Guide in Dog Food
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 rations, the following numbers may be used:
In dogs these values are considered estimates for predicting the metabolizable energy of a diet.
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.
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
Relative to an adult dog, a growing puppy requires as much as two to four times more energy per pound of body weight. As the puppy 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.
Typically, small breed dogs grow to a mature weight, which is up to 30 times greater than their birth weight. The small breed dog category includes those dogs whose mature body weight is less than 20 pounds. The medium breed dog category includes those mature dogs weighing between 20 and 50 pounds, whereas, the large breed dogs include mature dogs weighing 50 to 100 pounds. Giant breed dogs grow to a mature body weight of greater than 100 pounds.
Large, fast-growing dog breeds require less food per pound of body weight than small breeds. To relate energy needs to body size, energy standards for dogs are usually established by body weight. Individual animals can very greatly from these standards.
Environment and Activity
Dogs housed outdoors and exposed to extreme weather (both hot and cold temperatures) have changes in their caloric requirements. During hot weather, energy needs decrease and less food may be required. Conversely, during cold weather energy needs increase to maintain body temperature and more food may be required.
During seasons of conditioning and hard work, individual dogs' energy requirements will be increased above that of maintenance. Caloric standards for working dogs were developed using treadmill dogs running the equivalent of 20 miles per day in a 75 degree Fahrenheit environment with relative humidity of approximately 50%. Hardworking dogs require more energy intake per pound of body weight during the periods when they are training or working. Hardworking dogs include hunting dogs during the hunting season, racing dogs, sheepherding dogs or any animal regularly running long distances. When the animal is not training or working, they do not have elevated caloric requirements and a maintenance-type food may be fed. Feeding high-calorie, nutrient dense foods to dogs when they are not training or working could contribute to excessive weight gain unless appropriate adjustments are made in the amount fed.
Because energy is required for all body processes, animals eat to meet their energy needs. As a result, the intake of all nutrients is influenced by the amount of energy present in the diet. The energy content of the diet generally limits the amount of food an animal will consume. Reputable pet food manufacturers take this into account when formulating complete and balanced pet foods.
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.
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. A dog requiring 1000 kcals per day, therefore, would require 1000 ml of water or approximately 1 quart. Some animals would need more than this amount, while others would require less, which is why it is commonly recommended that dogs 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. Thus, a dog consuming 2000 metabolizable kcals of food per day may generate approximately 200 to 250g of water.
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 bloodstream. Amino acids are distributed to all cells of the body where they are utilized to build body proteins.
Over twenty amino acids are involved in the synthesis of proteins in the body. Ten of these are essential for dogs 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. 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. Consequently, they must be provided simultaneously in the proper proportions in a pet's diet. Essential amino acids include:
Protein Requirements of Dogs
A dog's protein requirement depends upon the life stage and activity of the dog. Generally, puppies need more dietary protein than do adult dogs. Caloric requirements are also high during growth phases, and protein needs of a puppy can be met by a high quality protein providing 20 to 25% of dietary calories.
Research has shown that the minimum protein requirement for geriatric dogs is about 50% greater than for younger adult dogs. However, diets formulated for adult maintenance usually provide adequate protein. Research has shown that the healthy geriatric dog utilizes protein in a manner similar to the young adult dog.
Severe protein deficiency in dogs results in poor food intake, growth retardation or weight loss, subnormal concentrations of blood proteins, muscle wasting, emaciation and death. Less severe deficiency can cause a rough, dull haircoat, compromised function of the immune system and poor milk production in reproducing bitches. Animals maintained with inadequate protein reserves may appear healthy, but are most susceptible to stresses, including increased susceptibility to infections as well as the effects of toxic compounds or cancer-causing agents.
During periods of stress, the protein requirement may be increased. For example, a dog food containing more than 1600 kcals per lb. of food and at least 21% protein (dry-type dog food) is recommended for reproduction. Hardworking dogs require a diet with higher levels of calories and fat. Normally, as the calorie level of a diet increases, the protein content is also increased.
Weight-reduction (or low-calorie) diets formulated for sedentary dogs are lower in fat and calories and may have a lower protein level. These diets may also contain a higher percentage of crude fiber. A pet food designed for weight reduction is not appropriate for young, growing puppies, or for pregnant or nursing females.
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 exception 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.
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 dog food which contains 21% protein with 85% digestibility would deliver equal amounts of protein as a diet containing 23% protein with 78% digestibility.
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 diets 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 bloodstream. Dietary fibers are carbohydrates which are not completely digestible.
