So you might have heard that cats need to eat wet food. You might have heard a dozen reasons why from generally well-meaning people. One of the reasons bandied about is that it prevents cats getting ‘kidney stones’.
Well, that’s not quite right. Some of the story has been lost in the retelling.
Above is the dried out urine sediment from a 3yo male cat who presented nearly dead. He had started looking ‘off’ 24 hours prior. All that stuff in the bottom of the jar is struvite crystals, and it had packed into his urethra so much that he couldn’t urinate. His blood urea and potassium soared, he went into acute renal failure with bradycardia, and I got to spend a Friday evening trying to coax him into life whilst cursing at a slippery penis that was far too small (which is not my usual recreational schedule).
Kidney stones, as in stones forming and getting stuck in the kidney, are rare. I’ve seen one in six years, and it actually caused no problem at all, and still hasn’t. These little things are a combination of bladder stones, and their precursor: urinary crystals or crystalluria.
Bladder stones can get seriously big, and are irritating but usually not life threatening. The acute, life threatening scenario happens when these little guys, tiny uroliths and sand-like crystals, get stuck in the urethra.
While any animal could have this happen, male cats are particularly prone. This is because their urethra is relatively small and narrow (the aforementioned tiny penis) and has a bit of a bend in it. Female urethras are generally broader, straighter and shorter. Female cats can still get crystals, but they usually end up irritated and angry rather than comatose and dead.
Now, these crystals are made of particular salts. When the urine gets more concentrated, more of these salts will precipitate out if solution to form a sediment that you see above, and if they do this over a long enough period of time, you will get a bladder stone.
So more dilute urine means less sediment forming, which is why certain people will tell you to feed wet food only to your cats, because it contains more water. But it’s not that simple.
Your cat should always have fresh water to drink, ideally from multiple locations. Set your water sources up right, and your cat should drink adequately. Wet food is useful when you need to force the cat to consume more water, like after a blockage or bladder surgery.
The more reliable way to prevent these crystals forming is to control the acidity of the urine, which makes them more likely to dissolve than form (remember equilibrium equations from chemistry, kids?), and to control their components.
You know how salt is made of sodium (Na+) and Chloride (Cl-)? If you don’t have enough sodium or enough chloride, then you physically can’t get salt crystals to form.
Struvite is made from ammonium, magnesium and phosphate. If you can reduce the concentration of one or all of these in the diet, then there are less building blocks available to make these potentially life threatening crystals. Prescription diets aim to control both the acid-base balance and the amount of these nutrients available, even the dry food formulas. So as long as you have plenty of water available for you cat, you can use certain dry foods to treat crystalluria, and not all are going to predispose to it.
But you what food will cause higher levels of phosphate and ammonium in the urine and is a good source of magnesium? Meat. Flesh of animals. Consequently, generic canned food or raw feeding is not going to be a magic fix for cats (or dogs) predisposed to producing these crystals. If you pet has these, they need lifelong* management and monitoring. They are not normal and need special nutrition to reduce their risk. (Unless struvite urolith is secondary to infection, but that’s a different topic)
Urinary crystals. Not kidney stones.