Here is that chart in question:

Short answer: It does matter. And it depends on the purpose of using the fluids as well.

In a healthy cat, with no loss of moisture from the body other than what we lose from our lungs, saliva, metabolism (which relates to muscle mass and digesting food), and a minimal amount sweat, the total amount of water in the body stays the same more or less. So if this cat drinks his maintenance amount of fluid to cover all of those things mentioned above, he shouldn't need any other fluids at all.

The second we introduce vomiting, diarrhea, lack of appetite, excessive peeing, infection,  or other illness that effect the amount of water they have in the body, we often have a fluid deficit.

Some cat’s will drink enough to cover this deficit, but many cats will not.

Some times it’s easy to know this: a 5k cat needs 220ml in a day, and he ate or drank nothing the whole day. So I know at minimum, he needs 220ml. Now what if I measured his water bowl, and found he actually drank 100ml only his own? And he didn't eat anything at all. Then theoretically I would only have to supplement his with 120ml at minimum.
So as they drink or eat more wet food to cover their deficit, the fluids I should need to give them will decrease. If they drink or eat less, I need to give them more.

The trick is that some of their fluid intake comes in through food (if eating moist foods), and calculating food and water intake, combined with what their needs are during illness can get complicated. It can be done. By measuring body weight, water intake, food intake, urine output, and measuring plus estimating other losses of fluid. But then the kids have to go to bed and I feel like a beer by the end of the night, and running the calculator and assessing the moisture level of poop is the last thing I feel like doing!

In the chart, I describe 1, 2, 3 and 5% BW fluid levels that could be given, and correlate it with what might be going on with the kitty. These estimations literally comes from clinical experience of administering thousands of doses of SQ fluids to different cats, of varying ages, going through dozens of clinical conditions or illness. For all those cats, I used knowledge of their behaviour and stresses they faced to estimate their internal fluid needs. Just like we do in animal welfare science when trying to figure out their emotional states.

Sometimes, it doesn't matter that they are drinking enough water to cover their daily amount or deficit. They have some illness, high calcium, fever, some other electrolyte or acid-base issue, or be at risk of dehydration while taking a certain medication, just to name a few.  I want to address this via fluids. The fluids are not just to replace what they are missing, but it is a treatment in and of itself.


Dr. Kris