Fuel Metabolism. We obtain our fuel primarily from the macronutrients (i.e., carbohydrates, fats, and proteins) in our diet. As we eat, our foodstuffs are digested and absorbed. The products of digestion circulate in the blood, enter various tissues, and are eventually taken up by cells and oxidized to produce energy. To completely convert our fuels to carbon dioxide (CO₂) and water (H₂O), molecular oxygen (O₂) is required. We breathe to obtain this oxygen and to eliminate the carbon dioxide (CO₂) that is produced by the oxidation of our food.

Fuel Stores. Any dietary fuel that exceeds the body’s immediate energy needs is stored, mainly as triacylglycerol (fat) in adipose tissue; as glycogen (a carbohydrate) in muscle, liver, and other cells; and, to some extent, as protein in muscle. When we are fasting, between meals and overnight while we sleep, fuel is drawn from these stores and is oxidized to provide energy (Fig. 1.1).

Fuel Requirements. We require enough energy each day to drive the basic functions of our bodies and to support our physical activity. If we do not consume enough food each day to supply that much energy, the body’s fuel stores supply the remainder and we lose weight. Conversely, if we consume more food than is required for the energy we expend, our body’s fuel stores enlarge and we gain weight.

Other Dietary Requirements. In addition to providing energy, the diet provides precursors for the biosynthesis of compounds necessary for cellular and tissue structure, function, and survival. Among these precursors are the essential fatty acids and essential amino acids (those that the body needs but cannot synthesize). The diet must also supply vitamins, minerals, and water.

Waste Disposal. Dietary components that we can utilize are referred to as nutrients. However, both the diet and the air we breathe contain xenobiotic compounds, compounds that have no use or value in the human body and may be toxic. These compounds are excreted in the urine and feces together with metabolic waste products.

THE WAITING ROOM

The major fuels we obtain from our diet are the macronutrients, namely carbohydrates, proteins, and fats. When these fuels are oxidized to CO₂ and H₂O in our cells (the process of catabolism), energy is released by the transfer of electrons to O₂. The energy from this oxidation process generates heat and adenosine triphosphate (ATP) (Fig. 1.2). Carbon dioxide travels in the blood to the lungs where it is expired, and water is excreted in urine, sweat, and other secretions. Although the heat that is generated by fuel oxidation is used to maintain body temperature, the main purpose of fuel oxidation is to generate ATP. Adenosine triphosphate provides the energy that drives most of the energy-consuming processes in the cell, including biosynthetic reactions (anabolic pathways), muscle contraction, and active transport across membranes. As these processes utilize energy, ATP is converted back to adenosine diphosphate (ADP) and inorganic phosphate (P_i). The generation and utilization of ATP is referred to as the ATP–ADP cycle.

The oxidation of fuels to generate ATP is called respiration (Fig. 1.3). Prior to oxidation, carbohydrates are converted principally to glucose, fat to fatty acids, and protein to amino acids. The pathways for oxidizing glucose, fatty acids, and amino acids have many features in common. They first oxidize the fuels to acetyl coenzyme A (acetyl CoA), a precursor of the tricarboxylic acid (TCA) cycle. The TCA cycle is a series of reactions that completes the oxidation of fuels to CO₂ (see Chapter 23). Electrons lost from the fuels during oxidative reactions are transferred to O₂ by a series of proteins in the electron transport chain (see Chapter 24). The energy of electron transfer is used to convert ADP and P_i to ATP by a process known as oxidative phosphorylation.

In discussions of metabolism and nutrition, energy is often expressed in units of calories. A calorie in this context (nutritional calorie) is equivalent to 1 kilocalorie (kcal) in energy terms. “Calorie” was originally spelled with a capital C, but the capitalization was dropped as the term became popular. Thus, a 1-calorie soft drink actually has 1 kcal of energy. Energy is also expressed in joules. One kilocalorie equals 4.18 kilojoules (kJ). Physicians tend to use units of calories, in part because that is what their patients use and understand. One kilocalorie of energy is the amount of energy required to raise the temperature of 1 L of water by 1°C at atmospheric pressure.

