A nutrient is a chemical or substance that an animal requires to live and grow. Nutrients can be found in food, synthesized in the body, or taken in from the environment. Nutrients that must be consumed in the diet because the animal cannot produce them or cannot produce them in sufficient quantities are considered essential nutrients. Those the animal can synthesize to meet its needs or does not need from its diet are considered nonessential nutrients.
All animals require six nutrient types: water, proteins, fats, carbohydrates, vitamins, and minerals. Often, energy is confused for a nutrient, but energy itself is not a nutrient. Energy (measured by calories) is provided in the diet by proteins, fats, and carbohydrates.
Water
Water is the most crucial nutrient that many people overlook. Water makes up more than half of the body mass of a dog. An animal’s water requirements change depending on environmental temperature, humidity, and the animal’s activity level, among other factors. A typical dog may consume 
approximately one ounce of water per one pound of body weight. This means that a 35-pound dog consumes at least 35 ounces of water a day. Water must be available at all times for dogs to reach their intake levels. Nursing dogs require an even higher water intake to meet the needs of milk production. To ensure proper hydration, change and provide water daily.
Insufficient water intake can lead to dehydration in dogs. Dehydration can cause serious problems with the gastrointestinal tract and urinary and respiratory systems. Signs of dehydration include dryness of the mouth and loss of skin elasticity. Dryness of the mouth is indicated by dry, tacky gums instead of wet and glistening gums. Loss of skin elasticity can be detected by pulling up the skin along the dog’s back. If the skin stays in a ridge rather than springing back into place, this demonstrates a loss of elasticity due to dehydration. If the dog is properly hydrated, the skin will spring back to into place.
Diets that have a higher water content will decrease the amount of water the dog will drink. Canned foods are made up of 70% to 75% water, so a dog fed a canned food ration will drink less water than a dog consuming a dry diet. Dry diets usually contain 8% to 12% water.
Proteins
Proteins are compounds comprised of amino acids. They make up an animal’s DNA and enzymes. Proteins play a role in most cellular functions. In addition, proteins have an important structural role in the body. They make up muscle, hair, toenails, and skin.
Protein is an essential nutrient for dogs. Protein requirements are defined as the minimum amount of protein to promote optimal performance. A dog’s protein requirements increase during times of growth, pregnancy, or lactation (milk production). Protein needs are lowest for adults at maintenance. Adult dog diets should be composed of 10% to 14% protein on a dry matter basis, whereas diets for growing puppies should be composed of 22% to 25% protein on a dry matter basis.
If the dog consumes more protein than is needed, the extra protein will be metabolized and used for energy. There is only a limited amount of amino acid storage available in the dog’s body. If these amounts are fulfilled, extra protein energy will be used in fat production.
If dogs are fed diets that are too low in protein, they may develop a deficiency. Deficiency symptoms can include stunted growth, weight loss, roughening of the coat, decreased reproductive performance, and decreased milk production. Dogs also have requirements for specific amino acids. If you feed your dog a complete and balanced diet, protein deficiency will be unlikely.
Fats
Fats are organic compounds that are insoluble in water and serve several important functions in the body. Fats supply energy, provide essential fatty acids, carry fat-soluble vitamins, and are an important part of cell membranes. 
Dog diets should be composed of 5% to 15% fat on a dry-matter basis for adult dogs, and 8% to 20% fat for growing puppies. There are a wide range of fat contents in commercial diets because the diets are often formulated for different purposes: growth, lactation, and work. Since fat can add more calories to the diet, it is important that any dog food contains a proper balance of protein and energy for the life state of the animal. If a dog consumes too much fat, obesity will become a problem. Obesity can cause health problems and decrease reproductive performance.
Since dogs cannot insert the omega-3 and omega-6 bonds, they cannot produce linoleic or alpha-linolenic acid, which makes these fatty acids essential for all dogs. Dogs do have a dietary requirement for linoleic acid. This acid is an unsaturated essential fatty acid that is found in corn or soy oil, and commercial dog foods contain it in ample amounts. It is also recommended to give dogs fatty acid supplements when they have dry, flaky, or dull coats.
Carbohydrates
Carbohydrates are the major components of plant material. The varied carbohydrates found in plant material are categorized on the basis of their structure and how they are utilized in the body. Carbohydrate types include sugars, starches, and fiber. Some carbohydrates, such as simple sugars, are easily digested in the mammal’s gastrointestinal tract.
Even though carbohydrates are found in large quantities in dog diets, there is no specific carbohydrate requirement for dogs. Starch is the most common carbohydrate in diets because it is easy for a dog’s digestive system to break down. Common soluble carbohydrates found in large quantities in dog diets include wheat, rice, barley, corn, and oats. These carbohydrates are excellent sources of energy.
