Nutrients in Pet Foods

A pet's nutritional health depends on receiving the correct amounts and proportions of nutrients from the six required groups, which are water, protein, fat, carbohydrates, minerals and vitamins. With the exception of water, commercial pet foods which are identified as 100% complete and balanced contain these required nutrients. This is what is meant by complete. These nutrients are also present in the proper proportions; that's what is meant by balanced. As important as these nutrients are, it is equally important for pet owners to provide clean, fresh drinking water for their pets so that all of their nutrient needs are met.


Although energy is not a nutrient, animals have a requirement for energy and meet that requirement by consuming dietary carbohydrates, protein and fats. Energy is measured in calories and a calorie is defined as the amount of heat required to raise the temperature of one gram of water from 14.5 degrees Celsius to 15.5 degrees Celsius. Because this amount of heat is so small, it is common to describe energy requirements and the energy content of foods in kilocalories (1000 calories = 1 kcal). The term calorie is often used to refer to the amount of energy in 1 kilocalorie.

Energy is a prime regulator of food consumption in most species. Energy in the form of calories provides the driving force in metabolic reactions and allows for the utilization of all other nutrients. It also provides heat to maintain normal body temperature. To estimate the energy values of nutrients for dog and cat rations, the following numbers may be used:

carbohydrates 3.5 kcals/gram
protein 3.5 kcals/gram
fat 8.5 kcals/gram

In dogs and cats these values are considered estimates for predicting the metabolizable energy of a product.

Measuring Energy

Animals are not capable of extracting all of the total or gross energy of a food. Because of that, the energy content of a food is measured in several ways.

  • Gross Energy (G.E.) = total energy content of food measured by bomb calorimetry
  • Digestible Energy (D.E.) = G.E. - energy in the feces
  • Metabolizable Energy (M.E.) = D.E. - energy in the urine

The gross energy (G.E.) content of a food is determined by completely burning that food to its ultimate oxidation products: carbon dioxide, water and other gases. The heat given off is considered to be the G.E. of that food.

The digestible energy (D.E.) content of a food is the amount of energy in the food which the animal is able to absorb. It is determined by animal feeding trials in which the G.E. in the food an animal eats is measured along with the G.E. lost in the animal's feces. The difference between these two represents the amount of energy the animal digested and absorbed.

The metabolizable energy (M.E.) content of a food represents the amount of energy in the food which the animal actually utilizes. This is determined by an animal feeding trial in which the G.E. in the food the animal eats, along with the G.E. in the animal's feces and urine, are measured. The difference between G.E. in the food consumed and the G.E. excreted in the feces and urine combined represents the amount of energy available for the animal's use. When the energy content of a pet food is stated on the packaging, it is in terms of M.E.

Influences on Caloric Requirements

Physiologic State

Relative to an adult cat, a growing kitten requires as much as two to four times more energy per pound of body weight. As the kitten approaches adulthood, caloric requirements for maintenance are reduced. For reproducing females, caloric requirements at the end of gestation and during early lactation can be two to four times greater than that of adult maintenance requirements.


All animals depend on water for life processes. Water is found inside and outside cells, and is involved in most biochemical reactions within the body. Water deficits are incompatible with good health. Water is the most important nutrient for survival on a short-term basis, and it is one that is too frequently neglected.

Water is essential in helping regulate body temperature, lubrication of body tissues and as a fluid medium for the blood and lymph systems. Because water is involved in practically every reaction within an animal's body, any large deviation will be associated with adverse effects. An animal's body, therefore, has several systems designed to maintain constant water balance.

Water intake is controlled by thirst, hunger, metabolic activity (work, gestation, lactation, growth), and the environment (humidity and temperature).

Animals obtain water from the water they drink, fluid ingested with food, and water generated from metabolic processes in the body.

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Water is lost in urine, feces, respiration, and to a small extent in flakes of skin, saliva, and nasal secretions. For nursing females, water will also be required for milk production.

An animal's water requirement is determined in large part by the amount of food they consume each day. A general guideline is that animals require 1 ml of water for each kcal of energy. Some animals might need more than this amount, while others would require less, which is why it is commonly recommended that cats have a ready source of fresh, clean water available at all times.

Sources of Water

Animals acquire water mainly by drinking water, the water content of food and as a result of metabolism of carbohydrates, proteins and fats. About 15g of water are produced for each 100 kcal of energy metabolized.

Food and Water Consumption

As food intake increases, an animal's water intake also increases. When the water content of a diet increases, the animal usually drinks less water. Therefore, animals consuming canned diets, which contain approximately 70-75% water, will generally drink less water than animals consuming dry diets, which contain about 8-12% water.


