Chapter 41 Animal Nutrition
Overview: The Need to Feed
• All animals eat other organisms—dead or alive, whole or by the piece
• In general, animals fit into one of three dietary categories.
1. Herbivores, such as gorillas, cows, hares, and many snails, eat
mainly autotrophs (plants and algae).
2. Carnivores, such as sharks, hawks, spiders, and snakes, eat
3. Omnivores, such as cockroaches, bears, raccoons, and
humans, consume animal and plant or algal matter.
• Humans evolved as hunters, scavengers, and gatherers.
• While the terms herbivore, carnivore, and omnivore represent the
kinds of food that an animal usually eats, most animals are
opportunistic, eating foods that are outside their main dietary
category when these foods are available.
• For example, cattle and deer, which are herbivores, may
occasionally eat small animals or bird eggs.
• Most carnivores obtain some nutrients from plant materials that
remain in the digestive tract of the prey that they eat.
• All animals consume bacteria along with other types of food.
• For any animal, a nutritionally adequate diet must satisfy three
0. A balanced diet must provide fuel for cellular work.
1. It must supply the organic raw materials needed to construct
2. Essential nutrients that the animal cannot make from raw
materials must be provided in its food. Concept 41.1 Homeostatic mechanisms manage an animal’s energy
• The flow of food energy into and out of an animal can be viewed as a
“budget,” with the production of ATP accounting for the largest
fraction by far of the energy budget of most animals.
• ATP powers basal or resting metabolism, as well as activity
and, in endothermic animals, thermoregulation.
• Nearly all ATP generation is based on the oxidation of organic fuel
molecules—carbohydrates, proteins, and fats—in cellular respiration.
• The monomers of any of these substances can be used as fuel.
• Fats are especially rich in energy, liberating about twice the
energy liberated from an equal amount of carbohydrate or
protein during oxidation.
• When an animal takes in more calories than it needs to produce ATP,
the excess can be used for biosynthesis.
• This biosynthesis can be used to grow in size or for
reproduction, or it can be stored in energy depots.
• In humans, the liver and muscle cells store energy as glycogen,
a polymer made up of many glucose units.
• Glucose is a major fuel molecule for cells, and its
metabolism, regulated by hormone action, is an important
aspect of homeostasis.
• If glycogen stores are full and caloric intake still exceeds
caloric expenditure, the excess is usually stored as fat.
• When fewer calories are taken in than are expended—
perhaps because of sustained heavy exercise or lack of
food—fuel is taken out of storage depots and oxidized.
• The human body expends liver glycogen first and then
draws on muscle glycogen and fat.
• Most healthy people—even if they are not obese—have enough
stored fat to sustain them through several weeks of starvation. • The average human’s energy needs can be fueled by the
oxidation of only 0.3 kg of fat per day.
• Severe problems occur if the energy budget remains out of balance
for long periods.
• If the diet of a person or other animal is chronically deficient in
calories, undernourishment results.
• The stores of glycogen and fat are used up, the body begins
breaking down its own proteins for fuel, muscles begin to
decrease in size, and the brain can become protein-deficient.
• If energy intake remains less than energy expenditure, death
will eventually result, and even if a seriously undernourished
person survives, some damage may be irreversible.
• Because a diet of a single staple such as rice or corn can often
provide sufficient calories, undernourishment is generally common
only where drought, war, or some other crisis has severely disrupted
the food supply.
• Another cause of undernourishment is anorexia nervosa, an eating
disorder associated with a compulsive aversion to body fat.
Obesity is a global health problem.
• Overnourishment, or obesity, the result of excessive food intake, is a
common problem in the United States and other affluent nations.
• The human body tends to store any excess fat molecules
obtained from food instead of using them for fuel.
• In contrast, when we eat an excess of carbohydrates, the
body tends to increase its rate of carbohydrate oxidation.
• Thus, the amount of fat in the diet can have a more direct effect
on weight gain than the amount of dietary carbohydrates.
• While fat hoarding can be a liability today, it probably provided
a fitness advantage for our hunting-and-gathering ancestors,
enabling individuals with genes promoting the storage of high-
energy molecules during feasts to survive the eventual famines. • The World Health Organization now recognizes obesity as a major
global health problem.
• The increased availability of fattening foods in many countries
combines with more sedentary lifestyles to put excess weight
• In the United States, the percentage of obese people has
doubled to 30% over the past 20 years, and another 35% are
• Obesity contributes to health problems, including diabetes, cancer of
the colon and breast, and cardiovascular disease.
• Research on the causes and possible treatments for weight-control
• Over the long term, feedback circuits control the body’s storage
and metabolism of fat.
• Several hormones regulate long-term and short-term appetite
by affecting a “satiety center” in the brain.
• Inheritance is a major factor in obesity.
• Most of the weight-regulating hormones are polypeptides.
• Dozens of genes that code for these hormones have been
• In mammals, a hormone called leptin, produced by adipose cells, is a
key player in a complex feedback mechanism regulating fat storage
• As adipose tissue increases, high leptin levels cue the brain to
depress appetite and to increase energy-consuming muscular
activity and body-heat production.
