Dry Food for Cats—An Historical Accident? By Randolph Baral Paddington Cat Hospital
This piece has several origins:
- The ISFM forum where a practitioner asked the maximum amount of dry food that can be fed to reduce the chance of recurrence of lower urinary tract diseases (LUTDs).
- My Science Week presentation last year where my clinical pathology centred talk showed the effect that dry food has on glycaemic load (then most of the discussion was more about nutrition than clinical pathology!).
- A recent weekend nutritional seminar where one speaker presented to the packed room that he wanted to ‘bust the myth’ that ‘cereal grains can cause food allergies’.
I have been in feline-only practice for almost 30 years. My experiences have echoed the growing body of evidence of the multiple problems caused by dry food in cats. Specific literature influences have been the Zoran ‘carnivore connection’ review,¹ the Mazzaferro et al diabetes paper² where nearly 2/3rds of cats had insulin discontinued after switching to a high protein canned diet and the Plantinga et al paper³ that assessed the nutrient profile of feral cat diets.
These all point to feeding cats a diet consistent with what cats have evolved to eat, which is ~70% water, with the remaining 30% dry matter comprising ~60% protein, 20-25% fat, ~12% total minerals (e.g. from bone) and next to no carbohydrate.
The difficulties that dry food has in achieving these metrics are:
- Too low moisture content (after all, it is ‘dry’ food).
- Too low protein content and too high carbohydrate content.
I will focus on these aspects as we explore them by clinical conditions:
LOW MOISTURE CONTENT
– Lower Urinary Tract Diseases
Putting aside infection and neoplasia (that require specific therapeutic approaches), LUTDs can be considered as those associated with uroliths or crystal formation which can lead to obstructive disease (almost entirely in male cats) and idiopathic LUTDs; in both cases, wet food diets have been proven to result in better outcomes.
It seems to me that many practitioners get concerned with urine pH (or magnesium or calcium content) and forget that the undisputed MAJOR factor associated with crystal and urolith formation is ‘relative supersaturation’.⁴ Wet food diets create more dilute urine,⁵ so we shouldn’t HAVE to prove that high moisture diets are less likely to result in less opportunity for crystals and uroliths to form, yet this has been demonstrated.⁶,⁷
Looking further at WHY wet food diets result in more dilute urine, it has been recognised for nearly 50 years (at least!) that cats eating a diet that contains >70% water barely drink.⁸ When cats eat dry food, they need to make up the deficit of water that they have evolved to ingest with their food. Consequently, a cat eating all dry food drinks 6-times as much as the same cat eating all wet food; yet, confoundingly, their total water intake from food and water is still considerably less than if they were eating all wet food.⁹
A lower total water intake results in more concentrated urine and thus provides the conditions to allow supersaturation to occur.⁴ A ‘small’ amount of crystalluria is normal in cats and so the mere presence of crystals does not necessarily cause clinical signs. However, we do know that large numbers of crystals can form a matrix with inflammatory protein to result in urethral obstruction.¹⁰
One study assessed the effect of dry and wet versions of a prescription diet designed for lower urinary tract diseases. The study described the cats as having ‘idiopathic cystitis’ and is not clear as to how many cats had substantial crystalluria. Over a 12-month study duration, wet food diet resulted in only an 11% recurrence compared to 39% recurrence for the dry food diet group.⁵ As well as crystal reduction, it has been postulated that more dilute urine ‘may decrease the concentration of substances in urine that are irritating to the urinary bladder mucosa.’¹¹
- Upper urinary tract uroliths
The studies noted above describing increased risk of urolith formation for cats eating dry food apply to uroliths generally and therefore upper urinary tract uroliths as well as those in the lower urinary tract.⁶,⁷ Additionally, dry food diets have proven to be a major risk factor for ureteral obstruction¹²,¹³ and upper urinary tract uroliths specifically.¹³
One of these studies determined that cats eating predominantly dry food were 15.9 (!) times more likely to develop ureteral obstruction and even cats eating a mix of wet and dry food were 3.4 times more likely to get ureteral obstruction compared to predominantly wet food.¹² The significant morbidity of the commonly resultant acute kidney injury as well as the high likelihood of subsequent chronic kidney disease,¹³ requiring lifelong management, can likely be significantly reduced by avoidance of dry food.
