Dr. Rachel Boltz is a veterinarian who specializes in feline health and uses an evidence-based approach. That means that when a question comes up, Dr. Boltz thoroughly reviews existing scientific studies, using her extensive knowledge of feline physiology and anatomy to come up with the right answers. When we approached her with questions about feline nutrition, Dr. Boltz provided us with a comprehensive and detailed analysis of the current scientific understanding of feeding solutions for cats.
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Her answers cover general feeding issues such as dry vs. wet and also address specific health conditions where a more specialized approach may be needed. The following article is not easy to digest (pardon the pun!). Dr. Boltz provides us with a lot of information - all of it evidence-based - and at a level which many cat owners will find fascinating and appealing. If you're looking for a "bottom line" instructional message, feel free to skip to the cat feeding recommendations Dr. Boltz offers at the bottom of this article.
Table Of Contents:
- Who is Dr. Rachel Boltz?
- Dry vs. wet cat food - what does science actually tell us?
- Weight concerns in spayed/neutered cats
- What about overall water intake and hydration?
- Feeding considerations in cats with kidney problems
- Feeding considerations for cats with cystitis (bladder inflammation)
- Feeding considerations in cats with hyperthyroidism
- Feeding considerations in diabetic cats
- Can a carbohydrate-rich diet cause diabetes in cats?
- In conclusion: Dr. Boltz's Recommendations
Who is Dr. Rachel Boltz?
Dr. Boltz was born and raised in the Detroit metropolitan area of Michigan, USA. She received a Bachelor’s Degree in Animal Science from Cornell University in 1994, a Master’s Degree in Biological Sciences from Oakland University in 1997 and graduated with honors from Michigan State University's College of Veterinary Medicine in 2001.
After graduation, she moved to the San Francisco Bay Area, USA, and entered general practice. The Morris Animal Foundation’s “Thank Your Vet for a Healthy Pet” contest named her the Best Veterinarian in America for 2008. Dr. Boltz became board certified through the American Board of Veterinary Practitioners in Feline Practice in 2009. That same year, Dr. Boltz was a Don-Low Practitioner Fellow at the University of California’s College of Veterinary Medicine at Davis in Small Animal Medicine (2008-2009). She recently graduated from UC Davis’s Hemodialysis Academy in 2016. She has authored several book chapters on feline infectious diseases and lectured on feline specific disease, handling and practice. Understanding and addressing the uniqueness of feline medical practice, from low stress handling techniques to the treatment of species-specific disease, has defined Dr. Boltz’s career.
Dr. Boltz lives in the Peninsula Bay Area of California with her family of cats. The cat featured in the photo (his name is Charlie) has been with her since her second year of veterinary school. Dr. Boltz is associated with Silicon Valley Veterinary Specialists located in San Jose, California.
Continue reading the article, as I hand the virtual microphone to Dr. Boltz herself so she can answer the questions we presented her with.
Dry vs. wet cat food - what does science actually tell us?
There is no consistent evidence that wet food diets are superior to dry food diets for the healthy, weight-appropriate cat.
Canned foods are in general lower in calories than equivalent dry food diets, which makes their use desirable in overweight cats as a means of weight loss and control. There are studies that suggest the relative lack of carbohydrates in canned diets is superior for weight loss (and most of these studies are in diabetic cats), but a few more recent studies have concluded total calorie intake, and not necessarily nutrient proportion, has the greatest effect on weight loss in healthy cats.
Weight concerns in spayed/neutered cats
An issue of concern has been the effects of neutering on the metabolism of cats. Much work has been done to define the consequences of neutering. Evidence clearly demonstrates neutered cats (male and female) have lower energy requirements than their intact counterparts and so tend toward obesity. Simply put, neutered cats require fewer calories per day.
A general statement can be made that if a healthy cat is not overfed, the formulation of food (canned versus dry) is not critical from a weight control standpoint. The energy density of the food must be balanced with the feeding recommendation of the manufacturer. If a cat is fed less than the manufacturer’s guideline for the current weight of the cat, then required daily nutrients (like taurine, for example) may not be met. For this reason, for significant weight loss purposes, it's best to choose canned food or specially formulated dry foods made for weight loss. Using such diets for weight loss is preferred to ensure that the nutritional requirements are met along with calorie restriction.
Many point to the higher satiety value of wet/moist foods, for which studies exist (in humans and dogs) that support this notion. "Satiety value" loosely refers to the perception of fullness and therefore satisfaction (and lack of “hunger”) for a period of time.
