Water requirements in cats can vary significantly between individuals depending on water losses, which are affected by a number of intrinsic and extrinsic factors including physical activity and ambient temperature.
Daily water needs for healthy cats are only marginally defined and are typically calculated by one of two methods: a bodyweight basis (millilitres of water per kilograms of bodyweight) or water:calorie ratio (millilitres per kilocalorie metabolisable energy ingested) (National Research Council (NRC), 2006; Zanghi, 2020). The NRC recommends calculating the energy needs of the pet on a kilocalorie per day basis and providing 1 ml water per 1 kcal (NRC, 2006) — although lower daily water to calorie ratios of 0.6–0.8 have been reported in healthy cats (Zanghi et al, 2018a; 2018b).
Given the variations between individuals, such equations only ever provide an approximation, which will require monitoring and adjustment depending on the individual. In healthy animals, water requirements are rarely estimated as they are adept at managing water balance and hydration, but such calculations can be useful in hospitalised patients to ensure fluid needs are met, and in studies to help assess the efficacy of different strategies used to encourage fluid intake.
Total dietary water intake is not calculated based on free water alone but on a combination of the three main sources of water for cats: dietary water, drinking water and metabolic water (Zanghi, 2020). In the hospital setting, if a cat is receiving fluid parenterally (for example, intravenous fluid therapy) this also needs to be considered. Dietary and drinking water compose the majority of fluid intake. Generally, pet owners are not aware of whether their cat is actually drinking ‘enough’ (Box 1). For veterinary professionals, a good medical and dietary history is important to try to accurately determine the water intake of an individual feline patient.
Box 1.Do owners know if their cat is drinking enough?*Do owners know how much their cat is drinking daily?
- Cat owners are more likely to assume their cat is drinking enough than actually know how much they drink. A recent study found 79% of owners believed their cat was drinking enough water, but only 37% claimed to know how much water their cat needed. 14% said they had no idea how much their cat drank daily.
- Veterinary surgeons have an opportunity to initiate hydration conversations. However, only 31% of owners said they had discussed their cat's hydration needs with their veterinary surgeon, and 45% were unaware that dehydration could be related to a secondary health condition.
(Nestle Purina Pro Plan, 2015)
Dietary water is affected by the moisture content of the food. Commercial dry feline diets typically contain less than 10% moisture, whereas wet diets (from cans, pouches or trays) contain over 75% moisture. In the EU, the water content of commercial feline diets must be labelled if it is higher than 14%. Where a diet does not provide all of a cat's water needs, they must drink water to meet their requirements, and all dry foods must be labelled with the advice to provide free access to fresh water.
Most cats fed commercial wet diets will still drink some water, but cats fed dry food will drink more as a result of the lower water content in the diet (Villaverde Haro, 2020). However, cats on a dry diet will drink less than the moisture consumed if on a wholly wet diet with a similar nutrient composition (Xu et al, 2014; Villaverde Haro, 2020). The daily water:calorie ratio for a healthy cat is estimated to be 0.6–0.8 when ingesting dry food, but 0.9 when ingesting wet food (Finco et al, 1986; NRC, 2006; Zanghi et al, 2018a; 2018b).
Metabolic water is composed of water produced in addition to adenosine triphosphate (ATP) during the metabolism of macronutrients to generate energy. Dietary macronutrient composition influences the amount of water produced: each gram of protein, carbohydrate and fat provides 0.41, 0.6 and 1.07 g of water, respectively (DiBartola, 2012). On average, the metabolism of 100 kcal results in 10–16 ml of metabolic water (NRC, 2006).
Why are cats susceptible to dehydration?
Multiple evolutionary, physiological, anatomical and behavioural reasons can make cats vulnerable to dehydration. Understanding these can be helpful to increase awareness of the risk of dehydration, and because we may be able to take advantage of these natural behaviours when trying to increase fluid intake.
Cats have evolved as desert dwellers with a very high urine concentrating ability because of their long medullary nephrons, enabling them to adapt to periods of scarce water intake. In the wild they would generally rely on prey, which contains over 60% moisture, to obtain their water, and would drink little or no water (Kremen et al, 2013). Domesticated cats tend to respond to eating foods low in moisture content by increasing their urine concentration rather than drinking more, and the amount of water cats drink on a dry diet does not equal the amount taken in if on a wet diet (Xu et al, 2014).
