Carbohydrates, Lipids, and Proteins PDF

Carbohydrates, Lipids, and Proteins c h a p t er o b j ec t ive s ⊑ Distinguish among monosaccharides, disaccharides, ⊑ Make prudent recommendations for dietary lipid and polysaccharides intake, including cholesterol and types...

Carbohydrates, Lipids, and Proteins c h a p t er o b j ec t ive s ⊑ Distinguish among monosaccharides, disaccharides, ⊑ Make prudent recommendations for dietary lipid and polysaccharides intake, including cholesterol and types of fatty acids ⊑ Identify the two major classifications of dietary fiber ⊑ Quantify the amount, energy content, and distribu- and their roles in overall health tion of fat within an average-sized woman ⊑ Discuss physiologic responses to different dietary ⊑ Outline the dynamics of fat metabolism during physi- carbohydrates in the development of type 2 diabetes cal activities of different intensities and durations and obesity ⊑ Discuss how aerobic training affects fat and carbohy- ⊑ Quantify the amount, energy content, and distribu- drate catabolism during exercise tion of carbohydrate within an average-sized man ⊑ Explain how aerobic training affects fat-burning ⊑ Summarize four major roles of carbohydrate in the adaptations within skeletal muscle body ⊑ Define the terms essential amino acid and nonessential ⊑ Outline the dynamics of carbohydrate metabolism amino acid and give two food sources for each during physical activities of various intensities and ⊑ Discuss the advantages and potential limitations of durations a vegetarian diet in maintaining good health and a ⊑ Contrast the speed of energy transfer from carbohy- physically active lifestyle drate and fat combustion ⊑ Outline the dynamics of protein metabolism during ⊑ Discuss how diet affects muscle glycogen levels and physical activities of various intensities and durations endurance exercise performance ⊑ Provide a credible rationale for increasing protein ⊑ For each of the diverse fatty acids (including trans- intake above the Recommended Dietary Allowance and omega-3 fatty acids), give an example of its food (RDA) for individuals who perform strenuous endur- source, its physiologic functions, and its possible role ance or resistance-exercise training in coronary heart disease ⊑ Describe the alanine–glucose cycle and how the body ⊑ List major characteristics of high- and low-density uses amino acids for energy during exercise lipoprotein cholesterol and discuss the role of each in coronary heart disease A n cill a rie s at-a-Glance Visit http://thePoint.lww.com/mkk8e to access the following resources. ⊑ References: Chapter 1 ⊑ Animation: Fat Mobilization and Use ⊑ Appendix D: The Metric System and Conversion ⊑ Animation: General Digestion  Constants in Exercise Physiology ⊑ Animation: Glycogen Synthesis ⊑ Interactive Question Bank ⊑ Animation: Hydrolysis ⊑ Animation: Alanine–Glucose Cycle ⊑ Animation: Transamination ⊑ Animation: Condensation ⊑ Focus on Research: Protein and Exercise—How Much ⊑ Animation: Digestion of Carbohydrate Is Enough? 7 8 Section 1â ‡ â ‡ Nutrition: The Base for Human Performance The carbohydrate, lipid, and protein nutrients provide energy Fructose (fruit sugar or levulose), the sweetest sugar, to maintain bodily functions during rest and physical activity. occurs in large amounts in fruits and honey. Fructose, like glu- Aside from their role as biologic fuel, these nutrients, called cose, also serves as an energy source but usually rapidly moves macronutrients, preserve the structural and functional integ- directly from the digestive tract into the blood to primarily rity of the organism. This chapter discusses each macronu- convert to fat but also glucose in the liver. Galactose does trient’s general structure, function, and dietary source. We not exist freely in nature; rather, it combines with glucose emphasize their importance in sustaining physiologic function to form milk sugar in the mammary glands of lactating ani- during physical activities of differing intensity and duration. mals. The body converts galactose to glucose for use in energy  metabolism. PART Oligosaccharides 1 CARBOHYDRATES Oligosaccharides form when 2 to 10 monosaccharides