Copyright © 2019 Robert Wildman PhD RD.

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ISBN: 978-1-4808-8345-1 (sc)

ISBN: 978-1-4808-8343-7 (hc)

ISBN: 978-1-4808-8344-4 (e)

Library of Congress Control Number: 2019917311

Archway Publishing rev. date: 03/02/2020

ABOUT THE AUTHOR

Dr. Robert Wildman is a native of Philadelphia and currently resides in Dallas, Texas, USA. Dr. Wildman received his PhD from The Ohio State University after earning his MS from Florida State University and BS from the University of Pittsburgh. He is also a registered and licensed dietitian and Fellow of the International Society of Sports Nutrition (ISSN). Rob currently serves as adjunct teaching and research faculty at Texas Women’s University and is also a member of the American College of Sports Medicine and the American Academy of Nutrition and Dietetics. Moreover, Dr. Wildman is the coauthor of Sport and Fitness Nutrition and Advanced Human Nutrition, as well as being the coeditor of The Handbook of Nutraceuticals and Functional Foods.

Dr. Wildman has contributed over thirty book chapters and one hundred research papers and abstracts and is the creator of TheNutritionDr.com as well as the founder of the International Protein Board (www.InternationalProteinBoard.org). The latter is a not-for-profit organization serving as the global authority on all matters related to protein. He continues to work with professional and elite athletes, as well as people from around the world, to achieve their performance, health, and fitness goals. Talking the talk and walking the walk, Rob has presented live on five continents. He enjoys a variety of sports and engages in weight training, running, and mountain biking.

Dr. Wildman’s website: TheNutritionDr.com

Social Media:

• Twitter: TheNutritionDoc

• Instagram: TheNutritionDoc

To – Gage, Bryn, Amber, Cindy, Connie, Gail,

Mark, Lou, Karyl, Chris and Perry.

Eternally—Dave, David, Carol, Karen, and Jack. I miss you daily!

INTRODUCTION

The seeming simplicity of our daily activities is greatly contrasted by the complexity of our true nature. Quite a paradox, no doubt. Simple in that, on the outside, the goals of our body appear few. We internalize food, water, and oxygen while simultaneously ridding ourselves of carbon dioxide and other waste materials. These operations support reproduction, growth, maintenance, and defense. Yet, on the inside, our bodies may seem very complex, as various organs participate in a tremendous number of complicated processes intended to meet the simple goals previously mentioned.

Nutrition is just one part of the paradoxical relationship mentioned above. The objective of nutrition is simple: to supply our bodies with all the necessary nutrients on a regular basis, and in appropriate quantities, to promote optimal health and function. However, in practice, nutrition is far from that simple. There are numerous essential nutrients and controversial nutrients, as well as different conditions, such as growth, pregnancy, weight management, aging, and exercise; all of this prevents nutrition from being a simple topic.

Although we have long appreciated food, it has only been in the more recent years that we have really begun to understand the finer relationship between food and our bodies. For instance, most nutrients have been identified within the last century or so, and right now, nutrition is one of the most prevalent areas of scientific research. Thus, our understanding of nutrition is by no means complete. It continues to evolve in conjunction with the most current nutrition research, and it seems that not a week goes by without hearing about yet another nutrient or rethinking of nutrition application.

Today, nutrition deeply penetrates many aspects of our lives, including preventative and treatment medicine, philosophy, exercise training, and weight management. Our diet has been linked to cardiovascular health, cancer, bowel function, mood, and brain activity, along with many other areas of function and health. We no longer eat merely to satisfy hunger; without doubt, nutrition has become a matter of great curiosity and/or concern for most of us today.

However, a few problems have developed along with this most recent illumination of nutrition. One such problem is that we may have generated too much knowledge too fast. Even though we, as humans, have been eating throughout our existence, the importance of proper nutrition seems to have been thrust upon us suddenly. We didn’t have time to first wade into the waters of nutrition science slowly, increasing our depth comfortably. The reality is that many people feel that they are in over their heads, barely treading water to keep up with the latest information. Sometimes, it’s all that we can do to follow the latest nutrition recommendations without really having the background or access to credible resources needed to truly understand the reasons behind them.

Although nutrition has become a very complex subject, many experts try to present it in an overly simplified manner. Maybe it’s because they believe that people are not interested in the scientific details and simply want to be told what to do. The Nutritionist: Food, Nutrition, and Optimal Health and its parent resource, TheNutritionDr.com, attempt to break that mold and provide more depth and detail. We will spend time laying a foundation for better understanding of nutrition by first covering the basic concepts of science and our bodies in hope that it will make nutrition a simpler subject.

The Nutritionist and TheNutritionDr.com believe that a scientist lurks within each of us. Every day, we ponder the effects of certain actions before performing them. This is the so-called cause and effect relationship, the very basis of scientific experimentation. Furthermore, since most of us give at least some thought to the foods we eat, we are all a special breed of scientist—nutrition scientists! A nutrition scientist is someone who ponders the relationship between food components and the body. Yet, you do not have to work in a laboratory to be a nutrition scientist. All you need is simple curiosity and the dedication of your time to pursue a greater understanding of nutrition. The Nutritionist and TheNutritionDr.com were created in a question-and-answer format to satisfy your curiosity. Please visit TheNutritionDr.com for video content, podcasts, articles, etc., for more information and exploration.