When cats become dehydrated they may be slow to initiate drinking. In humans and rats, the ageing process has been demonstrated to correlate with a lower total body water content and poorer thirst response that may predispose them to suboptimal hydration. The same is assumed to be true in cats, meaning older cats may be at increased risk of dehydration, particularly if pathological conditions are also interfering with normal water homeostasis (Sparkes, 2020).
Cats have excellent distance vision, but tend to have poor near-distance vision; this can make it difficult for them to see the meniscus on the surface of their water bowl (Greco, 2020). They can also feel vulnerable when drinking from a water bowl and are highly sensitive to the presentation and taste of water, all of which may influence the volume of fluid they drink (Greco, 2020). Adult cats are relatively inefficient drinkers of low-viscosity fluids such as water because they cannot create suction with their tongues. When they lap a bowl of water, only 3/100 of a teaspoon of water is consumed with each flick of the tongue. Consequently, a lot of lapping is required when they drink (Reis et al, 2010).
How can clinicians assess whether a feline patient is adequately hydrated?
It is well known that drinking water influences urine production in both volume and concentration. However, the specific relationship between daily water intake amount and urine concentration in cats is not well defined and is key in order to help assess whether a cat is adequately hydrated. Recent studies have demonstrated a curvilinear relationship between total water ingested by cats and their urine specific gravity (USG) (Zanghi et al, 2018b). This enables calculation of the total water intake required by a cat to exceed the water resorption mechanism in the kidney and promote a more dilute urine. A curvilinear relationship has also been reported between daily urine output volume (ml/kg) and USG, allowing clinicians to assess USG to estimate a healthy individual cat's urine output volume (Zanghi et al, 2018b). Additionally, this research demonstrated a linear relationship between daily urine output volume (ml/kg) and the total daily water intake (ml/kg/day), helping clinicians to estimate the volume of daily water to be ingested to achieve a targeted individual cat's urine output volume (Zanghi et al, 2018b). USG is a particularly useful parameter for clinicians to monitor to assess the efficacy of any strategies trialled in a feline patient to increase their moisture intake.
Strategies to encourage hydration
Increasing fluid intake is an important recommendation in the management of many disease conditions, including chronic kidney disease (CKD), diabetes mellitus, feline lower urinary tract disease, and constipation (Lulich et al, 2016; Sparkes, 2020). Older cats may also benefit from increased fluid intake as a result of their potentially reduced ability to adequately maintain hydration status (Sparkes, 2020). Measurement of USG and urine volume voided can be used to assess efficacy of any strategies recommended to improve hydration status, although this is not true in all conditions (for example, cats with CKD are likely to possess isothenuric USGs and a decrease in USG may not necessarily reflect improved hydration status).
Target USG may depend on the individual; for example, a USG below 1.030 is aimed for in cats with a history of urolithiasis (Villaverde Haro, 2020). The main strategies used to encourage hydration status are shown in Box 2 and discussed below.
Box 2.Key strategies to increase fluid intake and encourage hydration status
- Feeding a completely wet diet
- Feeding a soaked dry diet
- Feeding a dry diet with increased salt content to increase fluid intake
- Increasing intake of drinking water
- Nutrient enriched water supplements
Feeding a completely wet diet
Feeding a wet diet is often a relatively straightforward and highly effective strategy to encourage moisture intake, and results in a lower daily water:calorie intake ratio in healthy cats compared with those consuming a dry food diet only (Zanghi et al, 2018a) (Figure 1). For maximum efficacy, however, wet food should be fed exclusively. Feeding a combination of wet and dry food does not tend to be a successful alternative to affect USG and total body water in a meaningful way (Xu et al, 2014). One study conducted to determine the effect of feeding differing amounts of dry and canned foods on the water consumption and hydration parameters in cats fed a group of 24 cats either 100% dry food, 66:33% dry:wet, 33:66% dry:wet or 100% wet food with 4 weeks on each ration and a 2-week washout period in between. They found that water consumption increased significantly with increasing the amount of wet food in the diet, but only cats fed on the 100% wet diet showed enhanced hydration, increased urinary volume and a reduced urine concentration (Xu et al, 2014).