bond chemically. The major oligosaccharides, the disaccharides, or double sugars, form when two monosaccharide molecules KINDS AND SOURCES OF combine. Monosaccharides and disaccharides collectively are called simple sugars. CARBOHYDRATES Atoms of carbon, hydrogen, and oxygen combine to form a basic carbohydrate (sugar) molecule in the general formula (CH2O)n, where n ranges from 3 to 7 carbon atoms with What’s in a Name? hydrogen and oxygen atoms attached by single bonds. Except for lactose and a small amount of glycogen from animal ori- Simple sugars are packaged commercially under a variety of gin, plants provide the carbohydrate source in the human diet. names. This figure illustrates simple sugars with their percent- Carbohydrates classify as monosaccharides, oligosaccharides, age content of glucose and fructose. or polysaccharides. The number of simple sugars linked within each of these molecules distinguishes each carbohydrate form. Glucose or Dextrose 100% Monosaccharides The monosaccharide represents the basic unit of a carbohydrate. Corn syrup 100% Glucose, fructose, and galactose represent the three major monosaccharides. Maple syrup 48.5% 51.5% Glucose, also called dextrose or blood sugar, consists of a Brown sugar 49.5% 49.5% 6-carbon (hexose) compound formed naturally in food or in the body through digestion of more complex carbohydrates. Molasses 49.5% 47.5% Gluconeogenesis, the body’s process for making new sugar, Evaporated cane occurs primarily in the liver from the carbon residues of other juice (Sucrose) 50% 50% compounds (generally amino acids, but also glycerol, pyruvate, Raw sugar and lactate). After the small intestine absorbs glucose, it can (Sucrose) 50% 50% follow one of three pathways: Table sugar (Sucrose) 50% 50% 1. Become available as an energy source for cellular  metabolism 2. Form glycogen for storage in the liver and muscles Honey 50.5% 44.5% 3. Convert to fat (triacylglycerol) for later use as energy Orange juice concentrate 51% 49% See the animation “General Digestion” on http:// High-fructose corn syrup (HFCS) 55% 45% thePoint.lww.com/mkk8e for a demonstration of this process. Apple juice concentrate 66.5% 33.5% Figure 1.1 illustrates glucose along with other carbohy- Fructose 100% drates formed in plants from photosynthesis. Glucose con- sists of 6 carbon, 12 hydrogen, and 6 oxygen atoms (C6H12O6). 0 25 50 75 100 Fructose and galactose, two other simple sugars with the Fructose Glucose same chemical formula as glucose, have a slightly different C-H-O linkage and are thus different substances with distinct Source: US Department of Agriculture databases b  iochemical characteristics.  Chapter 1â ‡ â ‡ Carbohydrates, Lipids, and Proteins 9 H H C O H C O H H H C C O H C O O O O O C C H H H H H2O CO2 Chlorophyll H O H Reaction driven by energy from Glucose sun interacting with chlorophyll Leaves, wood, bark Fruits Grains Vegetables cellulose, hemicellulose sugars, starch, cellulose starch, cellulose starch, cellulose Figure 1.1â ‡ â ‡ Three-dimensional ring structure of the simple sugar glucose molecule formed during photosynthesis when energy from sunlight interacts with water, carbon dioxide, and the green pigment chlorophyll. Disaccharides all contain glucose. The three principal See the animation “Digestion of Carbohydrate” on disaccharides include: http://thePoint.lww.com/mkk8e for a demonstra- tion of this process. ⊑ Sucrose (glucose + fructose), the most common dietary disaccharide, contributes up to 25% of the total calories consumed in the United States. It occurs naturally in Polysaccharides most foods that contain carbohydrates, especially beet Polysaccharide describes the linkage of three or more (up to and cane sugar, brown sugar, sorghum, maple syrup, and thousands) sugar molecules. Polysaccharides form during the honey. chemical process of dehydration synthesis, a water-losing ⊑ Lactose (glucose + galactose), a sugar not found in plants, reaction that forms a more complex carbohydrate molecule. exists in natural form only in milk as milk sugar. The least Plant and animal sources both contribute to these large chains sweet of the disaccharides, lactose when artificially pro- of linked monosaccharides. cessed often becomes an ingredient in carbohydrate-rich, high-calorie liquid meals. ⊑ Maltose (glucose + glucose) occurs in beer, breakfast cere- Plant Polysaccharides als, and germinating seeds. Also called malt sugar, this sugar cleaves into two glucose molecules yet makes only a Starch and fiber are the common forms of plant polysaccha- small contribution to the carbohydrate content of the diet. rides. 