4: Metabolism and Nutrition
- Page ID
- 164275
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Chapter Overview
Nutrition and metabolism are words that are often used together—but what do they mean? Nutrition refers to the foods that we eat and the nutrients they contain. The Council on Food and Nutrition of the American Medical Association defines nutrition broadly as “the science of food; the nutrients and the substances therein; their action, interaction, and balance in relation to health and disease; and the process by which the organism (i.e., body) ingests, digests, absorbs, transports, utilizes, and excretes food substances. Metabolism refers to the complex, interactive set of chemical processes that make life possible. A good phrase to remember in connection with the word metabolism is “use of foods” because basically this is what metabolism is—the use the body makes of foods after they have been digested, absorbed, and circulated to cells.
Defining Metabolism, Catabolism, and Anabolism
Defines metabolism as the sum of all catabolic (breakdown) and anabolic (synthesis) reactions. Introduces metabolic rate as the energy used to maintain life, and explains that survival requires sufficient food intake to support this rate.
Catabolic Reactions: Releasing Energy from Nutrients
Describes how catabolic reactions break down carbohydrates, lipids, and proteins from food, and how ATP breakdown provides energy for cellular work throughout the body.
Anabolic Reactions and Redox Chemistry: Building and Powering the Body
Explains anabolic (biosynthetic) reactions that build bone, muscle, proteins, lipids, and nucleic acids using ATP. Introduces oxidation–reduction reactions as the basis for energy capture in ATP, and notes that metabolic errors in these pathways can cause disease.
Carbohydrate Metabolism: From Food to ATP
Outlines digestion and absorption of carbohydrates, the role of salivary amylase, and cellular processing of glucose. Introduces glycolysis, pyruvate, NADH, and ATP production, and distinguishes anaerobic (lactate formation) from aerobic pathways (entry of pyruvate into the Krebs/citric acid/TCA cycle and oxidative phosphorylation).
Anaerobic vs Aerobic ATP Production
Clarifies ATP yield under anaerobic conditions (net 2 ATP per glucose via glycolysis) versus aerobic respiration (up to ~36 ATP per glucose), and why oxygen availability is critical for efficient energy production.
Gluconeogenesis: Making Glucose in Low-Carbohydrate States
Describes synthesis of glucose from lactate, pyruvate, glycerol, alanine, or glutamate during fasting, starvation, or low-carbohydrate diets, emphasizing its importance for glucose-dependent organs such as the brain.
Lipid Metabolism: Storage, Mobilization, and Oxidation
Explains dietary, stored, and hepatic sources of lipids; intestinal digestion and absorption of triglycerides; re-synthesis and transport to liver and adipose tissue. Describes β-oxidation of fatty acids to acetyl CoA, entry into the Krebs cycle, and ATP production.
Ketone Bodies and Lipogenesis: Handling Excess or Limited Fuel
Describes ketone body formation when acetyl CoA exceeds Krebs cycle capacity or when glucose is limited, and their use as alternative fuels. Explains lipogenesis (fatty acid and triglyceride synthesis) from excess acetyl CoA, including roles in making steroid hormones, cholesterol, and bile salts. Defines lipolysis as triglyceride breakdown.
Protein Metabolism: Digestion, Use, and Nitrogen Disposal
Outlines protein digestion by HCl and pepsin in the stomach and by pancreatic and intestinal enzymes in the small intestine. Explains amino acid absorption, use for new protein synthesis, conversion of excess amino acids to glucose or ketones, urea formation and excretion, and use of amino acids as fuel (especially during starvation).
Metabolic States: Fed, Fasting, and Starvation
Introduces absorptive (fed), postabsorptive (fasting), and starvation states. Explains how metabolism shifts between these daily, and how fuel use changes in prolonged food deprivation.
Hormonal Regulation of Fuel Use: Insulin and Glucagon
Describes how rising blood glucose in the absorptive state stimulates insulin release, promoting uptake and storage of glucose as glycogen and fat. Explains how falling glucose in the postabsorptive state lowers insulin and raises glucagon, stimulating glycogen breakdown and alternative fuel use (fatty acids and proteins) when glycogen is depleted.
