Nutrient requirements in animals

Food is an energy source obtained from various plants, animals, and inorganics that provide living organisms with material to produce and repair tissue. Regardless of the source, food must provide its consumers with a sufficient amount of essential nutrients. A nutrient is any substance that serves as a source of metabolic energy, raw material for growth and repair of tissues, or general maintenance of body functions.

General Nutritional Requirements

Animals differ widely in their specific nutritional needs, depending on the species. Within any given species, those needs may vary according to variations in body size and composition, age, sex, activity, genetic makeup, and reproductive functions. A small animal requires more food for energy per gram of body weight than a larger animal because the metabolic rate per unit of body weight is higher in the smaller animal. Likewise, an animal with a cool body temperature will have fewer energy needs and require less food than an animal with a high body temperature. An egg-producing or pregnant female will require more nutrients than a male. A nursing female usually requires higher amounts of calories and nutrients to create plentiful and beneficial milk for her offspring. For an animal to be in a balanced nutritional state, it must consume food that will supply enough energy to supply power to all body processes, sufficient protein and amino acids to maintain a positive nitrogen balance and avoid a net loss of body protein, enough water and minerals to compensate for losses or incorporation, and those essential vitamins that are not synthesized within the body.

Activities such as walking, swimming, digesting food, or any other activity performed by an animal require fuel in the form of chemical energy. Adenosine triphosphate (ATP), the body’s energy currency, is produced by the cellular oxidation of small molecules, such as sugars obtained from food. Cells usually metabolize carbohydrates or fats as fuel sources; however, when these carbon sources are in short supply, cells will utilize proteins. The energy content of food is usually measured in kilocalories, and it should be noted that the term “calories” listed on food labels is actually kilocalories (1 kilocalorie = 1000 calories). Cellular metabolism must continually produce energy to maintain the processes required for an animal to remain alive. Processes such as the circulation of blood, breathing, removing waste products from the blood, and in birds and mammals, maintaining body temperature all require energy. The calories required to fuel these essential processes for a given amount of time in an animal at rest is called the basal metabolic rate (BMR). For a resting human adult, the BMR averages from thirteen hundred to eighteen hundred kilocalories per day. As physical activity increases, the BMR increases.

Energy balance requires that the number of calories consumed for body maintenance and repair and for work (metabolic and otherwise), plus the production of body heat in birds and mammals, be equal to the caloric intake over a period of time. An insufficient intake of calories can be temporarily balanced by utilizing storage fats, carbohydrates, or even protein and will result in a loss of body weight. On the other hand, an excessive intake of calories can lead to the storage of energy sources. Animals normally store glycogen, but when the glycogen stores are full, food molecules, such as carbohydrates and protein, will be converted to fats.

Nutrient Molecules

Proteins are composed of long chains of amino acids and serve several important functions in all living organisms, but they are primarily used as structural components of soft tissues and as enzymes. Proteins can also be utilized as energy sources if they are broken down into amino acids. Animal tissues are composed of about twenty different amino acids. The ability to synthesize amino acids from other carbon sources, such as carbohydrates, varies among species, but few, if any, animal species can synthesize all twenty required amino acids. Those amino acids that cannot be synthesized by an animal, but are required to synthesize essential amino acids, are the so-called essential amino acids and must be included in the diet. Humans, for example, require nine essential amino acids. Both plant and animal tissues can serve as protein sources, but animal protein generally contains larger quantities of essential amino acids.

Carbohydrates are primarily used as immediate sources of chemical energy, but they can also be converted to metabolic intermediates or fats. Some carbohydrates are also structural components of larger molecules. For example, the nucleic acids deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) contain the sugars deoxyribose and ribose, respectively, as an integral component of their structure. Most animals can also convert proteins and fats into carbohydrates. The principal sources of carbohydrates are the sugars, starches, and cellulose in plants and the glycogen stored in animal tissue.

Lipids are an important and essential component of all biological membranes. In addition, several animal hormones, such as sex hormones, are lipoidal in nature. Fats and lipids are also especially suitable as concentrated energy reserves because each gram of fat supplies twice as much energy as a gram of carbohydrate or protein and does not have to be dissolved in water. Hence, animals commonly store fat for times of caloric deficit when energy expenditure exceeds energy uptake. Some animals, such as migratory birds and hibernating mammals, store large quantities of fat to offset the times that they are not actively feeding. Lipid molecules include fatty acids, monoglycerides, triglycerides, sterols, and phospholipids.

All animals require an adequate supply of essential inorganic minerals. Carbonate salts of the metals calcium, potassium, sodium, and magnesium and some chloride, sulfate, and phosphate are important constituents of intra- and extracellular fluids. Calcium phosphate is present as hydroxyapatite, a crystalline material that gives hardness and rigidity to the bones of vertebrates and the shells of mollusks. Certain metals, such as copper and iron, are required for oxidation-reduction reactions and for oxygen binding and transport. The catalytic function of many enzymes requires the presence of certain metal atoms. Animals require moderate amounts of some minerals and only trace quantities of others.

Animals require a variety of vitamins—diverse and chemically unrelated organic substances. Vitamins primarily function as coenzymes for the proper catalytic activity of essential enzymes. As with amino acids, the ability to synthesize different vitamins from other carbon sources varies among species. Those essential vitamins that cannot be synthesized by the animal itself must be obtained from other sources, primarily from plants but also from dietary animal flesh or from intestinal microorganisms. Vitamin C (ascorbic acid) can be synthesized by many animals but not by humans. Vitamins K and B₁₂ are produced by intestinal bacteria in humans. Vitamins such as A, D, E, and K are fat soluble and can be stored in fat deposits within the body; however, water-soluble vitamins such as vitamin C are not stored and are excreted through the urine. Hence, the water-soluble vitamins must be consumed or produced continually in order to maintain adequate levels.

Dietary needs vary between species and may change with age, disease, pregnancy, or stress. For example, dogs require ten essential amino acids—methionine, histidine, isoleucine, leucine, arginine, lysine, phenylalanine, threonine, tryptophan, and valine. Cats require these same amino acids but also taurine. Additionally, cat and dog fat requirements may increase with age, during pregnancy, or during development, including alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA).

Although not commonly considered a nutrient, water is tremendously important and comprises up to 95 percent or more of the weight of some animal tissue. Animals lose their water content daily through urine, feces, evaporation from the skin or respiratory tract, sweat, drool, or milk in female mammals feeding their young. Salt and other vital minerals are also lost in these processes. Water is replaced in most animals by drinking fresh water, ingesting food, and, to some extent, by the metabolism of carbohydrates and lipids. Marine animals stay hydrated by drinking saltwater or absorbing it through their skin. They have specialized kidneys and gills, and some have glands in their intestines, eyes, or heads that allow these animals to safely retain the correct amount of salt for their bodies.

Principal Terms

Carbohydrate: an organic molecule containing only carbon, hydrogen, and oxygen in a 1:2:1 ratio; often defined as a simple sugar or any substance yielding a simple sugar upon hydrolysis

Lipid: an organic molecule, such as a fat or oil, composed of carbon, hydrogen, oxygen, and sometimes phosphorus, that is nonpolar and insoluble in water

Mineral: one of the many inorganic elements other than carbon, hydrogen, oxygen, and nitrogen that an organism requires for proper body function

Protein: an organic molecule containing carbon, hydrogen, oxygen, nitrogen, and sulfur and composed of large polypeptides in which over a hundred amino acids are linked together

Vitamin: an organic nutrient that an organism requires in very small amounts and which generally functions as a coenzyme

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