Protein is an essential nutrient and serves numerous functions in the body, including muscle growth, tissue repair, enzymes, blood, immune functions, hormones, and energy. Proteins are defined as groups of amino acids linked to each other in different quantities and sequences. Each protein has a precise combination of amino acids that is specific for that protein, and the arrangement of amino acids determines the specific nature of a protein. Dietary proteins that are digested in the stomach and small intestine are broken down to form free amino acids which are then absorbed into the blood stream. Amino acids are distributed to all cells of the body where they are utilized to build body proteins.

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Over twenty amino acids are involved in the synthesis of proteins in the body. Ten of these are essential for dogs and cats because they cannot be formed fast enough or in sufficient amounts to meet the requirements for growth and maintenance and, therefore, must be supplied in the diet. An additional amino acid, taurine, is required by cats. Nonessential amino acids are those that the body can produce in sufficient amounts from other nutrients and metabolites and, thus, do not need to be supplied in the diet.

Although essential amino acids are not stored as such in the body for any significant period of time, they are constantly metabolized by the dog or cat. Consequently, they must be provided simultaneously in the proper proportions in a pet's diet. Essential amino acids include:

  • arginine
  • histidine
  • isoleucine
  • leucine
  • lysine
  • methionine
  • phenylalanine
  • threonine
  • tryptophan
  • valine
  • taurine (cats)

Protein Requirements of Cats

All animals, as well as humans, need taurine, but the cat is the only known mammal which cannot make enough taurine to meet its metabolic needs. A taurine deficiency in the cat's diet can result in degeneration of the retina, and ultimately, blindness. A taurine shortage can also contribute to cardiomyopathy, a certain type of heart disease, in some cats. Because dogs do not require dietary taurine and dog foods are not fortified with taurine, it is recommended not to feed dog foods to cats or kittens for any length of time. An appropriate level of taurine is contained in high-quality, nutritionally complete and balanced cat foods.

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In addition to their unique requirement for taurine, cats have a higher protein requirement than dogs. The protein content in a complete and balanced dry cat food can vary from 25% to more than 36%.


Proteins are derived from both animal and plant sources. Most proteins contain inadequate amounts of one or more amino acids and are poorly utilized as the sole source for meeting protein needs. The exceptions to this rule are milk and egg proteins. Therefore, it is important to balance out these deficiencies and/or excesses of amino acids when selecting ingredients for use in pet food diets. For example, soybean meal and corn complement each other perfectly, because the amino acids which are deficient in one are present in the other. Neither meat nor soybean meal is an ideal protein, however, either can be adequate if fed in combination with another complementary source of amino acids.

Plant protein sources are completely satisfactory for all phases of a dog's life if they are properly processed and when balanced ratios of amino acids are present. Therefore, the factors of digestibility and amino acid levels determine protein quality.

Protein Digestibility

To evaluate the protein levels of different pet foods, two factors should be considered. One is the level of protein and the other is the protein digestibility, or availability of the protein to an animal, which can be determined only by controlled feeding studies. Two diets may have the same protein level listed on their packages, but the results of animal digestion studies may indicate very different levels of protein digestibility. For example, a food which contains 21% protein with 85% digestibility would deliver equal amounts of protein as a diet containing 23% protein with 78% digestibility.

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In addition to the protein level of a pet food, quality control during processing of pet foods is important. Proteins may be damaged by heat processing, but most reputable pet food manufacturers use proper cooking methods and employ quality control measures to ensure that products are made properly. Because information about protein digestibility cannot be listed on pet food labels, the manufacturer's reputation is important.

Excesses and Deficiencies

In animals fed products containing more protein than is needed, extra protein is metabolized and used for energy. Unlike fat, excessive protein is not stored as such in the body. Once the demand for amino acids is met and protein reserves are filled, protein energy could potentially go to the production of fat.

Protein is an essential nutrient. Animals fed diets too low in dietary protein may develop deficiency symptoms. These symptoms may include a depressed or decreased appetite, poor growth, weight loss, rough and dull haircoat, decreased immune function, lower reproductive performance, and decreased milk production.


Carbohydrates are sugars, starches and dietary fiber. Simple sugars are the smallest sugar molecules and are easily digested and absorbed. By contrast, complex carbohydrates, or starches, are combinations of simple sugars forming long chains which require more digestion before they can be absorbed into the blood stream. Dietary fibers are carbohydrates which are not completely digestible.

