this – contained WITHIN the body. An animal’s hydration status is a dynamic balance between water intake (through drinking and food) and water loss (through urine, faeces, respiration, panting, and sweat). Dehydration occurs when water loss exceeds water intake, leading to a deficit in total body water.
How much water do cats actually need?
Cats are desert animals, which contrary to popular belief means they require significantly less water to maintain water balance because they have developed remarkable adaptations to a low moisture environment. Cats excel at extracting and retaining moisture from their diet and are exceptionally efficient at conserving water; cats produce highly concentrated urine, minimising water loss in their urine.
Determining how much water a cat needs to consume can be calculated using various methods, including:
60 ml/kg/day
Or
(BW^0.75 x 70) x 1.2
Or
2.5 x % Dry Matter consumed (but let’s talk about this in a moment)
For example, a 4kg cat will need:
4 x 60 = 240ml
(4^0.75×70) x 1.2 = 237ml
As you can see, the amount is roughly the same regardless of which calculation you use.
Now when we talk about the type of food being fed, this changes things…
Are wet-fed animals ‘more hydrated‘ than those fed dry food? Are dry-only fed animals ‘chronically dehydrated’?
When animals consume a diet that is relatively low in dry matter and high in moisture content such as a wet diet, they drink less water because their water requirements have already been met through dietary moisture.
If we go back to the third calculation above, let’s use two examples:
1. A 4kg cat fed a dry diet that contains 85% Dry Matter (15% Moisture):
2.5 x 85 = 212ml of water consumed combined with dietary water (8.4ml) to meet requirements.
Total intake is 220ml
2. A 4kg cat fed a wet diet that contains 20% Dry Matter (80% Moisture):
2.5 x 20 = 50ml of water consumed combined with dietary water (204ml) to meet requirements.
Total intake is 254ml
Now before you start screaming ‘ahah! cats fed wet food get more water!‘ remember what I said up there about the amount of water contained within the body is what dictates hydration status?
Let’s take a look at this table from Small Animal Clinical Nutrition – notice the amount of urine and faecal water losses?
Wet fed cat: 278ml total water intake, with a total losses of 193ml. Net water balance = 85ml
Dry fed cat: 227ml total water intake, with a total losses of 123ml. Net water balance = 104ml
So quite clearly, the dry fed cat actually is more hydrated than the wet fed cat. Why? Because cats are adapted to a low moisture diet and environment, and water consumed in excess of requirements is rapidly excreted in urine. The wet fed cat ends up more dehydrated than they started.
Which brings me to my next point…
Doesn’t that mean it’s better for the kidneys?
When I explain the increased losses in wet fed pets, the argument then shifts to ‘but that just means it’s flushing the kidneys better’, assuming that increased urination is somehow a good thing for the kidneys…
Let’s unpack that – even in the human medical field opinions are divided if increasing water intake does anything for preventing or delaying the development of kidney disease. In fact, some studies suggest that individuals in the highest quartile of urine volume (>2.85 L/day) showed a faster eGFR decline than individuals in the lowest quartile of urine volume (<2 L/day) – which in simple terms means that the more you increase urine volume (by increasing water intake) you actually have a negative impact on glomerular filtration rate (how effectively the kidneys can filter and function). Simply put, increasing urine output by increasing water consumption may put undue pressure on the kidneys, rather than take the pressure off.
Anyone who has ever had to calculate intravenous fluids knows that over-hydration is equally as harmful to the body as dehydration. Overhydration not only can damage the kidneys due to increased blood pressure and decreased glomerular filtration rate, it rapidly dilutes the electrolyte balance in the body and has several other damaging effects on organ function:
In addition to this, studies time and time again have shown there is no association between the type of food fed to an animal and the development of Chronic Kidney Disease – in this study by Greene, et al (2014) it’s explicitly stated that “cats with a record of having been fed kibble were no more likely to develop CKD than those fed wet food, providing no support for this hypothesis.“
So what?
Increasing water intake is generally unnecessary and really should be done under close supervision. In healthy animals (who don’t have a urinary or renal issue) there is absolutely no benefit to increasing water consumption, and animals fed wet diets are no better off (but may actually be worse off) than those consuming normal levels of water.
Increasing water intake in healthy animals is not only unnecessary but probably isn’t benign – there’s evidence to suggest it might be harmful or at the very least creating expensive pee. So just don’t kid yourself thinking it’s having any benefit, or look down at people who just feed dry food. Because their pet is probably more hydrated than yours anyway!
References
Hand MS, Thatcher CD, Remillard RL, Roudebush P, Novotny BJ. Small Animal Clinical Nutrition. 5th ed. Topeka, KS: Mark Morris Institute; 2010.
Katharine F. Lunn, Anthony S. Johnson, Katherine M. James,
Chapter 5 – Fluid Therapy, Editor(s): Susan E. Little, The Cat, W.B. Saunders, 2012, pp 52-89, ISBN 9781437706604, https://doi.org/10.1016/B978-1-4377-0660-4.00005-3.
Anderson RS. Water balance in the dog and cat. Journal of Small Animal Practice 1982; 23: 588-598.
Gosolfi CV. Water and electrolyte metabolism in exercise. In: Fox EL, ed. Ross Symposium on Nutrient Utilization During Exercise. Columbus, OH: Ross Laboratories, 1983; 21-25.
Lulich JP, Osborne CA, Lekcharoensuk C, Kirk CA, Bartges JW. Effects of diet on urine composition of cats with calcium oxalate urolithiasis. J Am Anim Hosp Assoc. 2004;40(3):185-191. doi:10.5326/0400185
Zanghi, B. (2017). Water needs and hydration for cats and dogs. Proceedings, Nestlé Purina Companion Animal Nutrition Summit. Vancouver, BC, 15-23.
Choi HY, Park HC, Ha SK. High Water Intake and Progression of Chronic Kidney Diseases. Electrolyte Blood Press. 2015;13(2):46-51. doi:10.5049/EBP.2015.13.2.46
Greene, J. P., Lefebvre, S. L., Wang, M., Yang, M., Lund, E. M., & Polzin, D. J. (2014). Risk factors associated with the development of chronic kidney disease in cats evaluated at primary care veterinary hospitals. Journal of the American Veterinary Medical Association, 244(3), 320-327. Retrieved Apr 24, 2025, from https://doi.org/10.2460/javma.244.3.320
https://cat.life/en/desert-animal-cats-drink-water/
https://www.vpisglobal.com/2019/08/01/water-intoxication/
https://www.rspcapetinsurance.org.au/pet-care/health-and-wellbeing/hydrate-your-cat-or-dog?hl=en-AU
https://www.vin.com/apputil/content/defaultadv1.aspx?pId=22915&catId=124646&id=8896578&hl=en-AU
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