- Chronic Kidney Disease
Chronic kidney disease (CKD) has been recognised in 52-73% of cats with upper urinary tract uroliths.¹²⁻¹⁶ This demonstrates that avoiding uroliths reduces a high chance of developing CKD, and, repeating that stated above, avoiding dry food greatly reduces the risk of upper urinary tract uroliths.
Kidneys have many functions, one of the most important (I would argue THE most important) being conservation of total body water. The reduced kidney function in cats with CKD means that they have reduced ability to maintain adequate water in their bodies. This is why a typical clinical sign is polyuria, and why our typical measures to diagnose CKD (increased creatinine and urea, i.e. azotaemia) are measures of dehydration.
Polydipsia in CKD cats is an attempt to rehydrate. Even if dry food has not contributed to the initial development of CKD, as we’ve noted above, cats that eat dry food have a deficit of water that they have evolved to ingest with their food. With CKD, there is an additional water deficit. Therapies such as nutrient enriched liquids (i.e. Hydracare™)¹⁷ or using subcutaneous fluids are based around reducing the water deficit to improve clinical signs. Feeding wet food instead of dry food is the most straightforward strategy to reduce water deficits.
- Idiopathic Hypercalcaemia
There is much unknown about idiopathic hypercalcaemia, certainly the causes (as evidenced by the descriptor: idiopathic). We DO know that calcium-oxalate uroliths are a common consequence and this is borne out in papers that show ionized hypercalcemia in cats with uroliths,¹³,¹⁶ and we also know that many of the cats in these studies have eaten predominantly dry food diets.
We DO know that fluid therapy with 0.9% NaCl is recommended for emergency treatment of hypercalcaemia¹⁸,¹⁹ since most calcium is excreted in the urine. We also know that diuresis is promoted by reducing USG with a high moisture diet and thus it is not surprising that wet food diets have been recommended to reduce idiopathic hypercalcaemia.¹⁹,²⁰
Our experience at Paddington Cat Hospital is that this strategy is often, but not always effective on its own and other dietary strategies such as adding fibre²¹ may be additionally required. Of course, some cats require more therapy than diet change alone.
LOW MOISTURE AND HIGH CARBOHYDRATE/LOW PROTEIN
– Obesity: moisture content
If you compare any brand of dry food to its equivalent form of wet food, for the same volume (say, 100g), the dry form has approximately 5 times as many calories. If you compare a ‘light’ or ‘diet’ form of a dry diet, that factor reduces to 4 times the calories of a maintenance wet food (and increases back to 5 times if compared to the equivalent diet wet food). I invite the reader to look up the nutritional data from pet food companies to confirm this.
Given this large disparity in energy density, it is hardly surprising that multiple studies have identified dry food as a risk factor for obesity in cats.²²⁻²⁴ A large amount of this energy density disparity can be attributed to the water content which varies by a similar factor of approximately fivefold (in the opposite direction, of course).
We know that feral cats’ diets of whatever raw prey they can find comprises approximately 70% moisture,³ and it is not unreasonable to conclude that cats evolved to consume this amount of moisture. Since it has been demonstrated in many species that satiety is, at least in part, from degree of gastric dilation,²⁵,²⁶ we could infer that cats evolved to meet their caloric requirements from a diet much less calorie dense than dry food. Additional to simple caloric density, higher moisture content in food has been shown to promote increased activity,²⁷,²⁸ thus resulting in using more calories as well as ingesting less.
- Obesity: carbohydrate/protein proportions
Even dry food marketed for diabetics and obesity have 4 to 8 times the carbohydrate content as their wet food counterparts. Not only do cats have no dietary requirement for carbohydrate with glucose requirements (such as for the brain and red blood cells) being met by gluconeogenesis,²⁹ they also lack effective means to fully metabolise carbohydrates.³⁰,³¹
The precise amount that defines carbohydrate excess in cats has yet to be elucidated but one study assessing 12% carbohydrate as ‘low carbohydrate’ found significantly less glycaemic and insulin response than higher carbohydrate diets.³² Numerous dry diets marketed for diabetics and obesity have double this amount of carbohydrate.
In any species, exceeding the ‘carbohydrate economy’ results in oxidation to synthesise lipid.³³ Another consequence of high carbohydrate diets is that they inevitably result in a lower protein diet (since our only macronutrients are carbohydrate, protein, fat and moisture). High protein diets (>45% but preferably closer to 60%) not only help cats meet their high and specific amino acid requirements³⁴ but also leads to significantly higher heat production and therefore increased energy expenditure.³⁵ Regardless of if the driving factor of this aspect of obesity is high carbohydrate or resultant low protein or a combination of these, the macronutrient profile of dry food contributes to feline obesity.