This appears to be a good argument for the use of canned food in cats in a weight loss regimen. This does not mean, however, that feeding wet food is the only way or even the best way to effect weight loss for every cat. Several studies have compared calorie-matched wet and dry food preparations that showed no difference in weight loss or apparent satiety of the test subjects. Other studies have concluded that canned diets result in less “begging behavior,” which was taken as an expression of hunger.
Satiety is difficult to accurately assess in feline test subjects, so the effect of wet versus dry food on a client and their cat may not be adequately predicted by these studies. In short, wet food theoretically should make cats feel full for a longer period of time than a calorie equivalent portion of dry food. What we do not know for certain is that it actually does.
What about overall water intake and hydration?
Claims that canned foods boost total daily water intake relative to dry foods are frequently discussed. In fact, studies do reveal that total daily water intake in cats fed canned food is greater than those fed dry food. Healthy cats that consume dry foods drink more than cats that consume wet food (at least that is what most studies show). Overall, a cat on a canned food diet takes in more total daily water than a cat on dry food, even though the cat on dry food drinks more.
What is not known is if overall hydration status and renal water conservation of a healthy cat are significantly altered by the water concentration of the foodstuff. It is also not known how many healthy cats actually fail to meet daily water requirements on a dry food diet with free access to fresh water. For the healthy cat with well functioning kidneys, consumption of dry or canned diets results in similar urine concentrations. A study that objectively tracks hydration status of healthy cats on canned versus dry food diets is not known to this author. Such a study would be useful in assessing the utility of canned food in healthy cats from a water homeostasis standpoint.
There is also no study (to the author's knowledge) that demonstrates a renoprotective effect of increased total water intake above daily requirements in a healthy cat. A normal functioning feline kidney operates de novo to highly conserve water. Cats in general require less water than most mammals (about 60ml/kg/day), and they also have a lower thirst response than most mammals. The beneficial effect of increased water intake above daily requirements with canned versus dry food in the healthy cat is not known.
That said, for healthy cats not meeting their total daily water intake requirements, a canned diet may be the better way to feed. This author is not aware of a study that describes or even suggests what percent of healthy cats that consume a dry food diet fail to meet daily water requirements (and who would be, presumably, sub-clinically and chronically dehydrated). The effects of chronic dehydration, in general, are variable and nonspecific. In humans, chronic dehydration has been implicated in: fatigue, constipation, blood pressure alterations, acid base imbalances, elevated cholesterol, skin disorders, asthma, allergies, digestive disorders, joint stiffness, bladder/kidney disorders, and weight gain. It is not known if the same occurs in cats.
In constipated cats, an all-dry food diet is thought to contribute to the development of constipation in susceptible cats. A canned food diet, often with additional water added to the meal, is a cornerstone of therapy for these patients.
Feeding considerations in cats with kidney problems
Let's look at cats with kidney problems, such as in chronic renal failure (CRF). In these cases, the kidney itself is unable to absorb water back from urine and so the cat loses it. The result is dilute urine and a clinically dehydrated cat. Such cats are in negative water balance and so they drink more (because their brain signals them to by causing thirst). However, owing to a low thirst response that worsens with age, and in concert with increased water loss through the kidney, these cats are not able to rehydrate by consuming water alone. It is generally the case that such cats require parenteral means to achieve adequate hydration (such as SQ or IV fluids), and food preparations that boost overall total water intake have a place in nutritional support of these patients.
The majority of prescription renal foods available for cats have a variety of canned preparations to entice cats to eat them. These diets have relatively low protein content, boosted potassium, low phosphorus content, and (in general) a highly digestible carbohydrate source. Recommendations to add water to the wet food, ensuring adequate access to fresh water and the use of subcutaneous fluids are all meant to address the heightened hydration needs of cats with renal disease. Evidence supports canned food consumption increases daily water consumption over cats that eat dry food. This makes canned food (whether OTC or prescription diets) for renal failure cats desirable even if the canned food alone is not expected to correct the dehydration caused by decreased kidney function.
Protein restriction is as an important treatment point for CRF (chronic renal failure) cats, as protein catabolism results in nitrogenous waste that must be excreted (in large part) by the kidneys. Dysfunctional kidneys are not able to excrete these compounds as efficiently and so they build up in the blood. This leads to a uremic state, which (among other effects) makes the cat feel sick.