A. Carbohydrates

The major carbohydrates in the human diet are starch, sucrose, lactose, fructose, maltose, galactose, and glucose. The polysaccharide starch is the storage form of carbohydrates in plants. Sucrose (table sugar), maltose, and lactose (milk sugar) are disaccharides, and fructose, galactose, and glucose are monosaccharides. Digestion converts the larger carbohydrates to monosaccharides, which can be absorbed into the bloodstream. Glucose, a monosaccharide, is the predominant sugar in human blood (Fig. 1.4).

Oxidation of carbohydrates to CO₂ and H₂O in the body produces approximately 4 kcal/g (Table 1.1). In other words, every gram of carbohydrate we eat yields approximately 4 kcal of energy. Note that carbohydrate molecules contain a significant amount of oxygen and are already partially oxidized before they enter our bodies (see Fig. 1.4).

TABLE 1.1 Caloric Content of Fuels

B. Proteins

Proteins are composed of amino acids that are joined to form linear chains (Fig. 1.5). In addition to carbon, hydrogen, and oxygen, proteins contain approximately 16% nitrogen by weight. The digestive process breaks down proteins to their constituent amino acids, which enter the blood. The complete oxidation of proteins to CO₂, H₂O, and NH₄⁺ in the body yields approximately 4 kcal/g (see Table 1.1).

C. Fats

Fats are lipids composed of triacylglycerols (also called triglycerides). A triacylglycerol molecule contains three fatty acids esterified to one glycerol moiety (Fig. 1.6).

Fats contain much less oxygen than is contained in carbohydrates or proteins. Therefore, fats are more reduced and yield more energy when oxidized. The complete oxidation of triacylglycerols to CO₂ and H₂O in the body releases approximately 9 kcal/g, more than twice the energy yield from an equivalent amount of carbohydrate or protein (see Table 1.1).

D. Alcohol

Humans carry supplies of fuel within their bodies (Table 1.2), which are similar to the fuel supplies in the plants and animals we eat. These fuel stores are light in weight, large in quantity, and readily converted into oxidizable substances. Most of us are familiar with fat, our major fuel store, which is located in adipose tissue. Although fat is distributed throughout our bodies, it tends to increase in quantity in our hips, thighs, and abdomens as we advance into middle age. In addition to our fat stores, we have important, although much smaller, stores of carbohydrates in the form of glycogen located primarily in our liver and muscles. Glycogen consists of glucose molecules joined together to form a large, branched polysaccharide (see Fig. 1.4). Body protein, particularly the protein of our large muscle masses, also serves to some extent as a fuel store, and we draw on it for energy when we fast.

TABLE 1.2 Fuel Composition of the Average 70 kg Mana after an Overnight Fast

^aIn biochemistry and nutrition, the standard reference is often the 70-kg (154 lb) man. This standard was probably chosen because in the first half of the 20th century, when many nutritional studies were performed, young healthy medical and graduate students (who were mostly men) volunteered to serve as subjects for these experiments.

A. Fat

Our major fuel store is adipose triacylglycerol (triglyceride), a lipid more commonly known as fat. The average 70-kg man has approximately 15 kg of stored triacylglycerol, which accounts for approximately 85% of his total stored calories (see Table 1.2).

Two characteristics make adipose triacylglycerol a very efficient fuel store: the fact that triacylglycerol contains more calories per gram than carbohydrate or protein (9 kcal/g vs. 4 kcal/g) and the fact that adipose tissue does not contain much water. Adipose tissue contains only about 15% water, compared to tissues such as muscle that contain about 80%. Thus, the 70-kg man with 15 kg of stored triacylglycerol has only about 18 kg of adipose tissue.

B. Glycogen

Our stores of glycogen in liver, muscle, and other cells are relatively small in quantity but are nevertheless important. Liver glycogen is used to maintain blood glucose levels between meals, which is necessary for optimal functioning of the nervous system. Thus, the size of this glycogen store fluctuates during the day; an average 70-kg man might have 200 g or more of liver glycogen after a meal but only 80 g after an overnight fast. Muscle glycogen supplies energy for muscle contraction during exercise. At rest, the 70-kg man has approximately 150 g of muscle glycogen. Almost all cells, including neurons, maintain a small emergency supply of glucose as glycogen.