HIGH CARBOHYDRATE
- Diabetes
I have already referred to the Mazzaferro et al diabetes paper² in which 11 of 18 cats went into diabetic remission switching to a high protein canned diet. This paper has had a profound effect at Paddington Cat Hospital and we now not only have a remission rate of approximately 70%, consistent with subsequent studies,³⁶,³⁷ but many of our new diabetics now come from other practices where there are not the same recommendations to avoid dry food from kittenhood as we recommend.
Dry food creates a double threat for diabetes since it provides excess carbohydrate that cats are unable to efficiently metabolise³⁰,³¹ as well as increasing the risk of obesity (as noted above) which creates insulin resistance.³⁵,³⁸
At Science Week last year, I presented blood glucose results from a now 13-year-old male desexed Burmese cat at separate visits over a 10-year period, in which definitive peaks and troughs can be seen.
I then superimposed the cat’s body weight and body condition score at each visit which enables us to see that the blood glucose peaks and troughs correlate quite closely with body weight and body condition score as we would expect from our knowledge of insulin resistance; however, there are some additional peaks towards the right that cannot be explained by body weight/BCS alone.
Adding this cat’s dietary history explains those additional peaks, as well as being closely associated with the cat’s weight gains (as expected).
I have been asked about the impact of stress which most definitely can elevate blood glucose results; however, the consistency of blood glucose results with body weight and dietary history over a 10-year period reduces the likelihood that stress is affecting blood glucose in this case. We cannot make overall statements from only one cat but this cat’s lack of glycaemic control when faced with the increased carbohydrate of dry food being even part of his diet is entirely consistent with published work.³⁹
Gastrointestinal Diseases
Associations between high carbohydrate diets and gastrointestinal disease have also been made. Osmotic diarrhoea associated with carbohydrate dysfermentation³¹ and other ‘digestive disorders’³⁰ have been recognised for over 30 years with dietary intake of >5g/kg body weight of sugars and starches respectively.
High carbohydrate diets also reduce protein digestibility and incomplete carbohydrate fermentation by the small intestinal microbiota results in increased microbial fermentation in the large intestine.⁴⁰ The small intestinal incomplete microbial fermentation may alter small intestinal microbial diversity; independent to this, it has been shown that ‘the composition and number of mucosa-associated bacteria correlate with the presence and severity of inflammatory bowel disease in cats.’⁴¹
Food sensitivities demonstrated by gastroscopic food sensitivity tests as well as elimination-challenge trials found that wheat and corn gluten are among the most common allergens (as well as commercial dry diet in an additional 5 of 16 cats whose owners refused further testing).⁴² I am unaware of any assessment comparing efficacy of hydrolysed or novel protein diets to their wet food equivalents or nutritionist formulated home-made diets.
What about dental disease?
Dental disease is the biggest concern that clients and vets have when I advocate reduction or elimination of dry food from a cat’s diet. It has certainly been demonstrated in dogs that normal dry food offers no benefit to wet food for periodontal health⁴³,⁴⁴ and it is reasonable to assume that this hold for cats also.
Specific dental diets with larger kibble size and texture to promote chewing and maximal contact with teeth have been demonstrated to help periodontal disease⁴⁵ (again, at least for dogs; I was unable to find equivalent studies for cats). However, in cats, the most common dental disease is tooth resorption⁴⁶ for which no association of periodontal disease has been made.⁴⁷ I am unaware of any epidemiologic studies but I am sure that I am not alone in having seen many cats eating dental-style biscuits yet still requiring dental work to attend to tooth resorption.
Conclusion
I would like to think that, when introduced in the 1940s-1950s, and through its growth in sales starting in the 1970s, that dry food was considered appropriate for cats, much in the same way that ‘sliced white bread’ used to be considered healthy for people. However, the widespread evidence that I have shared here of disease processes that have increased in prevalence as dry food sales increased from the 1970’s, exemplified by diabetes,⁴⁸ has made me more and more certain that dry food for cats is an historical accident, far removed from how cats evolved to eat.
References
cve.edu.au/dry-food-cats
Contact and questions
Randolph can be contacted at the Paddington Cat Hospital in Sydney, 02 9380 6111.