Monosaccharides from foods are used for energy, stored as glycogen, or converted to fat.

Some excess glucose is polymerized to form glycogen (glycogenesis) and stored as a glucose reserve in the liver and muscles. Glucose can be rapidly mobilized from glycogen (glycogenolysis) when it is required to supply energy. However, only a certain amount of glycogen can be stored. Excess glucose beyond what is stored as glycogen is usually converted into fat and stored in adipose tissue (fig. 18.2).

Cells also use carbohydrates as starting materials for such vital biochemicals as the 5-carbon sugars ribose and deoxyribose. These sugars are required for the production of the nucleic acids RNA and DNA, and the disac-charide lactose (milk sugar), which is synthesized when the breasts are actively secreting milk.

Many cells can also obtain energy by oxidizing fatty acids. However, some cells, such as neurons, normally depend on a continuous supply of glucose for survival. (Under some conditions, such as prolonged starvation, other fuel sources may become available for neurons.) Even a temporary decrease in the glucose supply may seriously impair nervous system function. Consequently, the body requires a minimum amount of carbohydrate. If adequate carbohydrates are not supplied in foods, the liver may convert some noncarbohydrates, such as amino acids from proteins or glycerol from fats, into glucose—a process called gluconeogenesis. Thus, the requirement for glucose has physiological priority over the need to synthesize certain other substances, such as proteins, from available amino acids.

99 List several common sources of carbohydrates. ^9 In what form are carbohydrates utilized as a cellular fuel? ^9 Explain what happens to excess glucose in the body.

□ Name two uses of carbohydrates other than supplying energy.

An adult's liver stores about 100 grams of glycogen, and muscle tissue stores another 200 grams, providing enough reserve to meet energy demands for about twelve hours when the person is resting. Whether these stores are filled depends on diet. People live on widely varying amounts of carbohydrates, often reflecting economic conditions. In the United States, a typical adult's diet supplies about 50% of total body energy from carbohydrates. In Asian countries where rice comprises much of the diet, carbohydrates contribute even more to the diet.

Carbohydrate Requirements

Because carbohydrates provide the primary source of fuel for cellular processes, the need for carbohydrates varies with individual energy requirements. Therefore, physically active individuals require more carbohydrates than those who are sedentary. The minimal requirement for carbohydrates in the human diet is unknown. It is estimated, however, that an intake of at least 125-175 grams daily is necessary to spare protein (that is, to avoid protein breakdown) and to avoid metabolic disorders resulting from excess fat utilization. An average diet includes 200-300 grams of carbohydrates daily.

99 Why do the daily requirements for carbohydrates vary from person to person?

Q What is the daily minimum requirement for carbohydrates?


Lipids are organic compounds that include fats, oils, and fatlike substances such as phospholipids and cholesterol (see chapter 2, pp. 51-52). They supply energy for cellular processes and help build structures, such as cell membranes. The most common dietary lipids are the fats called triglycerides (tri-glis'er-idz) (see fig. 2.12).

Lipid Sources

Triglycerides are found in plant- and animal-based foods. Saturated fats (which should comprise no more than 10% of the diet) are mainly found in foods of animal origin, such as meat, eggs, milk, and lard, as well as in palm and coconut oil. Unsaturated fats are contained in seeds, nuts, and plant oils.

Cholesterol is abundant in liver and egg yolk and is present in smaller amounts in whole milk, butter, cheese, and meats. Foods of plant origin do not contain cholesterol. This is why a label on a plant-based food claiming that it is "cholesterol-free" states the obvious.

Be wary of claims that a food product is "99% fat-free." This usually refers to percentage by weight—not calories, which is what counts. A creamy concoction that is 99% fat-free may be largely air and water, and therefore in that form, fat comprises very little of it. But when the air is compressed and the water absorbed, as happens in the stomach, the fat percentage may skyrocket.

Fats from foods


Glucose ^

\S Glycerol

V Fatty acids

Lipid Utilization

Foods contain lipids in the form of phospholipids, cholesterol, or, most commonly, fats (triglycerides). A triglyceride consists of a glycerol portion and three fatty acids.

Lipids provide a variety of physiological functions; however, fats are mainly used to supply energy. Gram for gram, fats contain more than twice as much chemical energy as carbohydrates or proteins. This is why people trying to lose weight are advised to minimize fats in their diets.

Before a triglyceride molecule can release energy, it must undergo hydrolysis. Digestion breaks triglycerides down into fatty acids and glycerol. After being absorbed, these products are transported in the lymph to the blood, then on to tissues. As figure 18.3 shows, some of the resulting fatty acid portions can then react to form molecules of acetyl coenzyme A (acetyl CoA) by a series of reactions called beta oxidation, which occurs in the mitochondria.