It is not practical to store all of the stored energy as glycogen instead of triacylglycerol. Glycogen being a polar molecule with hydroxyl groups binds approximately 4 times its weight as water and will require significantly more intracellular space than the same amount of energy stored as triacylglycerol, which, being hydrophobic, contains much less water weight.

The BMR, which is usually expressed in kcal/day, is affected by body size, age, sex, and other factors (Table 1.3). It is proportional to the amount of metabolically active tissue (including the major organs) and the lean (or fat-free) body mass. Obviously, the amount of energy required for basal functions in a large person is greater than the amount required in a small person. However, the BMR is usually lower for women than for men of the same weight because women usually have more adipose tissue and less muscle mass. Body temperature also affects the BMR, which increases by 12% with each degree centigrade (7% with each degree Fahrenheit) increase in body temperature (i.e., “feed a fever; starve a cold”). The ambient temperature affects the BMR, which increases slightly in colder climates as thermogenesis is activated. Excessive secretion of thyroid hormone (hyperthyroidism) causes the BMR to increase, whereas diminished secretion (hypothyroidism) causes it to decrease. The BMR increases during pregnancy and lactation. Growing children have a higher BMR per kilogram body weight than adults because a greater proportion of their bodies is composed of brain, muscle, and other more metabolically active tissues. The BMR declines in aging individuals because their metabolically active tissue is shrinking and body fat is increasing. In addition, large variations exist in BMR from one adult to another, determined by genetic factors.

TABLE 1.3 Factors Affecting BMR Expressed per kg Body Weight

A rough estimate of the BMR may be obtained by assuming it is either 24 or 21.6 kcal/day/kg body weight (for men or women, respectively) and multiplying by the body weight. An easy way to remember this is 1 kcal/kg/hr for men and 0.9 kcal/kg/hr for women. This estimate works best for young individuals who are near their healthy weight. More accurate methods for calculating the BMR use empirically derived equations for different gender and age groups (Table 1.4). Even these calculations do not take into account variation among individuals.

B. Physical Activity

In addition to the BMR, the energy required for physical activity contributes to the DEE. The difference in physical activity between a student and a lumberjack is enormous, and a student who is relatively sedentary during the week may be much more active during the weekend. Table 1.5 gives factors for calculating the approximate energy expenditures associated with typical activities.

TABLE 1.5 Typical Activities with Corresponding Hourly Activity Factorsa

^aThe hourly activity factor is multiplied by the basal metabolic rate per hour times the number of hours engaged in the activity to give the caloric expenditure for that activity. If this is done for all of the hours in a day, the sum over 24 hours will approximately equal the daily energy expenditure.

National Research Council. Recommended Dietary Allowances. 10th ed. National Academy Press; 1989. https://doi.org/10.17226/1349. Adapted and reproduced with permission from the National Academy of Sciences. Courtesy of the National Academies Press, Washington, D.C.

A rough estimate of the energy required per day for physical activity can be made by using a value of 30% of the BMR (per day) for a very sedentary person (such as a medical student who does little but study) and a value of 60% to 70% of the BMR (per day) for a person who engages in about 2 hours of moderate physical activity per day (see Table 1.5). A value of 100% or more of the BMR is used for a person who does several hours of vigorous physical activity per day.

C. Diet-Induced Thermogenesis

Our DEE includes a component related to the intake of food known as diet-induced thermogenesis or the thermic effect of food (TEF). DIT was formerly called the specific dynamic action (SDA). After the ingestion of food, our metabolic rate increases because energy is required to digest, absorb, distribute, and store nutrients.

The energy required to process the types and quantities of food in the typical American diet is probably equal to approximately 10% of the kilocalories ingested. This amount is roughly equivalent to the error involved in rounding off the caloric content of carbohydrate, fat, and protein to 4, 9, and 4, respectively. Therefore, DIT is often ignored, and calculations are based simply on the BMR and the energy required for physical activity.

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