Protein restriction is therefore said to maintain quality of life. However, the protein requirements of cats over 11 years old (which coincides with increased incidence of renal failure) actually increase over time. In some aged cats, feeding a diet formulated for growing kittens may be the more appropriate way to feed. For example, a 14-year-old cat with loss of muscle mass and stage 2 CRF may actually do better with a canned kitten food with a phosphorus binder then a canned renal food.
Almost all prescription renal diets are protein restricted in both the wet and dry formulations. Recent formulations have increased calorie content by adding more fat, which is helpful for weight maintenance in these patients who tend to lose weight (especially in the later stages of CRF). However, fat is not a building block for muscle, only protein is. Some cats may need more protein than others, depending on their degree of disease and age- related catabolism. Therefore, the notion of decreasing protein consumption of every cat with renal failure is not always the best option, and blanket statements about how to feed every CRF cat are impossible to make.
The best diet recommendations for CRF cats are suggested by the stage of renal failure (see www.iris-kidney.com) and concurrent disease states. In general, phosphorus control has been shown to prolong the life of affected cats. For cats with IRIS Stage 2 and early 3 CRF, phosphorous may be the more important nutrient to control. Phosphorus control is accomplished by feeding diets low in phosphorous and the use of phosphate binders. All renal diets are formulated with low phosphorous, which makes them a good alternative to consider (even if the protein levels are lower).
Feeding considerations for cats with cystitis (bladder inflammation)
Canned diets have been advocated to manage cystitis cases in cats, as the enhanced water intake is thought to encourage urinary dilution. This is true of canned diets formulated for cats with urinary bladder disease (obstructive or non-obstructive cystitis with or without crystalluria).
Prescription diets use various methods to prevent super-saturation of elements within the urine by promoting renal diuresis. Though both wet and dry formulations are available, the canned food more reliably leads to urinary dilution. Most OTC diets do not approximate the prescription formations, but are better than more standard maintenance diets. In most cases, feeding OTC wet food (without added water) does not result in decreased urine concentration below 1.035 (which is the standard recommended therapeutic threshold concentration to prevent bladder stone formation).
That said, cats that eat the prescription dry food often do not reach that threshold either. If urinary dilution is the main goal, especially in cats with repeat bouts of obstructive cystitis with crystalluria, canned urinary prescription diets have the best chance of achieving the goal.
Feeding considerations in cats with hyperthyroidism
Daily water requirements have been shown to increase in cats with untreated hyperthyroidism. The induced hyper-metabolic state caused by excess thyroid hormones results in both increased water loss and decreased conservation of water through the GI tract, lungs and kidneys by direct and indirect actions. As a result, the cat shows signs of polyuria (PU=urinating more) and polydipsia (PD=drinking more).
The exact mechanism of PU/PD in the hyperthyroid cat has not been conclusively defined, but the combined result is often clinical dehydration. Additionally, the thirst response of cats is not robust, and this declines as cats age. Increased water loss and decreased consumption make it very hard for untreated hyperthyroid cats to accomplish water and fluid balance. For these patients, canned diets with 70-80% water would have expected benefit by increasing total daily water intake.
It is likely that relying on water intake from food and access to water for additional supplementation will not be enough to mitigate the increased water requirements of the untreated, hyperthyroid cat. These cats usually need exogenous fluid therapy. The good news is that treatment of hyperthyroidism with return of normal thyroid function helps to bring the cat back to a state of adequate hydration and lowers the daily water requirement back to a steady state. A cat with uncomplicated hyperthyroidism that receives medication to control, or treatment to resolve, the disease may or may not benefit from a canned food diet if their water requirements revert to normal.
This begs the question: How do we know what a cat's requirements are in real life and how can we predict the change as time moves on? The best alternative for monitoring a cat’s clinical hydration status is to assess for signs suggestive of dehydration (like skin turgor, coat quality, gum moisture, etc) . However, these signs can be subtle in the mildly dehydrated cat and perhaps a “best guess” is the only realistic answer. A cat with kidney disease, diabetes mellitus or hyperthyroidism that is PU/PD should be assumed to be dehydrated. In that case, increasing daily water consumption above the present water intake is helpful. Any means of accomplishing this, with canned food, supplemental water, or parenteral fluid administration, should be considered.
It is worth mentioning that canned diets have been implicated in the etiology of hyperthyroidism, as they may contain thyroid disruptor chemicals (goitrogens), improper iodine levels, or other nutrient imbalances. To date, no studies have demonstrated a direct correlation between specific environmental or food-sourced exposure and the development of hyperthyroidism. There is a general feeling that feline hyperthyroidism has a multi-modal etiology that cannot be simplified to a single causative agent. More research is needed to further define the role of nutrition in the pathogenesis of this disease, and this most certainly complicates our questions of how best to feed a hyperthyroid cat.
