Digestion and absorption of nutrients actually beginning in the mouth, with the mastication (chewing) of food and its mixture with saliva. Digestion continues through the gastrointestinal system and ends with the excretion of waste products and undigested food in the feces.
For an animal to survive, water is the single most important nutrient for the body. Water within the cells is necessary for most metabolic processes and chemical reactions, is important for temperature regulation, and is an essential component of normal digestion. Pets obtain water from food, metabolic water, and obviously by drinking water. If the water content of food is increased or decreased, most pets are naturally able to achieve water balance by increasing or decreasing their intake of drinking water.
Because carbohydrates are an excellent energy source for the body, they should be provided adequately in the diet so that protein will not be used for energy and can instead be used for tissue repair and growth.
Fat provides the most concentrated form of energy of all the nutrients, which is a source of essential fatty acids (EFAs), and allows the absorption of fat-soluble vitamins. Fat also contributes to the palatability and texture of food. As the fat content increases, so does the energy density of the diet.
Protein are the major structural components of hair, feathers, skin, nails, tendons, ligaments, and cartilage. Enzymes essential for nutrient digestion are proteins, as are many hormones, such as insulin and glucagon. Blood proteins, such as hemoglobin, act as important carrier substances. The antibodies that enable the body to resist disease are composed of large protein molecules. Because the protein in the body is in a constant state of flux, a regular intake of dietary protein is necessary to maintain normal metabolic processes and provide for tissue maintenance and growth. Like fat, protein content contributates to the palatability and acceptability of food.
Most vitamins can not be synthesized by the body and must be supplied in food. Well-balanced pet foods are formulated to provide the necessary supplementation. Vitamin C, however, is one vitamin that can be synthesized from glucose by dogs and cats: in contrast, humans must receive vitamin C from dietary sources.
Minerals are inorganic elements that make up only about 4% of an animal's total body weight; nonetheless, the essential minerals must be present to sustain life. Moderately fermentable fiber sources (as opposed to highly fermentable and nonfermentable fiber sources) that are source of bulk and provide adequate levels of short-chain fatty acids (SCFAs) in the large intestine are the best fiber sources for cats and dogs. These sources contribute to the health of the large intestine.