Everyone loves to blame dry food for all sorts of things, but one disease in particular is constantly accused of being the result of dry food only diets – but is this true? What is the evidence?
Let’s take a look at what, if any, studies have been conducted on dry food and it’s relationship with urinary disease in cats; is there an increased risk of LUTD if eating dry only?
“The majority of cats with signs of lower urinary tract disease do not, however, have urolithiasis; indeed, no specific cause can be established in most of these cats.” – Markwell et al, 1998
First, let’s take a look at what we know about Feline Lower Urinary Tract Disease; in the majority of cats presenting with urinary obstructions or LUTD signs (haematuria, crystalluria, stranguria or dysuria, frequent bathroom trips, or inappropriate urination etc), no inciting cause could be found. This is called Feline Idiopathic Cystitis (FIC), and these patients can have recurring episodes for little to no reason, but particularly stress appears to be a primary trigger. In these cases that make up the majority, diet is not believed to be a factor in the development of this condition.
Secondly, obstructive FLUTD also known as Feline Urologic Syndrome (FUS), is when cats obstruct completely due to the formation of crystals in the urine plugging the urethra and blocking the flow of urine. This condition is life threatening and can be fatal if untreated. This condition appears to be multifactorial (age, sex, neuter status, genetics/breed, activity level, behaviour and anatomy) however diet DOES play a role in FUS but maybe not in the way most people think.
Dry food is often blamed as the singular cause of FUS, purely because it’s ‘dry’ and the claim that high moisture diets won’t result in FUS. Not only is this a gross oversimplification of urinary disease in cats, it also doesn’t actually get to the core reason diet is implicated in FUS.
So what’s the evidence?
A number of epidemiological studies have looked at dry food consumption and it’s relationship with FLUTD, but few have actually looked at why dry food could be an issue. When we think about dry food, many jump to the conclusion that it’s the lack of moisture content that is the risk – but actually, this is not actually the case.
“The increased risks may lie in a combination of suboptimal composition of the dietary dry matter.” – Beynen et al (2015)
In my post about Hard Water & FLUTD, we discussed the mineral content of water being significantly lower compared to food – in the case of FUS, the mineral content of the diet, particularly in some dry foods can be very high which results in high excretion of these minerals into the urine, where the urine then becomes supersaturated (see below)
This is what is meant by the ‘suboptimal composition of dry matter‘ in some, but not all, dry cat foods; if the mineral content is very high in the diet, given the right conditions (urinary pH, urine concentration, the combination of minerals present, etc), crystals will form in the solution and this subsequently may result in a urinary obstruction (particularly in male cats) if the perfect storm of other risk factors are present.
So assuming the dry food is low in mineral content, is there another reason dry food could be a cause of FUS?
If we look at composition again and think about urinary pH for a moment, the ingredients in some dry foods can be problematic. Why? This is due to the overall urinary pH produced by the formulation. We know urinary pH is a factor in the formation of a number of crystal types, and a more acidic or basic pH can help to dissolve stones or crystals by increasing their solubility.
Some dry foods may use ingredients that are overly alkalizing, particularly those that use a lot of plant based ingredients which can result in an increased risk of struvite crystal formation. Conversely, dry diets that have very high levels of animal protein or use a large amount of different animal proteins, can create a more acidic urinary pH thus increasing risk of calcium oxalate crystals – which is even more challenging to manage as these cannot be dissolved like struvites can.
Urine pH aside, certain foods and ingredients contain higher levels of minerals that contribute to crystalluria (magnesium, potassium, phosphorus, calcium, oxalates etc), that can result in creating a high risk dry food diet.
A word on additions and treats
Many studies looking at dry food as a risk factor for FLUTD neglect to look at pet owners adding treats, toppers or human foods alongside their cat’s dry food, or if the pet has access to the outdoors where they are supplementing their diet with prey.
So even if the dry diet was low in mineral content, once supplemented, the overall diet composition is entirely changed with additions.
