Nutrition is the study of which compounds are required to be ingested by a given animal and how the animal breaks down and uses those compounds. Nutrients are compounds taken in by animals to provide necessary energy and the building blocks that allow tissue growth and repair. Nutrient requirements vary between species and may vary between individual animals within a species, especially as they progress through different life stages (e.g., growth, pregnancy, old age). Much remains unknown about canine nutrition, and most research is performed by large pet food companies. Most of the research to date has worked to determine minimally acceptable levels of various nutrients, which is not the same as knowing the optimal amounts of given nutrients; it may well be that future research will allow us to refine how we feed dogs so as to maximize the effect of nutrition on health and performance. The information presented here reflects what has been reported in the veterinary literature.
A. PHYSIOLOGY OF NUTRITION
What happens when an animal eats food? The food is broken down by the teeth and by enzymes in the saliva. Further physical breakdown of the food happens as it passes through the muscular activity of the stomach and into the intestinal tract. Other substances added to the food as it passes through the intestinal tract include acids, bicarbonate, bile, and water. By the time the digested food leaves the stomach and enters the small intestine, it is the consistency of watery mush. Enzymes are proteins secreted throughout the body that facilitate specific biological processes. In the mouth and intestinal tract, enzymes break down complex nutrients into smaller components that can then be absorbed through the intestinal wall directly into cells or into the bloodstream. Many of the nutrients absorbed into the bloodstream pass to the liver, where they are further processed. Any portion of the food that cannot be broken down into a usable nutrient passes into the large intestine, where it is excreted as feces. Some components of nutrients are passed into the urinary bladder and excreted as urine. Animals are required to ingest some compounds and can make others in their body. For example, glucose is a simple sugar that is vital to the function of brain cells and most other cells in the body. Animals can ingest sugars in the diet; make glucose by breaking down a more complex sugar or other carbohydrate; make glucose from its building blocks of carbon, hydrogen, and oxygen (a process called gluconeogenesis, which occurs in the liver); or make glucose from a storage product called glycogen (a process called glycogenolysis). Vitamin C is an example of a compound that must be ingested by one species (humans) but can be made in the body by other species (dogs). Compounds that must be ingested and cannot be synthesized in the body are called essential nutrients.
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.
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.