Feeding considerations in diabetic cats
Diabetic cats pose a special feeding challenge. Most diabetic cats have Type 2 diabetes mellitus, which is a disease centered on glucose toxicity. Dietary modification is the key to successful management for all diabetic cats, whether or not they require exogenous insulin.
There is copious literature on studies of diabetic cats and the effects of a high versus low carbohydrate diet. The work of Dr. Deborah Greco on this subject has shaped our current understanding of the nutritional connection to successful management of diabetes mellitus. She pioneered the notion that all pre-diabetic and diabetic cats should eat canned food.
When Dr. Greco first started her research, there was no such thing as protein rich, low carbohydrate kibble. Therefore the only low-carb alternative was canned diets. Her research repeatedly demonstrated that virgin diabetic cats fed a canned food diet lost weight and that a high proportion of them either never required insulin or were able to stop insulin. Central to her initial conclusions was that a low carbohydrate diet is the major factor that reverses glucose toxicity and therefore diabetes.
Over time, that conclusion has wobbled somewhat. Later research (both her own and others) has revealed that it is the relative lower calories of canned food diets - and the subsequent weight loss - that is the most important determining factor for blood glucose regulation in overweight cats.
Currently there are many high protein dry formulas available that were not available in the early days of Dr. Greco’s research. The take home message is that calorie control (regardless of protein level) through weight loss and maintenance is the major intervention point for control of feline diabetes.
It is important to understand the difference between a “low carbohydrate diet” and a “high protein diet.” A “low carbohydrate diet” can either be high protein, high fat or both. If calories are not supplied by carbohydrates, then fat or protein has to take up the bulk. Many “high protein” dry foods are in fact high in both protein and fat. The result is that many high protein dry foods are very high in calories. If portion control is not strictly exercised, cats on these types of dry food diets (whether prescription or OTC) will gain significant weight. For the average domestic cat, weight gain is not the goal. By far, canned diets are the most reliable source of a low carbohydrate meal that derives the majority of calories from protein and so does not supply an over-abundance of calories.
Protein and carbohydrate content is important in controlling daily blood sugar peaks in diabetic human patients. Dietary fat is important in calorie and weight control. All three act in concert to determine moment-to-moment and overall glucose control. Protein consumption, in general, does not spark a significant insulin response, and so a protein rich, low fat diet is the cornerstone therapy in the management of diabetes. Human diabetic patients are counseled to significantly limit ingestion of simple sugars in favor of fiber and other complex carbohydrates. The glycemic index of carbohydrates in food is a central issue to diabetes management in humans, and the role of fiber to slow carbohydrate digestion and blunt insulin peaks has been well established.
Glycemic index, per se, is not addressed in feline diabetes. The carbohydrate source of many cat foods contains the starch portion of plants like rice, wheat, oats and corn. Many pet foods now use whole and raw grains instead of the refined product. This would likely alter the glycemic impact of a “carbohydrate rich” diet. Indeed, prior to the advent and popularization of “high protein, low carbohydrate” diets, fiber-rich low fat or calorie controlled diets were the foods recommended for feline diabetic patients. One study in cats revealed that while diabetic cats fed a high fiber (12% fiber, dry matter) versus low fiber (1% fiber, dry matter) diet did have lower blood glucose values, there was no significant difference in glycosylated hemoglobin concentrations or in insulin requirements between the groups. In contrast, in a study that compared diabetic cats fed a low carbohydrate-low fiber canned diet with those fed a moderate carbohydrate –high fiber canned diet, subjects in both groups had significant decreases in serum glucose and fructosamine levels over a 16-week period, but significantly more cats who received the low carb-low fiber diet reverted to an insulin free state. These studies suggest that the role of fiber in the regulation of blood sugar in the diabetic cat may not be approximated by the effects noted in humans.
Can a carbohydrate-rich diet cause diabetes in cats?
So the question is: Does feeding an obligate carnivore a carbohydrate rich diet (like standard dry kibble) predispose them to developing diabetes?