• Conversely, loss of body fat decreases leptin levels in the
blood, signaling the brain to increase appetite and weight gain.
• Mice that inherit a defective gene for leptin become very obese.
• These mice can be treated by injection with leptin. • However, very few obese people have defective leptin
• In fact, most obese humans have abnormally high leptin
levels, due to their large amounts of adipose tissue.
• For some reason, the brain’s satiety center does not respond to
the high leptin levels in many obese people.
• One hypothesis is that in humans, in contrast to other
mammals, the leptin system functions to stimulate appetite and
prevent weight loss rather than to prevent weight gain.
• Most humans crave fatty foods. Although fat hoarding is a health
liability today, it may have been advantageous in our evolutionary
• Our ancestors on the African savanna were hunter-gatherers who
probably survived mainly on plant materials, occasionally
supplemented by meat.
• Natural selection may have favored those individuals with a
physiology that induced them to gorge on fatty foods on the
rare occasions that they were available.
• Perhaps these individuals were more likely to survive famine.
• Obesity may be beneficial in certain species.
• Small seabirds called petrels fly long distances to find food that
is rich in lipids.
• By bringing lipid-rich food to their chicks, the parents minimize
the weight of food that they must carry.
• However, because these foods are low in protein, young petrels
have to consume more calories than they burn in metabolism—
and consequently they become obese.
• In some petrel species, chicks at the end of the growth period
weigh much more their parents, are too heavy to fly, and need
to starve for several days to fly. • The fat reserves help growing chicks to survive periods when
parents are unable to find food.
Concept 41.2 An animal’s diet must supply carbon skeletons and
• In addition to fuel for ATP production, an animal’s diet must supply all
the raw materials for biosynthesis.
• This requires organic precursors (carbon skeletons) from its
• Given a source of organic carbon (such as sugar) and a source
of organic nitrogen (usually in amino acids from the digestion of
proteins), animals can fabricate a great variety of organic
molecules—carbohydrates, proteins, and lipids.
• Besides fuel and carbon skeletons, an animal’s diet must also supply
• These are materials that must be obtained in preassembled
form because the animal’s cells cannot make them from any
• Some materials are essential for all animals, but others are
needed only by certain species.
• For example, ascorbic acid (vitamin C) is an essential
nutrient for humans and other primates, guinea pigs, and
some birds and snakes, but not for most other animals.
• An animal whose diet is missing one or more essential nutrients is
said to be malnourished.
• For example, many herbivores living where soils and plants are
deficient in phosphorus eat bones to obtain this essential
• Malnutrition is much more common than undernourishment in
human populations, and it is even possible for an
overnourished individual to be malnourished. • There are four classes of essential nutrients: essential amino acids,
essential fatty acids, vitamins, and minerals.
• Animals require 20 amino acids to make proteins.
• Most animals can synthesize half of these if their diet includes
• The remaining essential amino acids must be obtained from food in
• Eight amino acids are essential in the adult human with a ninth,
histidine, being essential for infants.
• The same amino acids are essential for most animals.
• A diet that provides insufficient amounts of one or more essential
amino acids causes a form of malnutrition known as protein
• This is the most common type of malnutrition among humans.
• The victims are usually children, who, if they survive infancy,
are likely to be retarded in physical and perhaps mental
• In one variation of protein malnutrition, called kwashiorkor, the diet
provides enough calories but is severely deficient in protein.
• The protein in animal products, such as meat, eggs, and cheese, are
“complete,” which means that they provide all the essential amino
acids in their proper proportions.
• Most plant proteins are “incomplete,” being deficient in one or more
essential amino acid.
• For example, corn is deficient in the amino acid lysine.
• Individuals who are forced by economic necessity or other
circumstances to obtain nearly all their calories from corn would
show symptoms of protein deficiency.
• This is true from any diet limited to a single plant source,
including rice, wheat, and potatoes. • Protein deficiency from a vegetarian diet can be avoided by eating a
combination of plant foods that complement one another to supply all
essential amino acids.
• For example, beans supply the lysine that is missing in corn,
and corn provides the methionine that is deficient in beans.
• Because the body cannot easily store amino acids, a diet with all
essential amino acids must be eaten each day, or protein synthesis is
• Some animals have special adaptations that get them through
periods where their bodies demand extraordinary amounts of protein.
• For example, penguins use muscle proteins as a source of
amino acids to make new proteins during molting.
• While animals can synthesize most of the fatty acids they need, they
cannot synthesize essential fatty acids.
• These are certain unsaturated fatty acids, including linoleic
acids, which are required by humans.
• Most diets furnish ample quantities of essential fatty acids, and
thus deficiencies are rare.
• Vitamins are organic molecules required in the diet in quantities that
are quite small compared with the relatively large quantities of
essential amino acids and fatty acids animals need.
• While vitamins are required in tiny amounts—from about 0.01
mg to 100 mg per day—depending on the vitamin, vitamin
deficiency (or overdose in some cases) can cause serious
• So far, 13 vitamins essential to humans have been identified.
