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Recently I received a question from a reader named Marta. She was curious about a sudden change in her cat’s water consumption:

Your article on your website about increased thirst in cats lists a “change in diet” as the No. 1 cause. My 10-year-old cat has suddenly (as in overnight) gone from drinking about 1 oz. of water/day from her bowl to drinking 4-6 oz./day. Her increased thirst more-or-less coincides with a change in dry food. I called the food manufacturers and learned the old food had 0.25 percent sodium content, while the new food has 0.50 percent sodium content. So here’s my question: Could that increase in sodium (which seems small) cause this much of an increase in water intake? I will be taking her to the vet for blood tests this week.

Many years ago for dinner I split a meat lover’s pizza with a good friend. Evidently the sausage and pepperoni on the pizza were heavily salted. At about 1 a.m. I learned firsthand how thirsty one can become after consuming salt. I awoke desperately parched, to the point that I could barely wait for the faucet to fill my cup with water before consuming it. The process was repeated several times during the night, and the next day my friend reported that she, too, had suffered from severe thirst.

Here is where I’m going with this: Salt, which is made out of sodium and chloride, can induce thirst. Marta’s cat’s new diet does not merely contain 0.25 percent more sodium than the previous one; in fact, the new diet contains twice as much sodium.

Any change in diet may trigger a change in water consumption. The change is most likely to be noticeable when switching between wet food and dry food. The main difference between wet and dry food is in water content. Therefore, cats newly on dry food may be perceived to drink more water, and those switched to wet food will seem to drink less. In either case, however, actual water consumption remains the same — the change in thirst merely makes up for the different water contents of the foods.

And, as mentioned above, changes in dietary sodium consumption may trigger changes in thirst. It is therefore possible that Marta’s cat is drinking more water because of the diet change.

However, it is also possible that something else is going on. If you ever want to watch a vet come to attention, tell him that your cat is drinking more water. Increased water consumption is a hallmark of a number of common medical conditions in cats.

There is a scientific term for increased water consumption: polydipsia. When extra water is consumed, it has to go somewhere, and that somewhere ultimately is the urine. Therefore, increased urine production (also known as polyuria) inevitably occurs in conjunction with increased thirst.

In veterinary circles, any condition that manifests with increased thirst and urination is labelled in short hand as causing “PU/PD” — short for polyuria/polydipsia. Note that the urine is mentioned first. That is appropriate because increased urine production usually precedes increased thirst. Although cats may seem to urinate more because they drink more, in fact it is usually the other way around; they become thirsty because they pathologically are producing too much urine.

One of the most common and serious causes of increased thirst and urination in cats is chronic kidney disease. Cats with healthy kidneys can produce very strong urine. As the kidneys weaken the urine becomes dilute, and subsequently becomes more voluminous. Thirst increases to accommodate the increased urine output.

Diabetes mellitus, also known as diabetes, is another common cause of increased thirst. Diabetes is a disorder of blood sugar, and cats with the condition produce more urine because sugar in the urine draws in extra water through osmosis.

Other common causes of increased thirst and urination in cats include hyperthyroidism, reactions to certain medications (especially prednisolone), certain types of infections, behavioral anomalies, and some types of cancer.

Marta, I fully approve of your plan to run blood work on your cat. I also recommend urine testing. A change in thirst may occur after a change in diet, but a change in thirst also may be a sign of a major medical problem. Blood and urine tests generally can be used to determine whether there is anything to worry about.

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