1. Water

Water is the most vital compound taken in by animals. The body is composed primarily of water, and adequate hydration is necessary for normal blood pressure and heart function, kidney function, brain activity, and movement of dissolved compounds throughout the body. During lactation (nursing), water needs increase dramatically to support milk production.

2. Carbohydrates

Carbohydrates are a class of compounds made up of carbon, hydrogen, and oxygen (Figure 1-1). This class includes simple sugars and complex fiber. The body runs primarily on carbohydrates and is constantly making carbohydrates for fuel by gluconeogenesis and glycogenolysis. Because the animal’s body is capable of making carbohydrates and because carbohydrates are abundant in all types of food, no specific amount or type of carbohydrate is required in the diet.

Figure 1-1 Molecular structure of carbohydrates.

3. Fats

Fats are complex compounds. The basic structure of any lipid or fat is the compound glycerol attached to three fatty acid chains (Figure 1-2). Fats provide a greater number of calories when they are broken down than do any other nutrients and often are used as a primary source of energy in diets. Fats are necessary for movement in the body of those substances that dissolve in fat, such as the fat-soluble vitamins A, D, E, and K, and for the formation of fatty substances that enhance body processes, such as development of the myelin sheaths that encircle brain cells.

Figure 1-2 Molecular structure of fats.

The amount of fat required in the diet varies with the life stage of the animal. Because fats are necessary for the growth and development of body systems and are a concentrated source of energy, the amount of fat in the diet often is increased in growth diets and in diets used during pregnancy and lactation. Fats may be decreased in senior diets, in which a lower caloric density is preferred. Fats cannot be restricted entirely because the fats linoleic and alpha-linoleic acid, derived from plants, and arachidonic acid, derived from animal fat, are essential in dogs and because fat increases palatability of diets.

4. Proteins

Proteins make up most body tissues and enzymes responsible for normal physiologic processes. Proteins are made up of amino acids; the order and arrangement of the amino acids are what give specific protein its properties (Figure 1-3). Amino acids are made up of carbon, hydrogen, oxygen, and nitrogen and may also contain sulfur, sugar (glycoprotein), or fat (lipoprotein).

Figure 1-3 Molecular structure of proteins.

Some of the 20 amino acids are essential, and some can be synthesized in the body. For dogs, the 10 essential amino acids are arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Proteins are present in both plants and animals, and all the essential amino acids for dogs are present in plants. Therefore, dogs are not required to eat meat to live. If, however, the owner of a dog desires that animal to eat plant protein only, the diet must be carefully balanced to ensure that all the essential amino acids are present in the plants provided.

In general, animals on a maintenance diet require 13% to 19% protein. Growing and pregnant animals require higher levels of protein, perhaps as high as 25%. Increased levels of protein, when balanced in the diet for calories and other nutrients, are not associated with kidney damage in normal, healthy dogs. Increased amounts of dietary protein may spare muscle tissue in older dogs and may be beneficial in maintaining optimal health during weight loss. Diets specific to life stage are discussed later in this chapter.

5. Vitamins

Vitamins are carbon-based compounds that are required for life but only in very small amounts. Most vitamins bind to minerals and enhance the activity of biological processes in the body, often by altering enzyme function. Vitamins may dissolve in water (water-soluble vitamins: the B vitamin family, vitamin C) or in fat (fat-soluble vitamins: A, D, E, and K). Essential water-soluble vitamins must be ingested daily. Essential fat-soluble vitamins need not be ingested daily because they can be stored in body fat.

Vitamin A is a fat-soluble vitamin intimately associated with vision. Vitamin A also contributes to normal bone development, production of spermatozoa (spermatogenesis), and fetal development. The parent compound, retinol, is derived from beta-carotene. It is an essential nutrient and is found in fish liver oils, animal liver, egg yolk, carrots, and dark green or yellow vegetables.

The B vitamins are a diverse group of water-soluble vitamins that includes thiamin, niacin, riboflavin, and folic acid. B vitamins often are associated with neurologic function and the function of enzyme systems in the liver and other organs. These compounds are readily found in whole cereal grains, organ meats, egg yolk, and fish.

Vitamin C is a water-soluble vitamin that can be synthesized by dogs and therefore is not an essential nutrient. Anecdotal reports of the use of vitamin C for prevention or treatment of hip dysplasia or arthritis are unsubstantiated.

Vitamin D is necessary for calcium regulation and can be synthesized by the body with exposure to ultraviolet light. Compounds in the vitamin D family include cholecalciferol and ergocalciferol.

Vitamin E is a fat-soluble vitamin that acts as an antioxidant, preventing cellular damage from normal metabolism. Dietary requirement varies with the amount of the mineral selenium present.

Vitamin K is necessary for blood clotting. It can be synthesized by bacteria living in the canine gastrointestinal tract and is therefore not an essential nutrient.

6. Minerals

All minerals are essential nutrients, but different minerals are required in different amounts. Minerals required in the diet in large amounts are calcium and phosphorus, both of which must be present in balance to prevent either too low levels of calcium (hypocalcemia), associated with abnormal gait, disorientation, and possible seizures, or too high levels of calcium (hypercalcemia), associated with mineralization of tissues. Most of the calcium and phosphorus in the animal’s body are stored in the bones and teeth and can be drawn from those sources if the animal becomes deficient. Calcium is required for blood clotting and for neurologic and muscular functions. Phosphorus is an essential component of many enzyme systems. The optimal ratio of calcium to phosphorus in the diet varies from 0.8:1 to 1.5:1.

Minerals that are required in only small amounts are called trace minerals (e.g., iron, zinc). These compounds most commonly work with vitamins to effect biological processes. Trace minerals, although essential, are readily present in the diet.

7. Fiber

Fiber is not essential but often is added to diets. Fiber helps prevent constipation by drawing water into the large intestine, and it may help regulate blood sugar levels. In general, higher fiber content decreases the digestibility of carbohydrates, fats, and proteins and increases the volume of feces excreted. Fiber may be added to weight loss diets to make the animal feel “full” while ingesting fewer calories.