Unfortunately not all cats will accept wet diets. ‘Mixed’ feeding (offering both wet and dry foods) is a recommendation from leading feline organisations including icatcare (icatcare.org, 2018). Feeding a variety of both wet and dry food from a young age may minimise the risk of a cat developing a strong preference for one or the other, as well as enabling them to enjoy a variety of textures (Sparkes, 2020; icatcare.org, 2018). This may be useful later in life if a cat would benefit from a wet diet as a result of a medical condition. For example, feeding diets high in moisture is one of the cornerstones of urolith prevention strategies (Lulich et al, 2016).
Feeding a soaked dry diet
A soaked dry diet may be an option for cats not accepting a wet diet or where owners wish to avoid a wet diet — for example, a 100% wet diet may be too expensive for an owner, or they may have personal preferences to avoid a wet diet. Some cats will accept a soaked dry diet but decline a wet diet. The amount of water that needs to be added may vary according to the reasons for wishing to increase fluid intake in an individual cat (Figure 2).
In some cases, a very large volume of water may need to be added to the diet. For example, for pets predisposed to uroliths it is recommended that a diet containing over 75% moisture is used, equating to 300–500 ml/100 g dry food (Lulich et al, 2016; Villaverde Haro, 2020). This may spoil rapidly if left out, and some pets will refuse it. The volume of water added to the dry diet should be increased gradually to maximise the likelihood of acceptance, and it is usually most convenient for owners to mix this to a thick ‘soup’ in a blender (Villaverde Haro, 2020).
Feeding a dry diet with increased salt content to increase fluid intake
Although the extent of water intake and urine dilution achieved with increased dietary sodium is not as significant as in cats fed high-moisture foods, it can be considered in cats where there is refusal of high-moisture foods. A sodium-enriched diet may encourage increased voluntary intake of drinking water and promote diuresis (Hawthorne and Markwell, 2004; Lulich et al, 2016). Such diets should remain complete and balanced; appropriate examples may include some commercial therapeutic dry diets designed to minimise the risk of some uroliths, where dietary sodium content is 320–340 mg/100 kcal food (Nestle Purina Pro Plan, 2015; Royal Canin, 2020). It is important to highlight that not all commercial urinary dry diets adopt this approach to encourage voluntary fluid intake, so care should be taken to check sodium levels within any diet considered.
However, studies looking at the long-term efficacy of this strategy are still lacking (Lulich et al, 2016; Villaverde Haro, 2020). Although studies have not demonstrated adverse health effects in healthy cats as a result of increased dietary salt content (with no changes in glomerular filtration rate (GFR), blood pressure, or other routine clinical pathological variables over up to 2 years in a group of healthy aged cats) (Reynolds et al, 2013), this strategy should not be used in cats with sodium-sensitive conditions such as cardiac disease, hypertension or CKD (Villaverde Haro, 2020).
Increasing intake of drinking water
A number of recommendations exist to encourage increased intake of drinking water in cats. Water bowl or drinking receptacle type, placement and number should be considered (Box 3, Figures 3–5). While these are all important considerations, and take advantage of the natural behaviours of cats, scientific evidence to demonstrate that they actually increase voluntary water intake in cats has not been documented (Handl and Fritz, 2018; Sparkes, 2020). Cats do appear to show individual preferences, however, and any changes made should be done in a way that allows a cat to express their preference (Robbins et al, 2019; Westropp et al, 2019).
Box 3.Drinking receptacle type, material and placement considerations
- Consider water bowl type:
- Material — glass, ceramic or stainless steel; avoid plastic (may taint water taste)
- Size and shape — generally a wider, shallow bowl is preferred to prevent whiskers touching the edge, although some studies suggest a bowl <15 cm diameter is preferred
- Some cats may show preference for a glass or jug, rather than a bowl
- Consider water bowl placement:
- Place in all areas of the home a cat is in
- Place on multiple levels
- Ensure at least one water bowl per cat plus one extra in a multi-cat household
- Mild elevations may make drinking more comfortable in older cats with osteoarthritis in their cervical spine or forelimbs
- Avoid placement adjacent to the food bowl or litter tray
- Avoid placement in noisy or busy areas, or areas difficult to access
(Handl and Fritz, 2018; Caney, 2019; Westropp et al 2019)
The use of running water, a dripping tap or cat drinking fountains is often recommended to promote fluid intake (Figures 6 and 7) (Caney, 2019; Westropp et al, 2019). Studies have demonstrated clear individual preferences for running or still water, but no statistically significant differences in water intake between bowls and fountains (Pachel and Neilson, 2010; Handl and Fritz, 2018; Robbins et al, 2019). It does, however, support the advice that caregivers should experiment with different recommendations to see which of these is successful in an individual cat (Figure 8).