The introduction in S. Thiess et al’s. study (S. Thiees et al. Effect of high carbohydrate and high fat diet on plasma metabolite levels and on iv glucose tolerance test in intact and neutered male cats, Journal of Feline Medicine and Surgery (2004) 6, 207-218) nicely summarizes the general agreement and understanding regarding the unique feline metabolism and natural dietary adaptations:
“The cat during its evolutionary development has tightly adapted to a diet high in protein (~54% of DM) and low in carbohydrates (~8% of DM) (Scott, 1981), with its natural diet consisting of food of animal origin only (Lindemann, 1953; Rohrs, 1987). This adaptation is well reflected by its unique metabolism of nutrients, which makes the cat a true and strict carnivore. When compared to the omnivorous dog, cats have lower activities of carbohydrate digestive enzymes in the gastrointestinal tract (Kienzle, 1993a,c), slower glucose incorporation rate into glycogen (Ballard, 1965) and elongated glucose elimination time in the glucose tolerance test (Kienzle, 1989). These facts imply that the cat as a carnivorous animal is not well adapted to readily metabolize large glucose loads.
On a typical carnivorous diet, which is low in carbohydrates, the feline liver is able to provide sufficient amounts of glucose to fuel the glucose dependent tissues of the body (Ballard, 1965). The high protein content of the diet supplies the steadily high active gluconeogenic pathways with a continuous source of substrates (Rogers et al., 1977). On the basis of all these facts, no dietary requirement for carbohydrates was established for cats (MacDonald et al., 1984). On the other hand commercial cat foods often contain considerable amounts of carbohydrates, mainly as starch (De Wilde and D’Heer, 1982; Morris et al., 1977). According to the carnivore connection theory of Brand Miller and Colagiuri (1994), unnaturally high carbohydrate intake in carnivores--especially that with high glycemic index--may contribute to the development of diabetes mellitus. Such diets, through evoking higher postprandial insulin responses might lead to over- stimulation of the pancreatic b cells and to their exhaustion and eventually to diabetes mellitus…”
In the above-named study, the authors found no difference in the response to either diet between the neutered and intact test subjects. After a six-week feeding trial, cats fed the high fat diet (5.5g +/- 1.4 per 100g food, compared to 2.8 +/-0.8 g per 100g food) had significantly higher triglyceride and cholesterol blood levels, a slightly elongated glucose clearance and a statistically significant reduction in acute insulin response to glucose than those fed the high carbohydrate diet.
This suggests that diets rich in fat may diminish pancreatic insulin secretion and overall response to glucose, which may predispose these animals to developing diabetes. A plethora of similar studies in humans have led to the current understanding of glucose toxicity and the diabetic state. It may be that some of the concepts supported in the human literature are applicable to feline diabetic patients, even if one species is not a perfect model for the other.
In conclusion: Dr. Boltz's Recommendations
In this author’s opinion, the best way to feed a cat is to feed the best version of what the cat will eat. The foremost concern is that the cat eats. If the cat does not eat what you want it to eat, then feed it something that it will eat.
The next concern is the quality of food. When I think about cat food and pet food companies, I echo the thoughts of Dr. Mark E. Pederson (published on his blog, www.animalendocrine.com):
“For cat owners, I recommend that they choose two or more pet food companies known to have a good track record and feed those foods. I also choose foods that carry an AAFCO feeding claim to be complete and balanced for an adult or senior cat. I would be very careful in choosing a smaller company as the primary supplier for your cat's food.
Small pet food companies are less likely to have veterinary nutrition specialists on their staff, and therefore, their diets may not always balanced and could result in nutritional deficiencies.
In addition to rotating brands, I also like to feed a variety of different flavors. Why? I believe it's safer to rotate between brands because companies formulate their diets differently. It also helps to determine which brands and flavors and foods the cat prefers; since diet preferences may change over time, varying the food helps maintain a good appetite, especially as the cat ages.”
Third, I consider formulations (canned versus dry). Foremost in this consideration is if the cat likes canned food, or if it prefers dry food. With rare exception, I do not feel that cats that prefer dry food have to be forced into eating wet food. My exceptions are: overweight diabetic cats whose blood glucose in not well-managed and cats with a repeat history of obstructive bladder stones where a dilute urinary concentration cannot be achieved otherwise.
In general, I guide my patients to find balance. I personally feed both wet and dry foods daily, by several large pet food manufacturers. I feed a set amount to each cat per day and monitor his or her intake. I have cats in different life stages, from two years to 17.5 years old. I feed my two-year-old different formulations than my 17.5-year-old and in different proportions of wet to dry (according to their preferences).Dr. Rachel's cats
I encourage each of you to educate yourselves (which by reading this you clearly are trying to), seek guidance from professionals, and take it all with a grain of salt. Then go home and offer your cat foods that work for you both. For more science-based information about feline nutrition, try this online booklet too.