Long-Term Fasting and Starvation: Fuel Priorities and Tissue Survival
Details the progression from short-term fasting to starvation: continued gluconeogenesis for brain fuel, later shift to ketone bodies as preferred fuel for many organs, and eventual catabolism of body proteins, with initial preservation of muscle and later muscle breakdown when other stores are exhausted.
Energy Balance and Thermoregulation
Explains that much dietary energy is released as heat and used to maintain core temperature (~36.5–37.5 °C / 97.7–99.5 °F). Introduces hypothalamic control of temperature (sweating, shivering) and mechanisms of heat exchange: conduction, convection, radiation, and evaporation.
Nutrition, Caloric Balance, and Health
Links dietary intake to metabolism, noting that all excess calories (from carbohydrate, fat, or protein) are stored as fat. Discusses typical daily energy needs (about 1500–2000 kcal, higher with exercise) and consequences of chronic over- or under-eating, including obesity, cardiovascular disease, cancer, and diabetes.
Vitamins and Minerals: Micronutrients for Metabolic Pathways
Explains that vitamins and minerals are essential for proper function of metabolic pathways. Notes that most vitamins must be obtained regularly from the diet (limited storage), whereas many minerals are stored, especially in bone.
Factors Affecting Metabolic Rate
Describes how age, sex, activity level, fuel intake, and lean body mass influence metabolic rate. Explains that males typically have higher basal metabolic rates due to greater lean mass, that aging reduces metabolic rate, and that genetic factors (enzymes, regulatory proteins) help determine individual metabolic differences.
Modifying Metabolism: Diet, Exercise, and Body Composition
Discusses how changes in diet and exercise can alter lean body mass and metabolic rate, and how these changes impact muscle mass and overall health.
Integrating Metabolism: Oxygen, Mitochondria, Hormones, and Vitamins
Previews key themes explored in detail: why oxygen is required, how mitochondria transfer energy via oxidative phosphorylation, and the roles of “metabolic” hormones and vitamins in coordinating anabolic and catabolic reactions.
To achieve these chapter objectives, actively engage with the material. Learning tissues is not a passive process. Your understanding will grow as you interact with the content, peers, and your instructor. Regularly check your thinking against feedback. Revisit complex concepts until you can confidently explain and apply them on your own.
By the end of this chapter, you will be able to:
- Distinguish between metabolism, catabolism, and anabolism, and relate each to the body’s energy use and storage.
- Describe how carbohydrates are digested and metabolized, including glycolysis, anaerobic vs. aerobic ATP production, the Krebs (citric acid) cycle, oxidative phosphorylation, and gluconeogenesis.
- Explain how lipids are digested, transported, stored, and used for energy, including β-oxidation, lipogenesis, lipolysis, and ketone body formation and use.
- Outline protein digestion, absorption, and metabolic fates of amino acids, including protein synthesis, conversion to glucose or ketones, and nitrogen disposal via the urea cycle.
- Compare the absorptive (fed), postabsorptive (fasting), and starvation states, including the primary fuel sources, hormone regulation (insulin and glucagon), and shifts between carbohydrate, fat, and protein use.
- Describe how the body regulates core temperature (thermoregulation), including the role of the hypothalamus and the mechanisms of heat exchange: conduction, convection, radiation, and evaporation.
- Relate dietary intake (calories, macronutrient balance) to energy balance, fat storage, and the risk of metabolic and diet-related diseases.
- Explain the roles of vitamins and minerals in metabolism and why adequate dietary intake is necessary for proper function of metabolic pathways.
- Identify major factors that influence metabolic rate (age, sex, activity level, lean body mass, and genetics) and how lifestyle can modify metabolic rate and body composition.
- Recognize the central importance of oxygen, mitochondrial energy transfer, and key metabolic hormones and vitamins in sustaining life through anabolic and catabolic reactions.
- 4.1: Chemical Reactions Underlying Metabolism
- Metabolic processes are constantly taking place in the body. Metabolism is the sum of all of the chemical reactions that are involved in catabolism and anabolism. The reactions governing the breakdown of food to obtain energy are called catabolic reactions. Conversely, anabolic reactions use the energy produced by catabolic reactions to synthesize larger molecules from smaller ones, such as when the body forms proteins by stringing together amino acids. Both sets of reactions are critical.