Flavouring water or offering a broth is another key strategy which may help increase fluid intake (Box 4) (Sparkes et al, 2016; Caney, 2019; Westropp et al, 2019). A number of commercial complementary cat foods in ‘soup’ formulations are available, which may also help to encourage fluid intake. Experimenting with the type of water (for example, mineral water, tap water, rain water) may also be helpful (Handl and Fritz, 2018; Caney, 2019). One survey found that when given the option, cats with access outside preferred to drink from outdoor water points rather than bowls inside the home (Handl and Fritz, 2018). This could in part be as a result of the type of water, as well as the location. However, published evidence to support the efficacy of any of these suggestions is still lacking.
Box 4.Options to flavour drinking water, which may help increase fluid intake in some cats, although published research is lacking
- Add liquidised, cooked prawns
- Add spring water from tinned tuna
- Add water used to poach chicken or fish (may be offered as a fluid source alone, added to the water bowl, or frozen in an ice cube tray and added to the water bowl)
(Caney, 2019)
Nutrient-enriched water supplements
Published evidence to support the use of nutrient-enriched water containing osmolytes (including whey protein and glycerol) in cats exists (Zanghi et al, 2018a; 2018b). A nutrient-enriched water has been demonstrated to increase water intake, reduce USG and reduce urine osmolality in healthy cats consuming it, with these observations seen over a sustained period of time (Zanghi et al, 2018a; 2018b). Cats drinking the water supplement also demonstrated water:calorie ratios of over 1 and a preference for the nutrient-enriched water over tap water (Zanghi et al, 2018a).
Incremental increases in the volume of nutrient-enriched water offered resulted in incremental increases in the total volume of fluid ingested (Zanghi et al, 2018b). This nutrient-enriched water has been suggested as an alternative strategy to encourage cats to increase water intake above the level typically observed with wet food ingestion alone and without the added sodium in some foods (Zanghi et al, 2018a). It is now available as a commercial complementary nutritional supplement (PURINA® PRO PLAN® Hydra Care) in the form of a nutrient enriched highly palatable soft jelly.
Conclusion
There are a number of situations in which encouraging fluid intake in cats is desirable. A variety of different recommendations exist to help increase fluid intake and improve hydration status, although studies to support many of these recommendations are still lacking. Feeding of a completely wet diet and nutrient-enriched water currently have the greatest evidence base to support effective and measurable increases in hydration parameters.
Studies have demonstrated clear individual preferences in cats, supporting the recommendation that owners should experiment with different strategies in the home environment to help promote water intake. Clinicians can monitor the efficacy of strategies with regular urinalysis to examine parameters including USG, and adjust or combine strategies as appropriate if not achieving the target USG in that particular patient.
Further reading
You can lead a cat to water … but how do you make them drink? Vet Show Academy webinar. 2020. https://academy.vetshow.com/products/you-can-lead-a-cat-to-water-but-how-do-you-make-them-drink
KEY POINTS
- Promoting water intake can be beneficial in cats with increased water losses (for example, chronic kidney disease (CKD); diabetes mellitus; cases of vomiting or diarrhoea), decreased intake of water (for example, as a result of inappetance), and in cats with conditions such as feline lower urinary tract disease (FLUTD).
- Cats are often considered ‘poor drinkers’, and increasing their water intake can be challenging.
- The main strategies to increase fluid intake are feeding a completely wet diet, a soaked dry diet, or a dry diet with increased salt content; increasing intake of drinking water; and providing nutrient-enriched water supplements.
- Commonly provided recommendations to increase intake of drinking water include consideration of the drinking receptacle type, material and placement, use of moving water sources or flavouring water. However studies have not demonstrated statistically significant increases in fluid intake utilising these methods.
- Feeding a completely wet diet and nutrient-enriched water currently have the greatest evidence base to support effective and measurable increases in hydration parameters.