10 Section 1â ‡ â ‡ Nutrition: The Base for Human Performance A Straight chain linkage in amylose starch arranged in a helical coil o o White bread B Branch point in amylopectin starch Branch point Apple Figure 1.2â ‡ â ‡ The two forms of plant starch. (A) Straight-chain linkage with unbranched bonding of glucose residues (glycosidic linkages) in amylose. (B) Branch point in the highly branched amylopectin starch molecule. The amylopectin structure appears linear, but it exists as a helical coil. (Adapted with permission from McArdle WD, Katch FI, Katch VL. Sports and Exercise Nutrition, 4th ed. Philadelphia: Wolters Kluwer Health, 2013.) Starch, the storage form of carbohydrate in plants, occurs See the animation “Hydrolysis” on http://thePoint. in seeds, corn, and various grains of bread, cereal, pasta, and lww.com/mkk8e for a demonstration of this pastries. Starch exists in two forms (Fig. 1.2):  process. 1. Amylose, a long straight chain of glucose units twisted The term complex carbohydrate describes dietary starch, into a helical coil which represents the most important dietary source of carbo- 2. Amylopectin, a highly branched monosaccharide hydrate in the typical U.S. diet, accounting for approximately linkage 50% of the typical individual’s total intake. The relative proportion of each form of starch in a Fiber, classified as a nonstarch, structural polysaccha- plant species determines the characteristics of the starch, ride, includes cellulose, the most abundant organic mole- including its “digestibility.” Starches with a relatively large cule on Earth. Fibrous materials resist chemical breakdown amount of amylopectin digest and absorb rapidly, whereas by human digestive enzymes, although a small portion starches with high amylose content break down (hydrolyze) at ferments by action of bacteria in the large intestine and a slower rate. ultimately participates in metabolic reactions following  Chapter 1â ‡ â ‡ Carbohydrates, Lipids, and Proteins 11 intestinal absorption. Fiber occurs exclusively in plants; it comprises the structure of leaves, stems, roots, seeds, and fruit Recommended Daily Fiber  coverings. Table 1.1 Intake Health Implications of Fiber Deficiency.â ‡Much of the Recommended Daily Fiber Intake (g) interest in dietary fiber originates from studies that link high fiber intake, particularly whole-grain cereal fibers, with a Children 1–3 y 19 lower occurrence of obesity, systemic inflammation, insulin Children 4–8 y 25 resistance and type 2 diabetes, hypertension, the metabolic Boys 9–13 y 31 syndrome, digestive disorders, elevated blood cholesterol, Boys 14–18 y 38 colorectal cancer, and heart disease.1,16,46,48,58 Girls 9–18 y 26 Americans typically consume about 12 to 15 g of fiber Men 19–50 y 38 daily, far short of the Food and Nutrition Board of the National Academy of Sciences (http://www.iom.edu/reports/2002/ Men 51 y and older 30 dietary-reference-intakes-for-energy-carbohydrate-fiber- Women 19–50 y 25 fat-fatty-acids-cholesterol-protein-and-amino-acids.aspx) Women 51 y and older 21 recommendations of 38 g for men and 25 g for women up to Adapted with permission from McArdle WD, Katch FI, Katch VL. Sports age 50, and 30 g for men and 21 g for women over age 50.19 and Exercise Nutrition, 4th ed. Philadelphia: Wolters Kluwer Health, 2013, and US Department of Agriculture database. Appendix D, available online at http://the â … point.lww.com/mkk8e, shows the relationship The water-insoluble fibers cellulose, many hemicelluloses, between metric units and U.S. units, includ- and lignin and cellulose-rich products (wheat bran) do not ing common expressions of work, energy, and lower cholesterol. power. Heart disease and obesity protection may relate to dietary