• These can be grouped into water-soluble vitamins and fat-
soluble vitamins, with extremely diverse physiological functions.
• The water-soluble vitamins include the B complex, which consists of
several compounds that function as coenzymes in key metabolic
processes. • Vitamin C, also water soluble, is required for the production of
• Excessive amounts of water-soluble vitamins are excreted in
urine, and moderate overdoses are probably harmless.
• The fat-soluble vitamins are A, D, E, and K.
• They have a wide variety of functions.
• Vitamin A is incorporated in the visual pigments of the eye.
• Vitamin D aids in calcium absorption and bone formation.
• Vitamin E seems to protect membrane phospholipids from
• Vitamin K is required for blood clotting.
• Excess amounts of fat-soluble vitamins are not excreted but are
deposited in body fat.
• Overconsumption may lead to toxic accumulations of
• The subject of vitamin dosage has aroused heated scientific and
• Some believe that it is sufficient to meet recommended daily
allowances (RDAs), the nutrient intake proposed by nutritionists
to maintain health.
• Others argue that RDAs are set too low for some vitamins, and
a fraction of these people believe, probably mistakenly, that
massive doses of vitamins confer health benefits.
• Debate centers on the optimal doses of vitamins C and E.
• While research is ongoing, all that can be said with any
certainty is that people who eat a balanced diet are not likely to
develop symptoms of vitamin deficiency.
• Minerals are simple inorganic nutrients, usually required in small
amounts—from less than 1 mg to about 2,500 mg per day.
• Mineral requirements vary with animal species. • Humans and other vertebrates require relatively large quantities
of calcium and phosphorus for the construction and
maintenance of bone.
• Calcium is also necessary for the normal functioning of
nerves and muscles.
• Phosphorus is a component of the cytochromes that
function in cellular respiration.
• Iron is a component of the cytochromes that function in cellular
respiration and of hemoglobin, the oxygen-binding protein of
red blood cells.
• Magnesium, iron, zinc, copper, manganese, selenium, and
molybdenum are cofactors built into the structure of certain
• Magnesium, for example, is present in enzymes that split
• Iodine is required for thyroid hormones, which regulate
• Sodium, potassium, and chloride are important in nerve function and
have a major influence on the osmotic balance between cells and the
• Excess consumption of salt (sodium chloride) is harmful.
• The average U.S. citizen eats enough salt to provide about 20
times the required amount of sodium.
• Excess consumption of salt or several other minerals can upset
homeostatic balance and cause toxic side effects.
• For example, too much sodium is associated with high blood
pressure, and excess iron causes liver damage.
Concept 41.3 The main stages of food processing are ingestion,
digestion, absorption, and elimination
• Ingestion, the act of eating, is only the first stage of food processing. • Food is “packaged” in bulk form and contains very complex
arrays of molecules, including large polymers and various
substances that may be difficult to process or even toxic.
• Animals cannot use macromolecules like proteins, fats, and
carbohydrates in the form of starch or other polysaccharides.
• First, polymers are too large to pass through membranes and
enter the cells of the animal.
• Second, the macromolecules that make up an animal are not
identical to those of its food.
• In building their macromolecules, however, all organisms
use common monomers.
• For example, soybeans, fruit flies, and humans all
assemble their proteins from the same 20 amino acids.
• Digestion, the second stage of food processing, is the process of
breaking food down into molecules small enough for the body to
• Digestion cleaves macromolecules into their component
monomers, which the animal then uses to make its own
molecules or as fuel for ATP production.
• Polysaccharides and disaccharides are split into simple sugars.
• Fats are digested to glycerol and fatty acids.
• Proteins are broken down into amino acids.
• Nucleic acids are cleaved into nucleotides.
• Digestion reverses the process that a cell uses to link together
monomers to form macromolecules.
• Rather than removing a molecule of water for each new
covalent bond formed, digestion breaks bonds with the addition
of water via enzymatic hydrolysis.
• A variety of hydrolytic enzymes catalyze the digestion of each
of the classes of macromolecules found in food. • Chemical digestion is usually preceded by mechanical fragmentation
of the food—by chewing, for instance.
• Breaking food into smaller pieces increases the surface area
exposed to digestive juices containing hydrolytic enzymes.
• After the food is digested, the animal’s cells take up small molecules
such as amino acids and simple sugars from the digestive
compartment, a process called absorption.
• During elimination, undigested material passes out of the digestive
Digestion occurs in specialized compartments.
• To avoid digesting their own cells and tissues, most organisms
conduct digestion in specialized compartments.
• The simplest digestive compartments are food vacuoles, organelles
in which hydrolytic enzymes break down food without digesting the
cell’s own cytoplasm, a process termed intracellular digestion.
• This process begins after a cell has engulfed food by phagocytosis or
• Newly formed food vacuoles fuse with lysosomes, which are
organelles containing hydrolytic enzymes.
• Later the vacuole fuses with an anal pore, and its contents are
• In most animals, at least some hydrolysis occurs by extracellular
digestion, the breakdown of food outside cells.
• Extracellular digestion occurs within compartments that are
continuous with the outside of the animal’s body.
• This enables organisms to devour much