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Basically, carbohydrates are supplied in the diet by cereal grains and simple sugars, such as glucose, sucrose (table sugar), and lactose (milk sugar).

The primary site of carbohydrate digestion is in the small intestine, where these complex compounds are broken down to glucose (a simple sugar). Most of the carbohydrates in the foods of dogs and cats is broken down and absorbed as glucose. Glucose is the normal source of energy used by most cells in the body.

When animals consume products containing more carbohydrates than are needed, excess carbohydrate energy is stored in the form of glycogen in the liver and muscles and is converted to fat and stored in adipose tissues. During periods of fasting, stress, or exercise, glycogen is broken down to glucose and delivered to the bloodstream where it is distributed to all body tissues.


The primary function of carbohydrates is to provide energy. Carbohydrates are digested by enzymes in the small intestine or the gut. Most of the carbohydrate in the rations of dogs and cats is broken down and absorbed as glucose or other simple sugars before being used for energy.


Research has shown that pregnant dogs perform better with some carbohydrates in their diet, whereas pregnant dogs fed carbohydrate-free diets had problems whelping and did not deliver strong, healthy puppies. While no specific minimum requirements for carbohydrates have been determined for the diets of cats, they provide a readily digested and metabolized source of energy.

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Carbohydrate Ingredients

Carbohydrates may make up 40 to 55% of dry diets. A large portion of the carbohydrates in pet foods is derived from cereal grains. Cereal grains are usually processed by grinding, flaking or cooking. These processes improve palatability and digestibility.

Here is a list of typical sources of carbohydrates in pet foods:


  • Cereal Grain
  • Milling Products
  • Milk Products
  • Corn
  • Corn Gluten Meal
  • Dried Skim Milk
  • Oats
  • Oatmeal
  • Dried Whey
  • Wheat
  • Wheat Middlings
  • Rice
  • Rice Hulls
  • Barley


Fiber is the general term used to describe complex carbohydrates which are not digested by enzymes in the small intestine of dogs and cats. Some fibers can be partially degraded by normal microflora in the large intestine. The constituents of fiber include at least four major components which are distinctly different chemically: cellulose, hemicellulose, lignin, pectin, and others such as gums, waxes, mucilages, and cutin. These constituents are found in the cell walls of plants. In general, the more of these components there are, the stronger the plant cell walls become (i.e., more fibrous).

Dietary fiber has numerous effects within the gastrointestinal tract. Some fibers swell with water or have a high water-holding capacity (the amount of water that can be taken up by a unit weight of dry fiber to the point at which there is no free water). A high or low water-holding capacity can change the speed with which the diet passes through the intestinal tract. The increased dietary bulk of high-fiber foods contributes to stomach distention and causes an animal to eat fewer calories. Fiber influences the rate of passage of food through the intestine by slowing stomach emptying, but the specific effects vary with the type of fiber, how it is processed, and the amount fed. In general, fiber has a normalizing effect on the rate of passage of food through the intestine, slowing the rate in animals with diarrhea and increasing it in constipation. Dietary fiber also slows or decreases digestion and absorption of nutrients, including fat, vitamins and minerals. As a protection mechanism, fiber can bind to some toxins and prevent their absorption into the bloodstream.

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Excessive dietary fiber is associated with adverse effects such as the production of loose stools, flatulence (gas), increased stool volume and frequency, and decreased dietary caloric density.

Diabetes Mellitus

Diabetes Mellitus is a disease of carbohydrate metabolism resulting from inadequate insulin secretion from the pancreas. The disease is characterized by increased thirst, increased appetite, weakness, weight loss and increased urination. This disease is not the result of eating too much carbohydrate, rather it is caused by too little insulin secretion from the pancreas. Low levels of insulin prevent sugar in the blood from entering muscle and fat cells where it would be used for energy. This leads to an increased use of fat for energy while sugar accumulates in the blood and eventually spills over into the urine. Although the use of fat for energy may seem advantageous, it generates by-products the body can't use or easily get rid of.

Owners who have a diabetic animal or suspect their pet may be suffering from diabetes should consult their veterinarian. Diabetes mellitus is managed by insulin therapy, regulation of food intake and exercise. In general, the insulin dosage is adjusted until glucose disappears from the urine. The dietary energy and protein requirements of diabetic dogs and cats are not different from non-diabetic animals. However, a moderate increase in dietary fiber may be beneficial. Perhaps the most important consideration in feeding diabetic animals is to provide a balanced diet on a regular and constant basis.