Correlation not causation
When we think about variables in studies on dry food and FLUTD, there’s another glaring problem. Cats kept indoors typically eat dry food, and have increased risk of developing urinary obstruction. Unfortunately, this presents a problem – is it the cat eating dry food, or the cat living indoors? Buffington et al (2006) states:
Lower urinary tract signs in indoor-housed cats may be more closely associated with cat-related factors than with demographic or environmental factors.
So therefore, dry food being a risk is more likely to be a case of correlation rather than causation – indoor cats eat dry food and by being indoors, their environment and lack of activity is more likely to be the causative factor, than dry food itself.
Can they still get blocked on wet food?
Absolutely – If we are talking about suboptimal dry matter as per above, wet food still contains some level of dry matter. By design, wet foods are high in protein and moisture, meaning for a wet food to prevent crystal formation, the moisture content must be high enough to overcome the high mineral content provided by the high level of protein in the diet. If we look at the Key Nutritional Factors for FLUTD, and compare this to an average wet food, we get the below:
As you can see, the therapeutic wet and dry diets sit well within the necessary values – but look at the protein in the therapeutic and standard wet diets. They exceed the safe range; now with the therapeutic diet, all other values are within limits so this means that despite the protein slightly exceeding, the other DM values are controlled – meaning the urine will not become oversaturated with the inclusion of moisture.
However, as you can see a standard wet food is nowhere near what we need for a urinary safe diet, and unfortunately many of the key nutrients aren’t available for us to compare. Unfortunately this is where many people rely on ‘Ash’ content, but this also does not actually give us any indication as to what minerals the Ash is composed of, in what levels. Meaning even if you did find a “low ash” diet, it’s still unlikely to be safe for urinary disease or urolithiasis.
For this reason, yes cats absolutely do obstruct with wet food only diets – because it’s not just about moisture. It appears that in about 11% of cases, cats with FIC fed a wet only diet relapsed; which while lower than their dry fed counterparts (39%) the wet food in this study did have lower protein content than the dry food (as discussed above). So once again, this is another case for the dry matter issue – not purely the moisture content.
So what should we do?
Given what we know about FLUTD, I still wouldn’t avoid dry food all together because there is little evidence that the food simply being dry is an issue.
What is an issue, is the mineral content of the diet in question – whether it’s wet or dry – combined with the presence of other risk factors and the type of FLUTD we are dealing with.
There are a number of diets available that are designed specifically to be lower in mineral content, to encourage drinking and urinary dilution and to keep the urine pH in a stable region that will prevent crystal formation – they are available in both dry and wet varieties and both are equally as effective when it comes to managing risk or treating FLUTD;
“Clinical signs of struvite urolithiasis resolved in an average of 19 days in cats fed either a canned or dry urine-acidifying, magnesium-restricted diet, and stones completely dissolved in 31 of 39 cats in an average of 30 days.” – Houston et al, (2004)
Do some cats with FLUTD improve on a wet only diet? Potentially – the only urinary disease in cats that’s shown improvement with increased water intake is FIC, so unless your cat has this condition, I wouldn’t stress too much about adding copious amounts of water into their diet or only feeding wet food.
If you want to prevent urinary disease in your cat, I’d consider a urinary ‘care’ diet (dry or wet!) and manage their other risk factors (activity level and weight).
References
Beynen, A. C. Dry food for cats.
Beynen, A. C. (2016). Diet and feline idiopathic cystitis. Dier en Arts, 1(2), 18-21.
Buffington, C. T., Westropp, J. L., Chew, D. J., & Bolus, R. R. (2006). Risk factors associated with clinical signs of lower urinary tract disease in indoor-housed cats. Journal of the American Veterinary Medical Association, 228(5), 722-725.
Peter J. Markwell, C. Tony Buffington & Brigitte H.E. Smith (1998) The Effect of Diet on Lower Urinary Tract Diseases in Cats. 1, The Journal of Nutrition, Volume 128, Issue 12, 1998, Pages S2753-S2757, ISSN 0022-3166,
https://doi.org/10.1093/jn/128.12.2753S.
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