In the first phase of beta-oxidation, fatty acids are activated. This change requires energy from ATP and a special group of enzymes called thiokinases. Each of these enzymes can act upon a fatty acid that has a particular carbon chain length.

Once fatty acid molecules have been activated, other enzymes called fatty acid oxidases that are located within mitochondria break them down. This phase of the reactions removes successive two-carbon segments of fatty acid chains. The liver converts some of these segments into acetyl coenzyme A molecules. Excess acetyl CoA is converted into compounds called ketone bodies, such as acetone, which later may be changed back to acetyl coenzyme A. In either case, the citric acid cycle can oxidize the acetyl coenzyme A molecules. The glyc-erol portions of the triglyceride molecules can also enter

Glucose ^

\S Glycerol

V Fatty acids

We digest fats from foods into glycerol and fatty acids, which may enter catabolic pathways and be used as energy sources.

We digest fats from foods into glycerol and fatty acids, which may enter catabolic pathways and be used as energy sources.

metabolic pathways leading to the citric acid cycle, or they can be used to synthesize glucose.

When ketone bodies form faster than they can be decomposed, some of them are eliminated through the lungs and kidneys. Consequently, the ketone acetone may cause the breath and urine to develop a fruity odor. This sometimes happens when a person fasts, forcing body cells to metabolize a large amount of fat, and in persons suffering from diabetes mellitus who may also develop a serious imbalance in pH called aci-dosis, which results from an overaccumulation of acetone and other acidic ketone bodies.

Shier-Butler-Lewis: I V. Absorption and I 18. Nutrition and I I © The McGraw-Hill

Human Anatomy and Excretion Metabolism Companies, 2001

Physiology, Ninth Edition



Fatty acids + Glycerol





Fatty acids (except linoleic acid)


Figure 18.4

The liver uses fatty acids to synthesize a variety of lipids.

Glycerol and fatty acid molecules resulting from the hydrolysis of fats can also be changed back into fat molecules by anabolic processes and stored in fat tissue. Additional fat molecules can be synthesized from excess glucose or amino acids (fig. 18.4).

The liver can convert fatty acids from one form to another. However, the liver cannot synthesize certain fatty acids called essential fatty acids. Linoleic acid, for example, is an essential fatty acid that is required for the synthesis of phospholipids, which, in turn, are necessary for constructing cell membranes and myelin sheaths, and for transporting circulating lipids. Linoleic acid can be converted to another essential fatty acid, arachidonic acid, used in the synthesis of prosta-glandins. Good sources of linoleic acid include corn oil, cottonseed oil, and soy oil. Linolenic acid is also an essential fatty acid.

The liver uses free fatty acids to synthesize triglycerides, phospholipids, and lipoproteins that may then be released into the blood. Thus, the liver regulates circulating lipids (see fig. 18.4). It also controls the total amount of cholesterol in the body by synthesizing cholesterol and releasing it into the blood or by removing cholesterol from the blood and excreting it into the bile. The liver uses cholesterol to produce bile salts. Cholesterol is not used as an energy source, but it does provide structural material for cell and organelle membranes, and it furnishes starting materials for the synthesis of certain sex hormones and hormones produced by the adrenal cortex.

Adipose tissue stores excess triglycerides. If the blood lipid concentration drops (in response to fasting, for example), some of these stored triglycerides are hy-drolyzed into free fatty acids and glycerol and then released into the blood.


Lipid Requirements

The lipid content of human diets varies widely. A person who eats mostly burgers, fries, and shakes may consume 50% or more of total daily calories from fat. For a vegetarian, the percentage may be far lower. The American Heart Association advises that the diet not exceed 30% of total daily calories from fat.

The amounts and types of fats required for health are unknown. Since linoleic acid is an essential fatty acid, to prevent deficiency conditions from developing, nutritionists recommend that infants fed formula receive 3% of the energy intake in the form of linoleic acid. A typical adult diet consisting of a variety of foods usually provides adequate fats. Dietary fats must also supply the required amounts of fat-soluble vitamins.