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 is broken down and absorbed as glucose. Glucose is the normal source of energy used by most cells in the body.
When animals consume diets 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 carbohydrates in the rations of dogs 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 dogs, they provide a readily digested and metabolized source of energy.
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 found in pet foods:
Fiber is the general term used to describe complex carbohydrates which are not digested by enzymes in the small intestine of dogs. 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 protective mechanism, fiber can bind to some toxins and prevent their absorption into the bloodstream.
Excessive dietary fiber is associated with adverse effects such as the production of loose stools, flatulence, increased stool volume and frequency, and decreased dietary caloric density.
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 carbohydrates, 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 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.
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. Puppies have the ability to digest the lactose found in milk, but many adult animals suffer from different degrees of lactase deficiency. Most dogs 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 carbohydrates, 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.
Fat digestion is more complex than that of protein or carbohydrates. Still, healthy dogs and cats can digest fats with great efficiency, approximately 90-95%.
Dogs require linoleic acid only. This essential fatty acid cannot be made in the body and is 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.
Fat serves several functions in the diets of dogs. First, it is a concentrated source of energy. For example, one pound of ground corn contains approximately 1585 calories while one pound of animal fat contains approximately 4100 calories. So adding a little bit of fat adds a lot of calories. Fat also supplies the essential fatty acids required by dogs 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.
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 puppy 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 dog'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 and phosphorus are essential minerals in the diet of dogs and are necessary for normal bone development. These minerals provide rigidity to bones and teeth, aid in normal blood coagulation, aid in controlling passage of fluids to cell walls, and are necessary for nerve excitability. A deficiency of calcium or phosphorus, especially during the first year of a puppy's life, will produce bone weakness and/or serious skeletal deformities like rickets. In addition to calcium and phosphorus, bone contains small amounts of magnesium, sodium, potassium, chloride, fluoride, and trace elements.
Before complete and balanced pet foods were so widely available, rickets was commonly seen in young, growing animals. This bone condition is associated with a calcium, phosphorus, or vitamin D deficiency and results in soft and deformed bones because the bones do not calcify or become hard. These conditions are rarely seen with today's commercial pet foods.
Some pet owners and breeders believe that additional calcium (and possibly other minerals) should be added to the diets of pregnant or nursing females, as well as the diets of growing puppies. They feel that during these life stages a pet needs more of these minerals. It is true that more minerals are needed at these times, but all nutrients are needed in greater amounts during these same life stages. Additional dietary minerals are best obtained through increased consumption of a good quality complete and balanced diet, rather than individual supplements. For example, many puppies are fed a mixture of milk, baby cereal, vitamins, eggs, and/or meat prior to weaning. This type of diet is both expensive and time-consuming to prepare. Most important, it may not be nutritionally complete and balanced, so these animals are much more likely to suffer from a health problem during the period of rapid growth.
Calcium and phosphorus should be included in the diet at a balance or ratio of 1.0 to 2.0 parts calcium to each 1.0 part phosphorus by weight. Larger ratios may be detrimental to bone calcification. For example, when feeding supplements (or pet foods that are not complete and balanced), the amount of dietary phosphorus may exceed the amount of dietary calcium, and bone abnormalities may occur.
Sodium and Chloride
Sodium and chloride serve largely as fluid-regulating minerals to help maintain the balance between fluids inside and outside individual cells of the body. Sodium aids in the transfer of nutrients to cells, the removal of waste material, and the maintenance of water balance among the tissues and organs. Chloride is required for the formation of hydrochloric acid (HCl) in the stomach which helps in the digestion of protein.
A dietary deficiency of sodium and chloride would be extremely rare because most pets today are fed commercial pet foods, but sodium and chloride deficiencies can result from prolonged (or chronic) severe diarrhea and/or vomiting. Animals suffering from diarrhea or vomiting should be examined by a veterinarian. Sodium and chloride consumed in excess of the needs of healthy animals are, for the most part, filtered through the kidneys and excreted into the urine. Toxicity from consumption of excess sodium and chloride would be highly unlikely to occur so long as animals have access to good quality drinking water.
Potassium and Magnesium
Potassium is found in high concentrations within cells and is required for proper enzyme, muscle, and nerve functions, as well as helping to maintain fluid balance throughout the body. Potassium is widely distributed in foodstuffs and occurrence of deficiencies in the diet are not likely when pets are fed complete and balanced pet foods. Like sodium and chloride, potassium deficiency can occur in animals that have chronic diarrhea and/or vomiting or other illness. Symptoms of deficiency in dogs would include poor growth, loss of appetite, weakness, weight loss, and dehydration.
