40.2 ANIMAL NUTRITION AND DIET

Running, jumping, moving, growing, reproducing, and all other activities performed by animals require energy. Autotrophs like plants and certain microbes are able to capture energy from the sun or from inorganic compounds (Chapters 8 and 26), but animals must acquire food in the form of plants or other animals to support their growth, reproduction, and other activities. Food does not consist just of carbohydrates, fats, and proteins, however. To enable various cell processes, animals must also obtain in their diet certain minerals and chemical compounds that they cannot synthesize on their own. In this section, we explore what animals must obtain from their food, and how much of it, in order to sustain their cells and tissues.

40.2.1 Energy balance is a form of homeostasis.

Like core body temperature (Fig. 40.6), blood-glucose levels (Chapter 37), and blood pressure (Chapter 39), the energy balance of an organism is often maintained at a constant level. An animal in energy balance takes in the same amount of calories of energy from food that it uses over time to meet its metabolic needs. Energy balance can be thought of as a form of homeostasis. We consider sources of energy, or energy intake, and ways in which energy is expended, or energy use.

For animals, the source of energy is the diet. In turn, energy is used to do work (maintain tissues, grow, move about, and the like). Energy required for basic life processes accounts for the majority (about 70%) of energy use. The amount of energy used also depends on the level of physical activity, with higher levels of activity using more energy, as we saw. Finally, digestion and absorption of food from the diet itself requires energy.

When energy intake does not equal energy used, there is an energy imbalance. If an animal eats more food than it requires, energy stores such as fat deposits grow over time. The result is that the body shifts its metabolism mostly to anabolic processes that build energy stores. Many animals achieve a net positive energy intake during the late summer and fall when food is plentiful, before it becomes scarce in winter. Other animals maintain a constant energy intake throughout the year by migrating to areas with more abundant food and avoiding colder temperatures that require increased energy expenditure to remain active and warm. Still other animals hibernate, or become less active, to conserve their energy use over the winter when food is scarce or unavailable.

Animals that cannot acquire enough food are in negative energy balance and become undernourished. During prolonged periods of an inadequate food supply or starvation, an animal consumes its own internal fuel reserves. Starvation forces animals to deplete their glycogen and fat reserves first, and then, if no food is found, to resort to protein stores, primarily in muscle tissue. This ultimately leads to muscle wasting. Undernourishment and starvation are particularly serious human health problems in many developing countries, especially those ravaged by war and internal strife.

Humans, like most other animals, store excess food calories as fat. This is because over much of our evolutionary history food was less abundant and was more unpredictable in its availability than it is today. With the development of agriculture and domestication of livestock, food supplies rapidly increased, allowing many human populations to grow and consume increasing amounts of food. With the rise of mechanized agricultural food production and highly processed foods developed by the modern food industry, excessive intake of food calories has led to an increasing and now critical public health problem: obesity.

Obesity is now an epidemic in many industrialized nations, including the United States, where about 36% of the adult population is considered obese. Obesity is a major public health concern because it increases the risk of diabetes, heart disease, and stroke and contributes to a shorter life-span. For most animals, however, acquiring food and storing its products efficiently in the body allow them to have a fuel reserve to meet seasonal energy requirements. This rationing of stored energy remains an essential part of their metabolic and digestive physiology.

40.2.2 An animal’s diet must supply nutrients that it cannot synthesize.

Animals’ metabolic pathways enable them to obtain energy from the environment as well as to synthesize many of the compounds needed to sustain life. However, many nutrients necessary for life cannot be synthesized by an animal’s metabolism and therefore must be acquired in the food that they eat.

Recall that amino acids are the basic units of proteins (Chapter 4). There are 20 different amino acids that are typically found in proteins (see Fig. 4.2). Although all of them are necessary for life, an essential amino acid is formally defined as one that cannot be synthesized by cellular biochemical pathways and instead must be ingested. Most animals can synthesize about half of their amino acids. Humans are unable to synthesize 8 of the 20 amino acids (Table 40.1). We have to obtain these eight essential amino acids in our diets. Infants require additional amino acids. The most reliable source of all eight essential amino acids is meat. This is not to say that only omnivorous humans can lead healthy lives, though. While most plant proteins lack at least one of the eight amino acids essential to humans, and diets heavy in a particular food, such as corn (which lacks lysine), can lead to protein deficiency, vegetarians can achieve a healthy intake of all essential amino acids by combining plant foods. For example, beans supply the lysine that corn lacks, and corn supplies the methionine that beans lack.

Dietary minerals are chemical elements other than carbon, hydrogen, oxygen, and nitrogen that are required in the diet and must be obtained in the food that an animal eats (Fig. 40.7).

Figure 40.7: Commonly recognized minerals required by humans. Dietary minerals are chemical elements other than carbon, hydrogen, oxygen, and nitrogen that are required in the diet.

They include such elements as calcium, iron, phosphorus, potassium, zinc, and magnesium. Calcium is required for building skeletons (Chapter 37), and iron in hemoglobin binds oxygen and transports it in the blood (Chapter 39). Humans typically obtain the minerals sodium and chloride, which ionize to form salt crystals, through common table salt and other foods. Many wild animals seek exposed rock that they lick to obtain minerals and salts.

In addition to amino acids and minerals, animals must ingest essential vitamins, organic molecules that are required in very small amounts in the diet. Vitamins have diverse roles, some binding to and increasing the activity of particular enzymes, others acting as antioxidants or chemical signals. Different animals have different vitamin requirements, so a molecule that functions as a vitamin for one may not be a vitamin for another. Thirteen essential vitamins have been identified for humans (Table 40.2). Knowing which vitamins are required for an animal’s health is critical to proper medical and veterinary care. Animals have evolved diets that ensure that their need for vitamins, as well as for essential amino acids and minerals, is met through the combination of foods that they eat.

In humans as well as in other animals, vitamin deficiency can have serious consequences. Whereas most mammals can synthesize ascorbic acid (vitamin C), which is necessary for building connective tissue, primates (including humans) cannot. Without ingesting sufficient vitamin C, humans develop scurvy, a disease characterized by bleeding gums, loss of teeth, and slow wound healing. Until it was discovered that green vegetables and fruit supply vitamin C, scurvy was a prevalent disease that could lead to death for sailors who were deprived of fresh produce for long periods of time while at sea.

Deficiencies of vitamins B₁, B₆, and B₁₂ can cause nervous system disorders and various forms of anemia (evidenced by low levels of red blood cells or hemoglobin in the blood). Vitamin D is essential for the absorption of calcium in the diet and thus to skeletal growth and health. With adequate exposure to ultraviolet solar radiation, skin cells synthesize enough vitamin D to sustain a growing body. However, fairer-skinned people inhabiting more northern regions of the world produce less vitamin D and therefore require more of it in their diet. A clear role for vitamin E remains uncertain, but its absence is often linked to anemia.