fiber’s regulatory role in reducing insulin secretion by slow- Fiber retains considerable water and gives “bulk” to ing nutrient absorption by the small intestine following food the food residues in the intestinal tract. Fiber intake mod- intake. Fiber consumption may also confer heart disease estly reduces serum cholesterol in humans by lowering the  protection through beneficial effects on blood pressure,  insulin low-density lipoprotein fraction of the cholesterol profile. sensitivity, and blood clotting characteristics.43,79 On the nega- Particularly effective are the water-soluble, mucilaginous tive side, excessive fiber intake inhibits intestinal absorption of fibers such as psyllium seed husk, β-glucan, pectin, and guar the minerals calcium, phosphorus, and iron. Present nutritional gum present in oats, beans, brown rice, peas, carrots, corn- wisdom advocates a diet that contains 20 to 40 g of fiber (depend- husks, and many fruits.31,78 Dietary fiber exerts no effect on ing on age) per day (ratio of 3:1 for water-insoluble to soluble high-density lipoproteins (see the section on High-Density, fiber). Table 1.1 list the recommended daily fiber intake and Low-Density, and Very Low-Density Lipoproteins). Table 1.2 list the fiber content of some common foods. Table 1.2 Fiber Content of Common Foods (Listed in Order of Total Fiber Content) Food Serving Size Total Fiber (g) Soluble Fiber (g) Insoluble Fiber (g) 100% bran cereal 1/2 cup 10.0 0.3 9.7 Peas 1/2 cup 5.2 2.0 3.2 Kidney beans 1/2 cup 4.5 0.5 4.0 Apple 1 small 3.9 2.3 1.6 Potato 1 small 3.8 2.2 1.6 Broccoli 1/2 cup 2.5 1.1 1.4 Strawberries 3/4 cup 2.4 0.9 1.5 Oats, whole 1/2 cup 1.6 0.5 1.1 Banana 1 small 1.3 0.6 0.7 Pasta 1/2 cup 1.0 0.2 0.8 Lettuce 1/2 cup 0.5 0.2 0.3 White rice 1/2 cup 0.5 0 0.5 Adapted with permission from McArdle WD, Katch FI, Katch VL. Sports and Exercise Nutrition, 4th ed. Philadelphia: Wolters Kluwer Health, 2013, and US Department of Agriculture database. 12 Section 1â ‡ â ‡ Nutrition: The Base for Human Performance Added Sugar and the Blood Lipid linked together, much like a sausage link in a chain of sausages, with branch linkages for joining additional glucose units. Profile See the animation “Glycogen Synthesis” on http:// Researchers divided 6113 participants in the long-running thePoint.lww.com/mkk8e for a demonstration of National Health and Nutrition Examination Survey (NHANES) into this process. five groups based on the percentage of total calories consumed as added sugars. Groups ranged in added daily sugar intakes of Figure 1.3 shows that glycogen biosynthesis involves add- less than 5% (three teaspoons of sugar) to 25% or more (46 tea- ing individual glucose units to an existing glycogen polymer. spoons of sugar). Sugar intake varied inversely with the healthy Stage 4 of the figure shows an enlarged view of the chemi- levels of HDL cholesterol (58.7 mgâ ›∙â ›dL−1 [deciliter or 100 mL] in cal configuration of the glycogen molecule. Overall, glycogen the group consuming the least added sugar to 47.7 mgâ ›∙â ›dL−1 in synthesis is irreversible. Glycogen synthesis requires energy, as the group consuming the most) and directly with the unhealthy one adenosine triphosphate (ATP; stage 1) and one uridine levels of triglycerides (105 mgâ ›∙â ›dL−1 in the group consuming the triphosphate (UTP; stage 3) degrade during glucogenesis. least added sugar to 114 mgâ ›∙â ›dL−1 in the group consuming the most). The research was not designed to show cause and effect, How Much Glycogen Does the Body Store?â ‡ Figure 1.4 but it does argue for substituting the empty calories in sugars  illustrates that a well-nourished 80-kg man stores approxi- with foods containing a more nutritious package. mately 500 g of carbohydrate. Of this, muscle glycogen accounts Source: Welsh JA, et al. Caloric sweetener consumption and dyslipid- for the largest reserve (approximately 400 g), followed by 90 to emia among US adults. JAMA 2010;303:1490. 110 g as liver glycogen (highest concentration, representing 3 to 7% of the liver’s weight), with only about 2 to 3 g as blood glu- cose. Each gram of either glycogen or glucose contains approxi- Not All Carbohydrates Are Physiologically Equal.