Magnesium is important as a structural component of both muscle and bone, and it plays a key role in many enzymatic reactions throughout the body. Some attributes of magnesium are also common to calcium, potassium, and sodium. Calcium and phosphorus influence magnesium balance, because high amounts of calcium or phosphorus decrease the absorption of magnesium from the intestinal tract.
Magnesium content of pet food depends on the ingredients, and is usually not added in supplemental form. A deficiency of dietary magnesium is not probable for healthy dogs consuming commercial pet foods. Typical diets contain dietary magnesium within the range of 0.05 to 0.2 percent.
Although the bodies of animals contain only about 0.004% iron, it plays a central role in life processes. A small amount of iron (heme) combines with a large protein (globin) to make hemoglobin, the oxygen-carrying compound in red blood cells. Iron is a constituent of many different oxygen carriers and enzymes, but more than half the iron present in the body is in the form of the red blood cell pigment, hemoglobin. Iron is also a component of the enzymes needed for energy utilization.
Iron is absorbed primarily from the small intestine. The absorption of this mineral is rapid. Red blood cells and their hemoglobin are constantly being destroyed and replaced throughout life, especially during growth. In dogs, the average life span of red blood cells is about 110 days.
Anemia is a well-known result of a deficiency of iron. In anemia, the number or size of red blood cells is reduced. Changes in hemoglobin content also can occur. Symptoms of anemia include decreased growth rate, weakness, and increased susceptibility to stress or disease. The causes of anemia can be quite different, but nutritional anemias are unusual in pets fed complete and balanced pet foods. Certain factors can result in nutritional anemia due to iron deficiency during the nursing/suckling period of young puppies. A maternal diet deficient in iron during gestation influences the iron reserve (or stores) of the newborn. Feeding supplemental iron to the lactating or nursing female is not useful, since this treatment does not increase the iron content of the milk.
Iron and copper, along with vitamin B12, are all essential for prevention of anemia in dogs. Most commercial pet foods contain a highly available form of supplemental iron to help meet dietary requirements, so supplementation is not necessary. Too much iron in the diet can interfere with phosphorus absorption by forming an insoluble phosphate.
Zinc is important in the production of proteins and a functional immune system. Some enzyme systems are also dependent upon zinc including enzymes which protect cells from damage caused by oxidation. Zinc is present in natural feedstuffs, largely as zinc-protein complexes. However, its availability and level are such that it normally has to be supplemented in commercial dog foods. Compounds such as phytates, as well as some fibers, are known to decrease the availability of dietary zinc to the animal. Zinc absorption occurs primarily in the small intestine and is relatively inefficient with only 5 to 40% of zinc being absorbed.
Deficiencies of zinc are rarely seen in dogs fed complete and balanced diets. Zinc-responsive canine dermatoses are characterized by scaly skin around the eyes, lips, nails, and foot pads. This syndrome has been reported in dogs, especially puppies of several different breeds. It has been shown that the dermatosis responds dramatically within one to two weeks to therapeutic levels of zinc, but supplemental zinc should be administered only with the advice and consultation of a veterinarian. An adult version of this condition occurs in certain breeds. These animals cannot properly metabolize dietary zinc.
Zinc is considered relatively nontoxic, but the major effect of long-term excess dietary zinc is the decrease in copper absorption and storage. Therefore, copper deficiency and secondary iron deficiency can be induced by long-term excess zinc intake.
Manganese is a metallic element used by industry as an alloy in steel to give it toughness. In nutrition, manganese is an essential element for many animal species. The name, manganese, is derived from Latin for a form of magnetic stone, magnesia.
Manganese occurs in the body principally in the liver, but it is also present in appreciable amounts in the kidney, pancreas, and bone. The lowest concentrations are found in skeletal muscle. Despite the small total supply in the body, this element has several essential functions involving protein and carbohydrate metabolism and reproduction. More specifically, manganese is thought to be an activator of enzyme systems involved in the production of energy, fatty acid synthesis, and amino acid metabolism. The functions of manganese, copper, zinc, and iron may be interchangeable in certain enzyme systems.
In addition to the manganese contributed by typical ingredients in pet foods, manufacturers also add supplemental manganese in a trace mineral mix to their products. Therefore, most good quality, nutritionally complete and balanced pet foods contain adequate levels of manganese. As with magnesium, excess calcium and phosphorus have been known to interfere with the absorption of manganese from the intestinal tract.
