Dry Food vs Canned Food

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Economy and convenience have made dry food the most popular product to feed to pet cats. But is what's good for us also good for our cats?

A carnivorous diet is comprised of primarily protein and fat from animal tissue. They have teeth designed to tear flesh and a short and simple gastrointestinal tract, one suited for digestion and absorption of a concentrated, highly digestible diet. Dry foods typically contain 35-40% carbohydrate. Carbohydrates are nearly absent in the cat's natural diet. The cat obtains small amounts of carbohydrate through the stomach and intestines of her prey. Commercial dry foods, however, may contain as much as 45% - 50% carbohydrates. Since the cat metabolizes primarily fat and protein for energy, most of the excess carbohydrate is stored in the body as glycogen and fat. The primary adverse effect of excess carbohydrate is obesity. The effects of obesity are heart disease because of the increased workload on the heart; orthopedic problems are increased because of increased physical stress on the frame, leading to arthritis and early debilitation; diabetes mellitus, a condition in which the pancreas doesn't produce the amount of insulin that it should to help metabolize blood sugar, is one of the most common problems in obese cats; several liver disorders occur more frequently in overweight cats. Surprisingly, the deadliest one, hepatic lipidosis, happens when the cat stops eating. Changes in the operation of the liver cause fat to be deposited there, which eventually can shut down the liver altogether[1].

In the cat's liver, gluconeogenic amino acids and fat in the diet are deaminated and converted to glucose for the maintenance of blood glucose levels. The cat has evolved to maintain normal blood glucose levels and health on a carbohydrate-free diet, a capacity inherited from her desert-dwelling ancestors. This ability is related to its different pattern of gluconeogenesis. In most animals, maximal gluconeogenesis for the maintenance of blood glucose levels occurs during the postabsorptive state, when dietary soluble carbohydrate is no longer available. However, carnivorous species, such as the cat, are similar to ruminant species in that they maintain a constant state of gluconeogenesis - the immediate use of gluconeogenic amino acids for the maintenance of blood glucose levels (these mechanisms are turned "on" and "off" in other animals).

There are differences between cats and omnivores in the relative importance of various gluconeogenic and carbohydrate-metabolizing pathways. Compared with omnivorous species, the cat has a high hepatic activity of the enzyme serine-pyruvate aminotransferase and low activity of the enzyme serine dehydratase. Thus the cat is able to convert the amino acid serine to glucose by a route that does not involve either pyruvate or serine dehydratase.

After glucose is absorbed into the body, it must be phosphorylated to glucose-6-phosphate before it can be metabolized. The liver of most omnivorous animals, including the domestic dog, has two enzymes that catalyze this reaction, glucokinase and hexokinase. Hexokinase is active when low levels of glucose are delivered to the liver, and glucokinase operates whenever the liver receives a large load of glucose from the portal vein. The feline liver has active hexokinase but does not have active glucokinase. Consequently, the rate of glucose metabolism in the liver of the cat cannot increase in response to high levels of soluble carbohydrate in the diet to the same degree as the rate in the liver of a species possessing both enzymes. Thus most of the carbohydrate in dry food ingested by the cat is converted and stored as fat.

Not all protein sources are of equal value to the carnivore, and the quantity of protein in a commercial dry cat food often says nothing about its quality. Before domestication, cats hunted their prey and consumed a diet very high in meat protein, low to moderate in fat, and very low in carbohydrates. This diet provided both the proper quantity and quality of protein for the carnivore's unique digestive system. Unlike an omnivore, whose digestive system consists of a fairly large small intestine and relatively large stomach, the carnivore's system consists of a fairly short small intestine and relatively small stomach. Thus, a carnivore's optimum diet must be concentrated, highly digestible, and low in residue because its body is designed to digest primarily protein. If an excess of carbohydrates is included in the diet, much of what the carnivore eats is only partially digested by the time it reaches the large intestine for fecal formation, overloading the digestive and excretory systems. ...