â ‡ mately 4 calories (kcal) of energy. This means that the average Digestion rates of different carbohydrate sources possibly person stores about 2000 kcal as carbohydrate—enough total explain the link between carbohydrate intake and diabe- energy to power a 20-mile continuous run at high intensity. tes and excess body fat. Foods containing dietary fiber slow The body stores comparatively little glycogen, so its quan-  carbohydrate digestion, minimizing surges in blood glucose. tity fluctuates considerably through dietary modifications. For In contrast, low-fiber processed starches (and simple sugars in example, a 24-hr fast or a low-carbohydrate, normal-calorie diet soft drinks) digest quickly and enter the blood at a relatively nearly depletes glycogen reserves. In contrast, maintaining a car- rapid rate (high glycemic index foods; see Chapter 3). The aver- bohydrate-rich diet for several days almost doubles the body’s age American currently consumes 22 to 28 teaspoons of added glycogen stores compared with levels attained with a typical, sugars daily (equivalent to 350 to 440 empty calories)—mostly well-balanced diet. The body’s upper limit for glycogen storage aver- as high-fructose corn syrup and ordinary table sugar. The blood ages about 15 g per kilogram (kg) of body mass, equivalent to 1050 glucose surge after consuming refined, processed starch and g for a 70-kg (154 lb) male and 840 g for a 56-kg (124 lb) female. simple sugar has three effects: it (1) stimulates overproduction Several factors determine the rate and quantity of glyco- of insulin by the pancreas to accentuate hyperinsulinemia, (2) gen breakdown and resynthesis. During exercise, intramuscu- elevates plasma triacylglycerol concentrations, and (3) accel- lar glycogen provides the major carbohydrate energy source for erates fat synthesis. Consistently consuming high intakes active muscles. In addition, liver glycogen rapidly reconverts of simple sugar reduces the body’s sensitivity to insulin (i.e., to glucose (regulated by a specific phosphatase enzyme) for peripheral tissues become more resistant to insulin’s effects); release into the blood as an extramuscular glucose supply for this requires progressively more insulin to optimize blood sugar exercise. The term glycogenolysis describes this reconversion levels.65 Type 2 diabetes results when the pancreas cannot produce of glycogen to glucose. Depletion of liver and muscle glyco- sufficient insulin to regulate blood glucose, causing it to rise. Individ- gen by dietary restriction of carbohydrates or intense exercise uals should minimize sugary beverage intake, including fruit stimulates glucose synthesis. This occurs through gluconeo- juices, to lower the risk of obesity, diabetes, heart disease, gout, genic metabolic pathways from the structural components of and dental cavities. Light to moderate physical activity per- other nutrients, particularly proteins. formed on a regular basis exerts a potent influence to improve insulin sensitivity, thereby reducing the insulin requirement for Important Carbohydrate a given glucose uptake.37 Chapter 20 discusses exercise, diabe- Conversions tes, and the associated risk of the metabolic syndrome. Glucogenesis—glycogen synthesis from glucose (glucose → Glycogen: The Animal Polysaccharide glycogen) Glycogen is the storage carbohydrate within mammalian muscle Gluconeogenesis—glucose synthesis largely from structural com- and liver. It forms as a large polysaccharide polymer synthe- ponents of noncarbohydrate nutrients (protein → g lucose) sized from glucose in the process of glycogenesis (catalyzed Glycogenolysis—glucose formation from glycogen by the enzyme glycogen synthase). Irregularly shaped, glyco- (glycogen → glucose) gen ranges from a few hundred to 30,000 glucose molecules  Chapter 1â ‡ â ‡ Carbohydrates, Lipids, and Proteins 13 Stage 1 Glucose Hexokinase Glucose-6-phosphate Stage 2 Glucose- 6-phosphate Glucose-1-phosphate isomerase Stage 3 Uridyl transferase (UTP) Prophosphate (PPI) UDP glucose Stage 4 Glycogen synthase Glycogen Glycogen Figure 1.3â ‡ â ‡ Glycogen synthesis is a four-step process. Stage 1, ATP donates