Copper absorption is generally greater (60 to 70%) in younger animals than in older animals (10 to 20%). This mineral is absorbed in the stomach and small intestine of dogs, and stored primarily in the liver, kidney, and brain. The availability of natural dietary copper is reduced by phytates, by high levels of ascorbic acid (vitamin C), by increased levels of calcium, zinc, iron and sulfur, and by some toxic metals such as cadmium, silver, or lead.
The importance of copper in the body is quite varied because it is involved in collagen and elastic connective tissue formation, the development and maturation of red blood cells, antioxidant functions, as well as providing pigmentation for hair and wool.
Deficiencies of copper are rarely seen in dogs. A specific problem in copper metabolism of some dog breeds can result in symptoms of copper toxicity. Bedlington terriers, West Highland White terriers, and Doberman pincher dogs have been shown to suffer from this genetic disorder which causes copper to accumulate in the liver. Dogs suspected of having this problem should be examined by a veterinarian.
This trace element is a member of the family which includes both oxygen and sulfur. It was one of the few nutrients to be identified as a toxic substance long before it was found to be an essential nutrient for animals. Although selenium is required in the smallest amount of any of the generally accepted trace elements, it is also the most toxic. Selenium works primarily in conjunction with vitamin E to act as an antioxidant in the body.
High-protein plants such as cereals are a better source of natural selenium than low-protein fruits and vegetables. Meat products in pet foods can also be used to meet the requirement for this trace element. Selenium deficiencies are extremely rare in dogs. Selenium toxicities in dogs are also rare, but could occur if the dietary intake exceeded 2 ppm (parts per million) for prolonged periods of time. Signs of toxicity include hair loss, weakness, soreness, and anemia.
The only known metabolic role of dietary iodine is in the production of thyroid hormones by the thyroid gland. The primary function of these hormones is to regulate and influence basal metabolic rates of the body (for example, how quickly an animal metabolizes or burns up energy after eating a meal). Without the thyroid gland, or the adequate function of these hormones, an animal would exhibit poor growth, hair loss, weight gain, and extreme weakness.
Many natural feedstuffs do not contain sufficient iodine to meet the requirements for dogs. Supplemental forms of inorganic iodine used in commercial pet foods include potassium iodide, potassium iodate, sodium iodide, and calcium iodate.
The scientific recognition of the existence of vitamins at the beginning of the 20th century resulted from the efforts of a number of researchers working independently in several countries. They recognized that diets composed of purified ingredients were not able to support the life of experimental laboratory animals and had the curiosity to find out why. The isolation of vitamins and the definition of their functions in the body, the discovery of the therapeutic value of minute quantities, and the nutrient profile of ingredients with respect to vitamins have profoundly affected animal (and human!) nutrition.
Compared to the other groups of nutrients, vitamins are required in the smallest amounts. And unlike minerals, vitamins are complex substances. Vitamins are classified as either fat-soluble (vitamins A, D, E, K) or water-soluble (B-vitamins and vitamin C). Fat-soluble vitamins depend on the presence of dietary fat and normal fat absorption for their uptake and utilization in the body. Water-soluble vitamins simply depend upon the presence of water for absorption.
Like so many of the other nutrients discussed thus far, vitamins work in concert with other vitamins and nutrients to nourish the animal. This makes it important to provide balanced amounts of vitamins and other nutrients in complete diets. Adding supplements to diets which are already complete and balanced may create imbalances with detrimental effects. To elaborate on this point, eggs contain an excellent source of protein, and normally dogs like them. But repeatedly adding raw eggs to a pet's diet can cause a deficiency of the vitamin biotin. Raw egg whites contain an enzyme which destroys biotin. Symptoms of biotin deficiency include loss of hair and poor growth. Cod liver and wheat germ oils are considered good sources of vitamins D and E. Excessive cod liver oil, however, can supply more vitamin D than an animal requires, and over a long period of time this could result in skeletal disorders or calcification (hardening) of soft tissues. Mineral or vitamin supplements or supplements of any kind are not necessary for healthy animals consuming a complete and balanced pet food. Adding supplements carries the risk of creating an imbalance of the mineral levels already in the pet food.
Vitamin C (Ascorbic Acid)
This vitamin is also a water-soluble vitamin and has a primary metabolic role in the body of all mammals involving the synthesis or production of collagen. While ascorbic acid is essential in the diet of humans, other primates and guinea pigs, dogs have no dietary requirement for this vitamin. Therefore, a deficiency of vitamin C known as "scurvy" is unlikely to occur in dogs. Supplementation of a commercial diet with vitamin C would not be necessary or useful.