Water intake in cats is important both in health and disease. However, cats are generally thought of as “poor drinkers”, and thus encouraging water intake can be a challenge. Whilst a healthy cat with free access to water can maintain a good water balance, cats can be susceptible to dehydration and there are some situations where promoting water intake can be beneficial. This article will explore the water requirements of cats, why they may be susceptible to dehydration and how we can encourage water intake successfully in cats who may benefit from this.
Why are cats considered poor drinkers?
A number of evolutionary, physiological and behavioural reasons make cats potentially susceptible to dehydration.
Cats have evolved as desert dwellers with a very high urine concentrating ability due to their long medullary nephrons, enabling them to adapt to periods of scarce water intake. In the wild they would generally rely on their prey, which have a water content of over 60 percent, to obtain their water (Kremen et al., 2013). Domesticated cats tend to respond to eating food low in moisture content by increasing their urine concentration rather than drinking more, and while cats on dry diets do drink more water than those on wet diets, the amount does not equal the water contained in a wet diet with the same nutrient composition (Greco et al., 2014). When cats do become dehydrated, they can be slow to initiate drinking. In humans it has been shown that the ageing process is associated with a lower total body water content and a poorer thirst response that may predispose them to suboptimal hydration. It is assumed the same is true in cats, meaning that older cats may have a greater risk of dehydration, particularly if pathological conditions are also interfering with normal water homeostasis (Sparkes, 2020).
Adult cats are not designed to be efficient 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 percent 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). Whilst cats have excellent distance vision, they have poor near-distance vision which 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 be very sensitive to the presentation and taste of water, all of which may impact on the volume of fluid they drink.
Water requirements of cats and sources of water
Several equations have been proposed to estimate the daily water requirement of cats. The National Research Council recommends calculating the energy needs of the pet on a kcal per day basis and providing 1ml water per 1kcal (National Research Council et al., 2006), although lower daily water to calorie ratios of 0.6 to 0.8 have been reported in healthy cats (Zanghi et al., 2018). However, this is only an estimate that will require monitoring and adjustment dependent on the individual. In healthy animals, water requirements are rarely calculated since they are able to self-regulate their water status, but this calculation is used 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.
There are three main sources of water for cats: dietary water, drinking water and metabolic water. Dietary and drinking water compose the majority of fluid intake. All of these should be considered when trying to estimate the water intake of an individual. Generally, pet owners are not aware of whether their cat is actually drinking enough (Box 1).
Dietary water is impacted by the moisture content of the food. Commercial dry feline diets typically contain less than 10 percent moisture, whereas wet diets (tins or pouches) contain over 75 percent moisture. 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).
Metabolic water is composed of water produced in addition to ATP during the metabolism of macronutrients to generate energy. The macronutrient composition will impact on the amount of water produced. On average the metabolism of 100kcal results in 10 to 16ml of metabolic water (National Research Council et al., 2006).
Strategies to encourage hydration
There are a number of disease conditions where increasing fluid intake may be beneficial, including chronic kidney disease (CKD), diabetes mellitus, feline lower urinary tract disease and constipation (Sparkes, 2020). Generally, measurement of the urine specific gravity (USG) and urine volume voided can be used to assess efficacy of the strategy. 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).
Feeding a wet diet is a particularly effective way to encourage moisture intake. In general, the daily water-to-calorie intake ratio for water need is estimated to be 0.6 to 0.7 and 0.9 for healthy cats consuming dry and wet food respectively (Zanghi et al., 2018). Although cats can consume sufficient calories to meet their daily needs regardless of whether dry or wet food is given, this difference in the daily water-to-calorie intake ratio is observed in healthy cats because they drink less water when fed dry food than the moisture they ingest if eating wet food. Feeding a combination of wet and dry food does not tend to be a successful alternative: studies show that feeding diets with ratios of 1:3 and 2:3 wet to dry food has no significant effect on urine volume, even though water volume increased (Greco et al., 2014). Unfortunately, not all cats will accept wet diets, although feeding a variety of both wet and dry food from a young age may minimise the risk of a cat developing a strong preference to one or the other (Sparkes, 2020).
For those cats that do not accept a wet diet, or if feeding a 100 percent wet diet is too expensive for a client, soaking the dry diet is an additional option. However, dependent on the requirements of the patient, a very large volume of water may be required. For example, for pets predisposed to uroliths it is recommended that a diet containing over 75 percent moisture is used, which requires 300 to 500ml/100g dry food (Villaverde Haro, 2020). This may spoil rapidly if left out and some pets will refuse it. There should be a gradual increase in the volume of water added to the dry food to increase the likelihood a cat may accept it, and it may be most convenient for the client to mix this in a blender.
In pets refusing the addition of moisture to their diet, a sodium-enriched diet is an additional option which may encourage increased voluntary intake of drinking water. This should not be used in cats with sodium-sensitive conditions such as cardiac disease, hypertension or CKD, and studies looking at the long-term efficacy of this strategy are still lacking (Villaverde Haro, 2020).
Encouraging increased drinking water intake can be done in a variety of different ways. Water bowl or drinking receptacle type, placement and number should be considered. Whilst these are all important considerations, whether they actually increase voluntary water intake in cats has not been documented (Sparkes, 2020). The use of cat fountains is often recommended to promote fluid intake. Studies have not clearly demonstrated statistically significant differences in water intake between bowls and fountains, but have demonstrated clear individual preferences (Pachel and Neilson, 2010), so it may be something for a caregiver to experiment with. Other individual cats may prefer outdoor water sources or a dripping tap.
Flavouring water, by adding things such as liquidised cooked prawns or spring water from tinned tuna, for example, is another key strategy which may help increase fluid intake although published evidence to support the efficacy of these suggestions is still lacking. Experimenting with the type or water (for example, mineral water, tap water, rain water) may also be helpful.
Published evidence to support the use of nutrient-enriched water containing osmolytes in cats exists. It was demonstrated that nutrient-enriched water could successfully increase water intake, reduce USG and reduce urine osmolality, and these observations were seen over a sustained period of time (Zanghi et al., 2018). Cats drinking the water supplement also demonstrated water:calorie ratios of over 1.
Many situations exist where encouraging fluid intake in cats is desirable. A number of different recommendations exist to help improve hydration status beyond the generic advice of ensuring fresh water is available, although we unfortunately still lack evidence for many of these recommendations. However, clear preferences of individual cats appear to exist and owners should be encouraged to experiment with different strategies to help promote water intake. Veterinarians can monitor the efficacy of strategies with regular urinalysis to examine parameters including USG, and if not achieving the target USG, should recommend changing or combining strategies, or becoming more aggressive with such strategies.