Protein digestibility in pet foods is about 80 percent for dry foods, 85 percent for semimoist and canned foods containing large amounts of cereal grains, and 90 percent for canned diets with meat as the primary protein source. Digestibility is influenced both by the source of the protein and by how it is processed. Protein in cat foods comes from both animal and plant sources. Animal protein is generally more expensive and often of higher quality than plant protein. The composition of canned foods allows the use of protein and fat sources of higher biological value than can be used in dry food. A recent survey (Morris, James G. and Quinton R. Rogers. 1994. Assessment of the nutritional adequacy of pet foods through the life cycle. Journal of Nutrition 124:252OS-2534S). compared a well-known canned food with the leading dry food, both of which claim to provide "balanced" nutrition. The digestibility claim of the canned food was approximately 90%, while the digestibility of the dry food was rated at 80%. The biological value of the protein content (in other words, how useful the protein is to the animal) was given as 70% for the canned food and 60% for the dry. Net utilization (the amount of food used by the animal in relation to the amount provided) can be calculated by multiplying digestibility by biological value. The results: 68 % net utilization for the canned food and 48% for the dry. This means a cat would have to eat nearly twice the volume of dry food to achieve the net utilization that higher, more digestible sources of nutrients, found in canned food, would provide. All those excess waste products must be filtered from the blood placing an extra workload on the kidneys. This may explain the high prevelence of chronic renal failure in middleaged cats.

The cat's natural diet, live prey, contains between 65%-75% water. The cat, having evolved on the plains of Africa, has adapted to obtain her water requirements almost entirely on the moisture content in her prey. Cats can live for long periods without drinking water when receiving food containing 67-73% water but become dehydrated when the water content of the food is 63% or less. The water content of the commercial foods commonly fed to cats varies from 8% in dry foods to over 75% in canned foods; thus the amount of drinking water required is affected substantially by the water content of the food.

When fed canned food (80% moisture) with access to drinking water, cats obtain over 90% of their total water intake from the diet, whereas on dry food, 96% of the total water intake is obtained by drinking. The total free water intake (from food and drinking water) decreases when cats are fed dry food only, so that the water to dry matter intake ratio when fed on commercial dry foods varies from 2.0 to 2.8: 1 whereas on canned foods it varies from 3. 0 to 5.7: 1. Thus for any given dry matter intake cats have a higher water turnover on canned than on dry foods. (National Research Council [National Academy of Science] Nutrient Requirements of Cats).

Diet moisture content is related to the observation that cats fed dry food drink more six times more water than cats fed canned food but that much of this water contributes to fecal moisture so that urine volume is lower and urine specific gravity higher in cats fed dry food. The urine concentration of all solutes, including potentially calculogenic crystalloids, depends on urine volume. Cats increase voluntary water intake when fed dry food but not in sufficient amounts to fully compensate for the lower moisture content of the food. In a recent study, cats consuming a diet containing 10% moisture with free access to drinking water had an average daily urine volume of 63 milliliters (ml). This volume increased to 112 ml/day when fed a canned diet with a moisture content of 75%. Urine specific gravity was also higher in cats that were fed the low-moisture food. Decreased urine volume may be an important risk factor for the development of urolithiasis in cats. Diets that cause a decrease in total fluid turnover can result in decreased urine volume and increased urine concentration, both of which may contribute to urolithiasis in cats.

Canned diets contain enough water that cats consuming them rarely need to drink. Daily water needs, in milliliters, often are "guesstimated" as equal to the metabolizable energy requirement in kilocalories or approximately 60 ml/kg. Once the diet is consumed, oxidation of nutrients produces an additional 10 to 13 grams of water for each 100 kcal of metabolizable energy. Thus a 4 kg cat consuming a 240 kcal canned diet containing 78% moisture will consume 237 ml or 98% of its daily water need directly from the diet. Thus the cat needs to drink less than 1 oz. of additional water per day whereas a cat consuming a 240 kcal dry diet needs to drink over 7 oz. of water per day. This can be difficult becausecats are not naturally big drinkers. Feeding a canned diet containing 78% moisture virtually guarantees homeostatic control of water balance in the cat.

In addition to canned food ensuring adequate hydration, a high water turnover helps eliminate crystallogenic substances before they grow to sufficient size to interfere with normal urinary function. This is a very important consideration for male cats. Cats that cannot urinate for more than 24 hours due to urinary tract obstruction can die from acute renal failure and/or severe damage to the urinary bladder. In addition to the removal of crystals, benefits of increased water intake include dilution of any noxious substances in urine, and more frequent urination to decrease bladder contact time with urine that may reduce the risks of urinary tract disease. For that reason, canned diets are usually prescribed as the first-line therapy for feline lower urinary tract disease.

The domestic cat is a carnivorous mammal. Compared with an omnivorous or herbivorous diet, a carnivorous diet has the effect of increasing net acid excretion and decreasing urine pH naturally. This urine-acidifying effect is primarily a result of the high level of sulfur-containing amino acids found in meats. Oxidation of these amino acids results in the excretion of sulfate in the urine and a concomitant natural decrease in urine pH. In addition, a diet that contains a high proportion of meat is lower in potassium salts than a diet containing high levels of cereal grains, which have been shown to produce an alkaline urine when metabolized. Therefore the inclusion of high levels of cereal grains commonly found in high-carbohydrate (>35%) dry cat foods has been shown to be a contributing factor in the development of struvite urolithiasis by producing an alkaline urine.

The "solution" to "correct" the alkaline urine-struvite dilemma was the advent of acidified, magnesium-restricted "urinary tract health" diets. Even though the maintenance of a urine pH of 6.4 or lower helps prevent the formation of struvite crystals, the production of urine that is too acidic can be detrimental to a cat's health. If more acid is consumed than an animal is capable of excreting, metabolic acidosis occurs. Several studies have shown that when some cats are fed an acidifying diet for several months, they develop metabolic acidosis, decreased levels of serum potassium, and depletion of body potassium stores. Other studies indicate that the long-term feeding of highly acidifying diets containing marginal levels of potassium cause hypokalemia and kidney disease in some cats. For example, three out of nine cats fed an acidifying diet containing 40% protein and marginal levels of potassium developed chronic renal failure within 2 years. (J Am Vet Med Assoc 1993 Mar 1;202(5):744-51) Feeding a diet that contains ingredients that naturally promote moderate urine acidification (sulfur-containing amino acids, phospholipids, and phosphoproteins, e.g., meat and animal fat - a carnivorous diet) present less risk for overacidification than does supplementing a cat's diet with a urine-acidifying agent. The alkalizing nature of carbohydrate-laden dry food requires more than twice to three times the amount of acidification than does canned food increasing the risk of acidosis and kidney damage proportionally.

Another effect of an acidified urine may be to promote the formation of another type of urolith. Although struvite is soluble in an acid urine, an acid pH may increase the likelihood of calcium oxalate formation. The prolonged feeding of a highly acidified diet leads to a loss of calcium in the urine, making this mineral available for the formation of calcium-containing uroliths. In addition, feeding a low-magnesium diet can exacerbate this problem because urine magnesium inhibits calcium oxalate formation. The incidence of calcium oxalate urolithiasis in cats has increased while struvite urolithiasis has decreased during the past several years. It is theorized that the widespread feeding of acidifying diets that contain low levels of magnesium may be a contributing factor to this trend.

Early studies reported that more than 95% of uroliths in cats were composed of struvite. However, the incidence of this type of urolith has changed significantly within the last 10 years since the advent of acidified, reduced-magnesium, "urinary tract health" diets. A study conducted in 1981 found that 78% of feline uroliths analyzed at the Minnesota Urolith Center were composed of struvite and only 1% of calcium oxalate. By the mid 90s, the incidence of struvite urolithiasis decreased to 43% of the cases, while the incidence of calcium oxalate urolithiasis increased to 43%. Struvite crystals and uroliths can be medically dissolved whereas calcium oxalate cannot be medically dissolved requiring surgical removal in many cases.

Acidification of the urine is not without potential toxicity. Dl-methionine causes hemolytic anemia, met hemoglobinemia, and Heinz body formation in cats. Dl-methionine is commonly used in dry foods as a urinary acidifier. Additional concerns about chronic acidification are its potentially detrimental effects on renal function and bone development. Dietary potassium content also may be important because chronic metabolic acidosis can cause potassium depletion which can contribute to renal dysfunction. A syndrome of hypokalemic nephropathy occurs in cats fed an acidifying diet low in potassium. Feeding a diet that has marginal amounts of potassium and that also contains excessive acidifying chemicals (e.g., dl methionine) may cause chronic metabolic acidosis and depletion of body potassium stores. Potassium depletion and hypokalemia may lead to renal dysfunction characterized by chronic tubulointerstitial nephritis (Chronic Renal Failure) and increased urinary fractional excretion of potassium, further aggravating potassium depletion.

Dry cat food is generally unpalatable to cats because of its dry nature. Palatability of dry cat foods is enhanced by animal fats, protein hydrolysates, meat extracts, acid, and the amino acids alanine, histidine, proline, and lysine. The preference for protein breakdown products and acidity may explain the use of "digest" as an ingredient in nearly all dry foods. Digest is "a microbiologically stable material resulting from digesting animal tissues. . . ." It is produced by enzymatic hydrolysis of animal tissues and by-products, which yield a viscous solution of amino acids, peptides, and fatty acids. Digest also contains significant quantities of phosphoric acid, which is added to stop the enzymatic degradation process and to preserve the product. Digest is sprayed onto the outside of cat foods at 4 to 10 percent of the final finished product or is incorporated directly into the food. Digest can enhance the palatability of foods by as much as two- to threefold over the uncoated product. Once incorporated into cat food, the phosphoric acid increases the amount of acid ingested by the cat. Because of this manufacturing practice, urine acidifiers should not be given to cats fed commercial cat foods, however, nearly all dry foods contain urine acidifiers, most commonly, dl-methionine. Chronic, overacidification leads to metabolic acidosis, demineralization of bone, calcium oxalate crystal formation and possibly renal damage. This may be a contributing factor in increase of incidence of calcium oxalate urolithiasis and high prevelence of chronic renal failure in middleaged cats.

The only benefit of feeding dry food is the marginal dental benefit. However, as is typical of carnivores, the teeth of the cat are appropriately modified for grasping, puncturing, and tearing (cutting), rather than for true mastication. With the exception of "crunching" dry food, cats do little, if any, actual chewing. The hinging of the lower jaw can only be moved up and down and possesses no ability for a lateral chewing motion.

The cat has no first premolars and no lower (inferior) first or second premolars; the molars consist of a single upper and lower tooth on each side. When the mouth is closed, the upper sectorial tooth slides across the vestibular surface of the lower sectorial tooth, producing an effective scissor-like cutting action, rather than a chewing action. Thus the dental benefits of feeding dry food are grossly overrated.

It has long been felt that feeding a cat or a dog a dry kibble diet is better for the teeth than feeding them a canned diet. The logic goes that dry food leaves less residue in the mouth for oral bacteria to feed on and so plaque would accumulate at a slower rate. Despite that, many animals fed on commercial dry diets still have heavy plaque and calculus accumulations and periodontal disease. This is because most dry pet foods are hard but brittle so that the kibble shatters without much resistance and so there is little or no abrasive effect from chewing. A small portion of dry food (no more than 25%) or so-called "tarter reducing" treats (no more than 10% of the cat's total daily caloric requirement) probably have the same slight dental benefit as an all-dry diet without the accompanying risks and adverse effects.

References

  1. Max's House