a phosphate to glucose to form glucoseâ ‚6-phosphate. This reaction involves the enzyme hexokinase. Stage 2, Glucose-6-phosphate isomerizes to glucose-1-phosphate by the enzyme glucose-6-phosphate isomerase. Stage 3, The enzyme uridyl transferase reacts uridyl triphosphate (UTP) with glucose-1-phosphate to form uridine diphosphate (UDP)-glucose (a phosphate is released as UTP → UDP). Stage 4, UDP-glucose attaches to one end of an existing glycogen polymer chain. This forms a new bond (known as a glycoside bond) between the adjacent glucose units, with the concomitant release of UDP. For each glucose unit added, 2 moles of ATP converts to ADP and phosphate. (Adapted with permission from McArdle WD, Katch FI, Katch VL. Sports and Exercise Nutrition, 4th ed. Philadelphia: Wolters Kluwer Health, 2013.) Hormones play a key role in regulating liver and muscle when blood sugar falls below normal, the pancreas’s alpha glycogen stores by controlling circulating blood sugar (α) cells secrete glucagon to normalize blood sugar con-  levels. Elevated blood sugar causes the beta (β) cells of the centration. Known as the “insulin antagonist” hormone pancreas to secrete additional insulin; this facilitates cel- (www.glucagon.com), glucagon elevates blood glucose by lular glucose uptake and inhibits further insulin secretion. stimulating the liver’s glycogenolytic and gluconeogenic This type of feedback regulation maintains blood glucose pathways. Chapter 20 contains further discussion of hor- at an appropriate physiologic concentration. In contrast, monal  regulation in  exercise. 14 Section 1â ‡ â ‡ Nutrition: The Base for Human Performance Sufficient daily carbohydrate intake for physically active Muscle glycogen individuals maintains the body’s relatively limited glycogen 400 g (1600 kcal) stores. Once cells reach their maximum capacity for glycogen stor- age, excess sugars convert to and store as fat. The interconver- sion of macronutrients for energy storage explains how body fat can increase when dietary carbohydrate exceeds energy requirements, even if the diet contains little lipids. Liver glycogen 100 g (400 kcal) 2.â ‡Protein-Sparer Adequate carbohydrate intake helps to preserve tissue protein. Plasma glucose  Normally, protein serves a vital role in tissue maintenance, 3 g (12 kcal) repair, and growth, and to a considerably lesser degree, as a nutrient energy source. Depletion of glycogen reserves—read- Total carbohydrate 503 g (2012 kcal) ily occurring with starvation, reduced energy and/or carbohy- drate intake, and prolonged, strenuous exercise—dramatically affects the metabolic mixture of fuels for energy. In addition Figure 1.4â ‡ â ‡ Distribution of carbohydrate energy in an to stimulating fat catabolism, glycogen depletion triggers glu- average 80-kg man. (Adapted with permission from McArdle cose synthesis from the labile pool of amino acids  (protein). WD, Katch FI, Katch VL. Sports and Exercise Nutrition, 4th ed. This gluconeogenic conversion offers a metabolic option Philadelphia: Wolters Kluwer Health, 2013.) for augmenting carbohydrate availability (and maintaining plasma glucose levels) even with insufficient glycogen stores. RECOMMENDED INTAKE OF The price paid strains the body’s protein levels, particularly CARBOHYDRATES muscle  protein. In the extreme, this reduces lean tissue mass and adds a solute load on the kidneys, forcing them to excrete Although there are no absolute minimum or maximum rec- the nitrogenous byproducts of protein breakdown. ommendations for total carbohydrate intake, a sedentary 70-kg person’s daily carbohydrate intake typically amounts to about 300 g or between 40 and 50% of total calories. For more physically active people and those involved in exercise training, carbohydrates should equal about 60% of daily calories or 400 to Discuss the rationale for recommending adequate carbohy- 600 g, predominantly as unrefined, fiber-rich fruits, grains, and drate intake rather than an excess of protein to increase mus- vegetables. During periods of intense training, carbohydrate intake cle mass through resistance training. should increase to 70% of total calories consumed or approximately 8 to 10 g per kg of body mass. Nutritious dietary carbohydrate sources consist of fruits, grains, and vegetables, yet this does not represent the usual 3.