What tastes like fat, feels like fat, but isn't fat? Fake fat, or more formally, a fat substitute. Because it is really a carbohydrate or protein, a fat substitute offers half the calories of fat. The first fake fats, introduced in the early 1990s, were made from egg, milk, or whey proteins, mixed and broken down into tiny particles that impart the texture of fat. A product called Olestra consists of a molecule of the sugar sucrose bound to six or more triglyceride molecules. It passes through the alimentary canal without being absorbed. Olestra cuts calories in snack products such as chips approximately in half. However, Olestra binds fat-soluble nutrients, including vitamins and carotenoids, taking them out of the body, and may cause abdominal cramps and "anal leakage."

U Which foods commonly supply lipids?

^9 Which fatty acid is an essential nutrient?

^9 What is the role of the liver in the utilization of lipids?

□ What is the function of cholesterol?

1 Reconnect to chapter 4, Cellular Respiration, pages 114-121.


Proteins are polymers of amino acids with a wide variety of functions. When dietary proteins are digested, the resulting amino acids are absorbed and transported by the blood to cells. Many of these amino acids are used to form r



Amino acids



Proteins are digested to their constituent amino acids. These amino acids are then linked together, following genetic instructions, to build new proteins. Free amino acids are also used to supply energy under certain conditions.



Proteins are digested to their constituent amino acids. These amino acids are then linked together, following genetic instructions, to build new proteins. Free amino acids are also used to supply energy under certain conditions.

new protein molecules, as specified by DNA base sequences. These new proteins include enzymes that control the rates of metabolic reactions; clotting factors; the keratins of skin and hair; elastin and collagen of connective tissue; plasma proteins that regulate water balance; the muscle components actin and myosin; certain hormones; and antibodies, which protect against infection (fig. 18.5).

Protein molecules may also supply energy. To do this, they must first be broken down into amino acids. The amino acids then undergo deamination, a process that occurs in the liver that removes the nitrogen-containing portions (—NH2 groups) from the amino acids. These —NH2 groups are converted later into a waste called urea. The liver produces urea from amino groups formed by deamination of amino acids. The blood carries urea to the kidneys, where it is excreted in urine.

The remaining deaminated portions of the amino acids are broken down by several pathways. Some of these pathways lead to formation of acetyl coenzyme A, and others more directly lead to steps of the citric acid cycle. As energy is released from the cycle, some of it is captured in molecules of ATP (fig. 18.6).

A few hours after a meal, protein catabolism, through the process of gluconeogenesis (see chapter 13, pp. 530-531), becomes a major source of blood glucose. However, metabolism in most tissues soon shifts away from glucose and toward fat catabolism as a source of ATP. Thus, energy needs are met in a way that spares proteins for tissue building and repair, rather than being broken down and reassembled into carbohydrates to supply energy. Using structural proteins to generate energy causes the tissue-wasting characteristic of starvation.

Protein Sources

Foods rich in proteins include meats, fish, poultry, cheese, nuts, milk, eggs, and cereals. Legumes, including beans and peas, contain lesser amounts of protein.

The human body can synthesize many amino acids (nonessential amino acids). However, eight amino acids the adult body needs (ten required for growing children) cannot be synthesized sufficiently or at all, and they are called essential amino acids. This term refers only to dietary intake since all amino acids are required for normal protein synthesis. Table 18.2 lists the amino acids found in foods and indicates those that are essential.

All twenty types of amino acids must be in the body at the same time for growth and tissue repair to occur. In other words, if the diet lacks one essential amino acid, the cells cannot synthesize protein. Since essential amino acids are not stored, those present but not used in protein synthesis are oxidized as energy sources or are converted into carbohydrates or fats.

Proteins are classified as complete or incomplete on the basis of the amino acid types they provide. The complete proteins, which include those available in milk, meats, and eggs, contain adequate amounts of the essential amino acids to maintain human body tissues and promote normal growth and development. Incomplete proteins, such as zein in corn, which has too little of the essential amino acids tryptophan and lysine, are unable by themselves to maintain human tissues or to support normal growth and development.

A protein in wheat called gliadin is an example of a partially complete protein. Although it does not contain enough lysine to promote human growth, it contains enough to maintain life.

Certain kidney disorders impair the ability to remove urea from the blood, raising the blood urea concentration. A blood test called blood urea nitrogen (BUN) determines the blood urea concentration and is often used to evaluate kidney function.

Aspartic Acid Structure Blood
Amino Acids in Foods


Leucine (e)

Arginine (ch)

Lysine (e)


Methionine (e)

Aspartic acid

Phenylalanine (e)



Glutamic acid



Threonine (e)


Tryptophan (e)

Histidine (ch)


Isoleucine (e)

Valine (e)

Eight essential amino acids (e) cannot be synthesized by human cells and must be provided in the diet. Two additional amino acids (ch) are essential in growing children.

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