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Carbohydrate Malabsorption

Dietary carbohydrates must be reduced to simple sugars by the action of enzymes in the intestine prior to absorption. Carbohydrates that are not completely digested cause (or are associated with) persistent gastrointestinal upset, including gas and/or diarrhea. The most common carbohydrate malabsorption problem is a deficiency of lactase which is the enzyme required to break down lactose, the sugar found in milk. Kittens have the ability to digest the lactose found in milk, but many adult animals suffer from different degrees of lactase deficiency. Most cats can tolerate the small amount of lactose found in pet foods (as skimmed milk or dried whey) but may have difficulty digesting the lactose in a whole bowl of fresh milk.


Fats are concentrated forms of energy. Compared to protein and carbohydrate, fats contain approximately two and a half times the amount of energy per pound. Most dietary fat is made up of triglycerides, which is a group of three fatty acids. Fatty acids can be classified by the length of their carbon chain, by the presence or absence of double bonds, the number of double bonds, the position of those bonds along the carbon chain, and by their melting point.

Fats with no double bond at all are called saturated fats. Fats containing fatty acid chains with a double bond are called unsaturated fats. These may vary from a single double bond in the fatty acid molecule (monounsaturated) to fatty acids with many double bonds (polyunsaturated). Saturated fats are generally solid at room temperature and unsaturated fats are usually liquid.

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Fat digestion is more complex than that of protein or carbohydrates. Still, healthy cats can digest fats with great efficiency, approximately 90-95%.


Cats have a requirement for the essential fatty acids linoleic acid and arachidonic acid, while dogs require linoleic acid only. These essential fatty acids cannot be made in the body and are required in very small amounts in the diet. Fats from both animal and vegetable sources can be used with almost equal efficiency for the production of energy. However, vegetable oils are the most potent sources of essential fatty acids for the dog. Cats have a dietary requirement for arachidonic acid, which by nature is of animal origin (so cats require a small amount of animal fat in their diet).


Fat serves several functions in the diets of dogs and cats. First, it is a concentrated source of energy. For example, one pound of ground corn contains approximately 1,585 calories while one pound of animal fat contains approximately 4,100 calories. So adding a little bit of fat adds a lot of calories. Fat also supplies the essential fatty acids required by dogs and cats for maintaining healthy skin and haircoat and serves as a carrier for fat soluble vitamins. Finally, fat contributes to the palatability of a pet food.

Excesses and Deficiencies

In animals fed diets containing more fat than is needed, extra fat is generally stored in the body within the adipose tissue or fat stores. If enough fat is accumulated over time, animals will become obese. Animals carrying excessive amounts of weight may be at greater risk for complications with surgery, and several disease conditions including some orthopedic diseases, and Diabetes Mellitus.

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Although fatty acid deficiencies are rare, animals fed diets too low in fat may eventually develop deficiency symptoms. Signs would include dry, coarse hair and flaky, dry and thickened skin. A synthetic diet completely devoid of fat would not support a growing kitten for more than a few weeks.


Minerals are relatively simple molecules compared to other nutrients which can be large and complex. Nutritional issues related to minerals include the amount of each in the diet, proper balance of all minerals, and the availability of minerals in the animal's food.

Minerals perform many different functions in the body such as bone and cartilage formation, enzymatic reactions, maintaining fluid balance, transportation of oxygen in the blood, normal muscle and nerve function, and the production of hormones. While the function of some minerals can be separated from that of others, it is impossible to adequately nourish an animal without providing all the minerals in their proper proportions. This is due to the fact that minerals interact in many aspects of body function and maintenance.

Supplementation of any one specific mineral can create imbalances and possibly disrupt an animal's nutritional health. Manufacturers producing good quality pet foods maintain a safety margin for all essential nutrients in the product formulation to compensate for any loss during normal processing and storage and for the variation in the needs of individual animals.

Situations requiring supplementation should be addressed by a veterinarian. Supplementation may sometimes be needed to correct a specific deficiency due to a cat's inability to utilize the normal level of a particular nutrient.

The minerals are usually grouped into macro and micro categories. Macro-minerals are needed in greater amounts in the diet, and found in larger amounts in the body than micro-minerals.


  • Calcium (Ca)
  • Phosphorus (P)
  • Sodium (Na)
  • Chloride (Cl)
  • Potassium (K)
  • Magnesium (Mg)
  • Sulfur (S)


  • Iron (Fe)
  • Zinc (Zn)
  • Copper (Cu)
  • Manganese (Mn)
  • Selenium (Se)
  • Iodide (I)

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