Featured ingredient part two: Water
How much water does your pet need?
The daily water requirement for any animal is defined as the amount of extra water required each day to maintain a stable body water balance for that animal. Your pet uses up a significant quantity of water each day in hydrolysis reactions, through losses during respiration, sweating through the mouth and the paws, and in the urine and faeces. These losses need to be replaced each day and this is achieved for the most part through the diet and from drinking free water. Eating and drinking account for about 90% of your pet’s daily water requirement. In addition, your pet also produces a significant quantity of water of his own when oxygen and hydrogen are combined as part of the energy- producing metabolic pathways. This ‘metabolic water’ accounts for up to 10% of your pet’s daily water requirement.
So, how much water does this all add up to? Luckily, there is a reasonably accurate rule-of-thumb that states that any healthy animal needs to consume 1ml of water every day for every kilocalorie of energy it consumes, at rest and in an environment that is neither too hot, nor too cold, but just right, i.e. thermo-neutral. This can be put into an equation - ml/day = (70 x (body weight in kg)0.75) x 1.6.
This equation makes a lot of assumptions of course. Sick animals and especially animals that are vomiting and have diarrhoea need a lot more water, either through drinking, but more commonly straight into a vein if they are very unwell. Healthy dogs also need more water on days when they are busy and running about. The equation thus represents an absolute minimum daily water requirement – according to this equation, a 1 kg dog required 112 ml per day, a 4 kg cat required 317 ml / day and a 15 kg Spaniel requires 854 ml / day.
The nature of the pet’s diet determines how much fresh water he needs to drink each day, which may include from ponds, puddles etc. Pets on exclusively dry commercial food need to drink a lot more than those fed moist foods, which themselves consist of around 75% water.
From what has been said above, estimating daily water requirements for pets is really quite straightforward, but there are 1 or 2 other caveats that need to be taken into account. First, cats generally produce more concentrated urine than dogs, therefore they may need to drink a bit less water. Some pets drink over and above what they actually need because they simply enjoy it. This is fine as long as they do not do it to an excess. It is possible (but very rare) for an animal to drink so much in one go that it induces water intoxication which can be fatal.
And what about pets that don’t drink enough? Well, water losses of just 15% to 20% of total body weight is rapidly fatal, and this can happen in pets deprived of water completely over just a few days. Much more common are pets that don’t drink enough because of chronic illness, for example kidney disease. Cats are particularly susceptible to this. Other diseases in cats linked to not drinking enough include feline lower urinary tract disease (cystitis) and urolithiasis (bladder stones).
Dr. Robert Falconer-Taylor BVetMed DipCABT MRCVS