- 4.2: Carbohydrate Metabolism
- Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen atoms. The family of carbohydrates includes both simple and complex sugars. Glucose and fructose are examples of simple sugars, and starch, glycogen, and cellulose are all examples of complex sugars. The complex sugars are also called polysaccharides and are made of multiple monosaccharide molecules. Polysaccharides serve as energy storage and as structural components.
- 4.3: Lipid Metabolism
- Fats (or triglycerides) within the body are ingested as food or synthesized by adipocytes or hepatocytes from carbohydrate precursors (Figure 24.3.1). Lipid metabolism entails the oxidation of fatty acids to either generate energy or synthesize new lipids from smaller constituent molecules. Lipid metabolism is associated with carbohydrate metabolism, as products of glucose (such as acetyl CoA) can be converted into lipids.
- 4.4: Protein Metabolism
- Much of the body is made of protein, and these proteins take on a myriad of forms. They represent cell signaling receptors, signaling molecules, structural members, enzymes, intracellular trafficking components, extracellular matrix scaffolds, ion pumps, ion channels, oxygen and CO2 transporters (hemoglobin). That is not even the complete list!
- 4.5: Metabolic States of the Body
- You eat periodically throughout the day; however, your organs, especially the brain, need a continuous supply of glucose. How does the body meet this constant demand for energy? Your body processes the food you eat both to use immediately and, importantly, to store as energy for later demands. If there were no method in place to store excess energy, you would need to eat constantly in order to meet energy demands.
- 4.6: Energy and Heat Balance
- The body tightly regulates the body temperature through a process called thermoregulation, in which the body can maintain its temperature within certain boundaries, even when the surrounding temperature is very different. The core temperature of the body remains steady at around 36.5–37.5 °C. In the process of ATP production by cells throughout the body, approximately 60 % of the energy produced is in the form of heat used to maintain body temperature.
- 4.7: Nutrition and Diet
- The carbohydrates, lipids, and proteins in the foods you eat are used for energy to power molecular, cellular, and organ system activities. Importantly, the energy is stored primarily as fats. The quantity and quality of food that is ingested, digested, and absorbed affects the amount of fat that is stored as excess calories. Diet—both what you eat and how much you eat—has a dramatic impact on your health.
- 4.8: Terms and Definitions
- This page offers definitions of key metabolism and digestion terms, such as absorptive and postabsorptive states, metabolic reactions, and hormones like insulin. It covers metabolic processes like glycolysis and the Krebs cycle, highlighting essential molecules including ATP and glucose. Additionally, it underscores the significance of enzymes, energy expenditure, and the role of vitamins and minerals in bodily functions.
- 4.9: Chapter Review Questions
- This page outlines strategies for using end-of-chapter multiple-choice questions as a self-assessment tool to enhance understanding and retention of key concepts in biochemistry and metabolism. It emphasizes attempting questions without notes, reviewing missed answers, and revisiting them later.
- 4.10: Applied Thinking Self - Assessment
- This page explores metabolism and energy production, detailing glucose digestion to ATP synthesis, insulin's role in glucose uptake, and the conversion of excess carbs to fat. It addresses diabetes symptoms, mechanisms against pancreatic self-digestion, and ketosis. Additionally, it examines environmental effects on metabolic rate, the consequences of low-fat high-sugar diets, and the significance of vitamins and minerals.
- 4.11: Alternate Text Descriptions
- This page provides an overview of cellular metabolism, detailing the ATP–ADP cycle and the stages of cellular respiration, including glycolysis, the Krebs cycle, and oxidative phosphorylation, which produce significant ATP. It contrasts aerobic and anaerobic glucose metabolism, highlights lipid metabolism and associated pathways, and discusses the synthesis of proteins and urea. The regulation of metabolic states by hormones, such as insulin and glucagon, is explained.
Thumbnail: Poultry, eggs, nuts and legumes provide protein. (CC BY 4.0; via Menu Planning and Food Safety).