â ‡ Metabolic Primer/Prevents Ketosis source of carbohydrate intake for all people. The typical Amer- Components of carbohydrate catabolism serve as “primer” substrate ican consumes about 50% of carbohydrate as simple sugars. for fat oxidation. Insufficient carbohydrate breakdown—either This intake comes primarily from sugars in the form of sucrose through limitations in glucose transport into the cell (e.g., and high-fructose corn syrup added in food processing. These diabetes where insulin production wanes or insulin resis- sugars do not come in a nutrient-dense package characteristic tance increases) or glycogen depletion through inadequate of the sugar found naturally in fruits and vegetables. diet or prolonged exercise—causes fat mobilization to exceed fat oxidation. The lack of adequate byproducts of glycogen ROLE OF CARBOHYDRATES IN THE catabolism produces incomplete fat breakdown with accumu- lation of ketone bodies (acetoacetate and β-hydroxybutyrate, BODY acetone-like byproducts of incomplete fat breakdown). In  Carbohydrates serve four important functions related to energy excess, ketones increase body fluid acidity to produce a poten- metabolism and exercise performance. tially harmful acid condition called acidosis or, specifically with regard to fat breakdown, ketosis. Chapter 6 continues the discussion of carbohydrate as a primer for fat catabolism. 1.â ‡ Energy Source Carbohydrates primarily serve as an energy fuel, particularly dur- 4.â ‡ Fuel for the Central Nervous System ing intense physical activity. Energy derived from the catabo- lism of bloodborne glucose and muscle glycogen powers the The central nervous system requires an uninterrupted stream of contractile elements of muscle and other forms of biologic  carbohydrate for proper function. Under normal conditions, the work. brain metabolizes blood glucose almost exclusively as its fuel  Chapter 1â ‡ â ‡ Carbohydrates, Lipids, and Proteins 15 source. In poorly regulated diabetes, during starvation, or with a 1. Long-chain fatty acid oxidation by skeletal muscle prolonged low-carbohydrate intake, the brain adapts after about 2. Free fatty acid (FFA) liberation from adipose tissue. 8 days and metabolizes larger amounts of fat (as ketones) for fuel. Adequate carbohydrate availability (and resulting increased Chronic low-carbohydrate, high-fat diets also induce adaptations catabolism) can inhibit transport of long-chain fatty acids into in skeletal muscle that increase fat use during  low-to-moderate the mitochondria, thus controlling the metabolic mixture. physical activity levels and spares muscle glycogen. Blood sugar usually remains regulated within narrow  limits for two main reasons: High-Intensity Exercise 1. Glucose serves as a primary fuel for nerve tissue metabo- Neural–humoral factors during intense exercise increase the lism output of epinephrine, norepinephrine, and glucagon and 2. Glucose represents the sole energy source for red blood decrease insulin release. These hormonal responses activate cells glycogen phosphorylase (indirectly via activation of cyclic adenosine monophosphate, or cyclic AMP; see Chapter 20), At rest and during activity, liver glycogenolysis (glycogen- the enzyme that facilitates glycogenolysis in the liver and to-glucose conversion) maintains normal blood glucose levels, active muscles. Think of glycogen phosphorylase as the con- usually at 100 mgâ ›∙â ›dL−1. In prolonged activity such as marathon troller of the glycogen–glucose interconversion to regulate running (or similar duration intense activities), blood glucose circulating glucose concentration in the bloodstream. Because concentration eventually falls below normal levels because liver muscle glycogen provides energy without oxygen, it con- glycogen depletes, while active muscle continues to catabolize tributes considerable energy in the early minutes of exercise the available blood glucose. Symptoms of clinically reduced when oxygen use fails to meet oxygen demands. As exercise blood glucose (hypoglycemia: