Feline Nutrition, The Basics

Dr. Kari Mundschenk 

Just For Cats Veterinary Hospital

As cat owners, most of us realize that our cat does not have the same nutritional requirements as our dogs or ourselves.  Kitties have evolved to meet the special needs of their environment and it is only in recent history that we have seen kitties become indoor only companions.  This change is good in some aspects because trauma injuries have decreased yet it has created an entirely new area of concern.

What makes the kitty different from other species?  The easiest answer is that they are obligate carnivores (meat-eaters) and have no dietary requirements for carbohydrates.  Their bodies were not designed to utilize carbohydrates and as such, kitties have a smaller stomach and shorter intestinal tract relative to their body size when compared to other species.  This anatomy is why they require small, frequent meals such as prey.  However, they can metabolize carbohydrates for energy.  Cats synthesize glucose only when they have eaten.  In most other species, this process occurs only when fasted.

Cats cannot “turn off” the liver enzymes that break down amino acids (building blocks of protein) for energy. Therefore, when fed a diet low in protein, a cat will utilize body protein (i.e. muscles) to generate energy.  This is called a negative energy balance and the process is catabolism.

Cats lack the liver enzyme, glucokinase, which initiates the break down and utilization of glucose in the body.  They also lack salivary amylase and have only 5% of the pancreatic amylase activity and 10% of intestinal amylase activity of dogs.  Kitties derive less energy per gram of carbohydrate than humans or dogs do.  Their anatomy also limits their ability to use poorly digestible starches and fibers through microbial fermentation seen in the large bowel of omnivores and herbivores.

This does not, however, imply that cats cannot use carbohydrates.  Our experience alone shows us not only that they can use carbohydrates quite efficiently despite a lack of dietary requirement for this energy source, but that they do.  When cats are fed diets high in simple starches and sugars, it is common to see their blood sugar elevate above normal levels.  This is significant especially in homemade diets high in sugar or starches (i.e. rice, pasta, highly processed flour) and when using human liquid diets such as Ensure in cats.  Care must also be taken when using baby food.  Additionally, if there is too much lactose or other sugars in the diet, then bloating, diarrhea and flatulence may result.

Diets should be balanced according to their energy content.  In other words, when a cat eats enough of a diet to meet its energy requirement, it should also meet its protein, vitamin, and mineral requirements, too.  Certain disease conditions alter the requirements for nutrients and so a diet that may be adequately balanced for a healthy patient may not be balanced for a sick patient.

The most common nutritional problem in cats is overeating.  Food available now tastes good.  Cats suffer from boredom that leads to over consumption.  And so, while cats would normally only eat to meet their energy needs, they often now overeat.

Metabolic rate and thus the energy requirement of cats decreases with age.  Stress and changes in activity levels will change energy requirements.  Additionally, there is a 25-33% decrease in energy requirements within 2 months of spaying/neutering.

Dietary energy can be derived from carbohydrates, fat and protein.  Carbohydrates include starches and sugars. Protein is made up of chains of amino acids and is found in muscle as well as vegetable sources.

Energy is measured in kilocalories.  Gross energy (GE) is the total energy released by a food when it is completely burned.  Digestible energy (DE) content of food is the amount of energy that is digested and absorbed by an animal.  Metabolizable energy (ME) is the energy in a food source that is actually used by the animal.

Carbohydrates contain 3kcal/gm, protein contains 3.9kcal/gm, and fat contains 7.7kcal/gm when metabolized by the cat (ME).

The remainder of the article will focus on the different aspects of a diet.

PROTEIN:

Dietary protein is required to supply specific essential amino acids that cannot be synthesized in sufficient quantities by tissues in the cat's body to allow for optimal growth and performance.  In addition, protein in the diet provides the nitrogen building blocks to allow the body to make or synthesize the non-essential amino acids and other nitrogen-base compounds found in the body and are used for energy.

Cats have a relatively higher protein requirement than other species.  Essential amino acids include:  Arginine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylolonine, Threonine, Tryptophan, Valine, and Taurine.  Cat foods are primarily designed to meet the total protein requirements of the cat but rather should be designed to meet the essential amino acid requirements of the cat.

Protein sources are rated by their biologic value.  Biologic value reflects the essential amino acid content and ability of the animal to retain and utilize the amino acids from the source.  The highest value is 100% and egg protein is the standard.  As a rule, proteins of animal origin have a higher biological value than those of plant origin.

All proteins are not created equal.  Many factors affect the availability of amino acids from a protein source including the digestibility of the source, amino acid balance, processing and storing losses, the energy density and palatability of the diet and the health, functional demands and environment of the animal eating the diet.  If a diet contains protein of high biologic value that is highly digestible, then the total protein of the diet could actually be less than the recommended levels. 

Notable Deficiencies:

• Arginine:  high blood ammonia levels, neurologic signs
• Taurine:  central retinal degeneration, blindness, reproductive failure, abnormal growth in kittens, dilated cardiomyopathy.  Canned foods need to contain higher levels than dry.  Low potassium diets that are acidified need higher levels of Taurine.

VITAMINS

Vitamins are micronutrients that act as cofactors for specific enzymatic reactions in the body although several vitamins behave as hormones (Vitamin D).  Vitamins are either fat or water soluble.  The fat soluble vitamins can be stored in the body while the water soluble ones are less easily stored.  This makes it important to meet the daily requirements of water soluble vitamins while daily intake of fat soluble vitamins is less important.  Because fat soluble vitamins can be stored, excess intake can lead to toxicity.  Actual requirements for cats for many of the vitamins have not been established.

Fat Soluble Vitamins: 

• Vitamin A:  Cats cannot convert beta-carotene from plants to active vitamin A.  Vitamin A is only found in animal tissues.  Important for vision, normal growth of epithelial cells, and remodeling of bone.  Toxicity: deforming cervical spondylosis, GI signs, lameness.
• Vitamin D:  Necessary for bone growth in kittens.  Important for intestinal absorption of calcium and growth and development of bone.  Deficiency: rickets; Toxicity: Hypercalcemia, dystrophic calcification.
• Vitamin E:  An anti-oxidant.  Stabilizes membranes.  Used as a natural antioxidant in pet foods.  Deficiency causes painful inflammation of the fat in the body (steatitis), muscle dystrophy, and immune suppression.  Toxicity: in high doses, can decrease clotting factors and lead to bleeding.
• Vitamin K1:  Cats obtain most if not all of their daily requirement from bacterial synthesis in the intestine.  Deficiency: blood coagulation disorders.  Toxicity: hypersensitivity reaction, Heinz body anemia to plant (K1) and synthetic (K3) sources.

Water soluble Vitamins: 

• Vitamin B1 (Thiamin):  inactivated by heat and enzymes in raw viscera of some species of fish, deficiency causes anorexia, neurologic signs and possibly cardiac signs before death.
• Vitamin B2 (riboflavin):  inactivated by light; deficiency causes anorexia, weight loss, poor coat, and cataracts in cats.
• Niacin:  deficiency causes weight loss, anorexia, and ulceration of the mouth.
• Pantothenic Acid:  abundant in most food sources; deficiency causes anorexia, poor growth and fatty liver.
• Pyridoxine:  Deficiency includes increased excretion of oxalates in the urine.
• Folic acid:  Deficiency is only seen when cats are fed a folic acid-free diet and given antibiotics (ie sulfas).  May also be seen in certain IBD cases.
• Biotin:  destroyed by an enzyme found in raw egg whites.
• Vitamin B12 (Cobalamin):  absorption occurs in the distal small intestine (ileum) and requires a protein for absorption called intrinsic factor, synthesized in the stomach and pancreas of cats.  Deficiency seen in IBD involving the distal SI.
• Choline:  Deficiency includes anorexia, poor growth, fatty liver.
• Vitamin C:  no dietary requirement for cats.

MINERALS: 

• Calcium:  excess interferes with absorption of zinc, copper, iron and phosphorus, causes skeletal deformities.  Deficiency causes tetany and secondary hyperparathyroidism.
• Phosphorus:  excess causes calcium deficiency, rickets, bone density loss and possible kidney toxicity.
• Magnesium:  deficiency causes poor growth and skeletal abnormalities.  Excess can predispose to struvite crystal formation in cat’s urine especially if their urine is alkaline.
• Potassium:  deficiency causes muscle weakness and polymyopathy; excess does not occur from dietary intake in normal animals.
• Sodium:  deficiency causes fatigue. Excess causes neurological signs and is seen only when inadequate water is available.  Diets high in sodium chloride may accelerate the progression of chronic renal failure. Dietary supplementation is sometimes advocated as a means of increasing thirst but does not do so. 

Trace mineral deficiencies of interest: 

• Iron:  anemia, weakness, and fatigue
• Copper:  anemia, rusty coat color
• Manganese:  poor fertility, partial albinism in Siamese cats
• Zinc:  anorexia, poor growth, colorless hair, exuberant skin proliferation, testicular atrophy
• Selenium:  degeneration of skeletal and cardiac muscle
• Cobalt:  anemia, poor growth
• Iodine:  possible connection to hyperthyroidism
• Vanadium:  bypasses insulin receptors on cells allowing glucose to enter the cells and turns on glucose metabolism

FIBER:

Fiber is a generic term including indigestible carbohydrates and lignin in food.  Sources include beet pulp, guar gum, pectins, peanut hulls, and sawdust or cellulose.  Fiber is not a nutrient.

Addition of fiber to a diet will decrease the digestibility and absorption of most nutrients.  Manufacturers have increased the nutrient levels in fiber-added diets but there is no data on whether the nutrients are available to the cat.  Caution should be used when adding fiber (i.e. Metamucil, bran, canned pumpkin) to any diet.

Dietary fiber binds intestinal water, provides intestinal bulk, speeds intestinal transit time, and stimulates the defecation reflex.  Fiber is defined as soluble and insoluble.  Soluble fibers have a greater capacity to bind water in the intestinal tract than insoluble fiber.  Intestinal bacteria ferment soluble fiber producing butyrate, a short chain fatty acid that the colon lining (mucosal) cells use for energy.

When high fiber diets may be used: 

• Constipation
• Inflammatory Bowel Disease
• Diarrhea
• Hyperlipidemia
• Diabetes Mellitus
• Weight Loss

Next month, we will begin to look at specific requirements of certain diseases.  I wish to tahnk my colleagues in the AAFP for assisting with the composition of this subject.