Fiber is composed of carbohydrates found in plant materials that are not digestible by mammalian enzymes. Fiber is broken down only in certain organs of the gastrointestinal tract that contain high concentrations of bacteria. Bacteria are able to digest or ferment soluble fiber but are not capable of fermenting the insoluble fiber fraction. Soluble fiber serves as a key energy source for the bacteria and results in increased bacterial cell growth. The breakdown of soluble fiber by the bacteria results in the release of short chain fatty acids (SCFA). These SCFAs are then used by intestinal cells for energy, providing health benefits that lower the risks or symptoms of some diseases, such as colon cancers or gastrointestinal disease. Insoluble fiber, on the other hand, is not able to be fermented and is passed in the stool. Insoluble fiber plays an important role in keeping food moving through the gastrointestinal tract, improving satiety (creating a full feeling after a meal), and preventing constipation. Common fiber sources found in dog foods are grains, corn and corn byproducts, and rice bran.
Vitamins
Dogs, like humans, require a number of different vitamins. These vitamins can be divided into the fat-soluble vitamins (A, D, E, and K) and the water-soluble vitamins (B complex and C). Fat-soluble vitamins are stored in fat storage cells known as lipocytes, and water soluble vitamins
are stored in the body only in minute amounts. Vitamins are required in small amounts, and each one has its own function in the body. Lack of any one vitamin in the diet can cause deficiency symptoms or illness, while excesses of some vitamins can also be toxic.
Meeting the dog’s vitamin needs is relatively easy with a good quality diet. The main source of vitamin A is the yellow pigment (carotene) found in plants. This vitamin is easily digested and stored within fat cells in the liver. Deficiencies in vitamin A can lead to poor low-light vision, poor-quality skin and hair development, reproductive failure, and low growth rates. Vitamin A supplementation is a must, especially for lactating females. Pups receive their first vitamin A supplementation from their mother’s colostrum.
Vitamin D can be obtained through the sun’s ultraviolet radiation. This vitamin helps regulate calcium and phosphorus levels, which help maintain bone formations and muscle control. Vitamin D deficiencies are highly unlikely but can cause bone demineralization known as rickets. Supplementation is more common in puppies than adults, but is usually minimal.
Vitamin E can be found in many plant oils and in meats like liver and fat. It helps in the formation of cell membranes and the metabolism of fats. A deficiency in this vitamin can cause cell damage and death to liver, nerve, and muscle cells. Brown Bowel Syndrome is a condition that is commonly associated with vitamin E deficiency.
The final fat soluble vitamin that dogs require is vitamin K. Since this vitamin can be manufactured in the intestine by bacteria, the vitamin does not need to be found at high levels in a food source. Vitamin K is crucial for normal blood functions and allows the blood to clot. If the animal is deficient, it will begin hemorrhaging in its small intestine and require immediate veterinary care. Since vitamin K is produced within the animal’s intestine, the dietary requirement is unclear. Supplementation for fat soluble vitamins will not be needed if the dog is fed a properly balanced diet.
Water-soluble vitamins are not stored within the body, so excess vitamins are excreted through urine. Vitamin C is required in the diet to help prevent bladder infections and joint pain. Supplementation is only appropriate for fast-growing puppies and lactating females. Vitamin B is crucial in aiding metabolism and cellular functions. Since excesses of vitamin C and B are excreted, toxicity is not a concern. A well-balanced diet provides proper amounts of vitamin C and B.
Choosing the appropriate diet for the life stage of your dog will prevent vitamin imbalances. Consult with your veterinarian before adding any supplemental vitamins to the diet.
Minerals
Animals require about 22 different inorganic elements, known as minerals, in their diet. These may be required in relatively larger amounts (macrominerals) or relatively smaller amounts (microminerals or trace minerals). The macrominerals include calcium, phosphorus, sodium, chlorine, potassium, magnesium, and sulfur. Minerals have different functions in the body. Calcium and phosphorus are particularly important for their role in skeletal structure.
Calcium and phosphorus are important minerals to be aware of when it comes to your dog’s diet. You should consider both the amount of each of these minerals and the ratio of calcium to phosphorus in the diet. The ideal levels of these mineral are affected by the life stage of your dog. During growth and development and pregnancy, dogs need higher levels of calcium and phosphorus to allow for bone development. Adult dogs at maintenance require lower levels of calcium and phosphorus than at other life stages. During maintenance, the ideal ratio is 1.2 to 1.5 to 1. During growth or lactation, dogs will utilize a higher calcium-to-phosphorus ratio. Dogs are able to absorb adequate amounts of these minerals from the diet, and any excess is filtered out by the kidneys and excreted in the urine. If calcium levels are too high for too long, your dog may develop kidney damage and urinary stones.
The ingredients used in commercial dog foods should provide ample amounts of these minerals. There are special commercial diets to meet the requirements for lactating and large breed dogs that may have different calcium-to-phosphorus requirements than what a maintenance dog would need. If you are feeding your dog the correct diet, you need provide no additional calcium or phosphorus.
Jared Mracek and Lisa Karr-Lilienthal, Ph.D. – University of Nebraska-Lincoln
For More Information
For more information about dog nutrition, view the canine nutrition webinars at http://www.extension.org/pages/66407/canine-nutrition-webinars#.UgK2AW2FGAo.