So, here we go. Good luck and good science to you!

CONTENTS

CHAPTER 1     Being Human And Hungry

CHAPTER 2     The Nature Of Food

CHAPTER 3     Carbohydrates Are Our Most Basic Fuel

CHAPTER 4     Fats And Cholesterol Aren’t Really Bad Guys!

CHAPTER 5     Proteins Are The Structural And Functional Basis Of

The Body

CHAPTER 6     Water Is The Basis Of The Body

CHAPTER 7     Energy Metabolism, Body Weight, And Composition

And Weight

CHAPTER 8     Vitamins Are Vital Micronutrients

CHAPTER 9     The Minerals Of The Body

CHAPTER 10   Exercise And Sports Nutrition

CHAPTER 11   Nutrition Throughout Life

CHAPTER 12   Nutrition, Heart Disease, And Cancer

APPENDIX A   Dietary Reference Intakes

CHAPTER 1

BEING HUMAN AND HUNGRY

Have you ever wondered why we (humans) are as we are and do what we do? We are truly remarkable in our capabilities, operations, and functions. Yet, we are just one of millions of different species inhabiting this planet, each with a unique story to tell. And, like our fellow planet-mates, we must abide by the basic objectives of life, which include self-operating and defending ourselves, both externally and internally, as well as metabolizing and nourishing our existence. And let’s not forget the ultimate objective of all life-forms, which is to live long enough to participate in reproduction.

Yet, we are special in that we have large brains, affording us the intellectual capacity to think about ourselves and, in turn, how we are to be nourished. This book leverages that biological advantage. In this chapter, we will begin to explore the very basis of our being and the world we inhabit. This will provide a platform for understanding of what it takes to nourish our bodies for optimal health, fitness, and longevity. We will pose key questions regarding basic scientific concepts relevant to our bodies, such as what they are made of and how they function. To go deeper, visit TheNutritionDr.com.

What is nutrition?

We will start out as simply as possible. The shortest definition of nutrition is the science pertaining to the factors involved in nourishing our bodies. Nutrition hinges upon the special relationship that exists between our bodies and the world we live in. From the moment of conception to the waning hours of advanced age, we live in a continuum to nourish our bodies. We strive daily to bring nourishing substances into our bodies to serve their immediate and extended needs. These nourishing substances are called nutrients, which are chemicals that are used by our bodies for energy or other purposes. Proper nourishment supports body businesses, such as growth, development, movement, defense, injury recovery, disease prevention, and of course, reproduction.

All that we are, ever were, or are going to be, is borrowed from the environment that we inhabit. This unique state of indebtedness is primarily attributed to our nutrition intake. We must be grateful to the earth’s crust for lending us minerals that strengthen our bones and teeth and allow us to have electrical operations that drive nerve and muscle function. We must also pay homage to other life-forms for the carbohydrates that power our operations and for proteins that build our body structure and mechanics.

All too often, we do not truly appreciate the relevance of nutrition to our basic being. But again, please keep in mind that nearly everything we are, were, and will be is either a direct or indirect reflection of our past and current nutrition intake. No matter how much nutrition is oversimplified in advertisements and infomercials, it is without a doubt one of the most complex and interesting human sciences.

How do we begin to understand nutrition?

Any great building must be constructed upon a solid foundation. So, let’s commit to building a solid scientific foundation to begin our exploration of nutrition. To say that differently, before we begin to learn how to nourish our bodies, we need to have a better understanding of what needs to be nourished. Our bodies are the product of nature, and thus they must adhere to the basic laws of nature. In fact, you can think of nutrition as the scientific offspring of more basic natural sciences, namely chemistry and biology. Therefore, understanding the whats, whys, and hows of nutrition will be a lot easier once a few basic areas of chemistry and biology are appreciated.

ATOMS AND MOLECULES MAKE THE MAN—NOT CLOTHES

What is the most basic composition of our bodies?

Let’s say that we had access to fancy laboratory equipment capable of determining the most fundamental composition of any object. If we used it to assess a man or woman, it would generate some interesting data on our most basic level of composition—elements. Elements are substances that cannot be broken down into other substances. Scientists have determined that there are one hundred or so of these elements in nature. Some of the more recognizable elements include carbon, oxygen, hydrogen, nitrogen, iron, zinc, copper, potassium, and calcium. All the elements known to exist can be found on the periodic table of elements, which we have all come across at one point or another in our schooling. Now, imagine that everything that you can think of is merely a skillful combination of these same elements. This includes cars, boats, buildings, clouds, oceans, trees, and of course, our bodies. In fact, our bodies employ about twenty-seven of the elements as displayed in Table 1.1, Elements of Our Bodies.

What is the elemental composition of our bodies?

The late, great Carl Sagan, in his personal exploration of the cosmos, said that we are made up of star stuff. What he meant was that our bodies are made up of many of the very same elements that make up planets and other celestial bodies. Humans, as well as other life-forms on our planet, have borrowed these elements. Interestingly, four of these elements, namely oxygen, carbon, hydrogen, and nitrogen, make up more than 90% of our body weight. Since these elements make up water, proteins, carbohydrates, fats, and nucleic acids (DNA and RNA), it only makes sense that these substances must be the major materials of our bodies. For example, a lean, young adult male’s body weight may be approximately 62% water, 16% protein, 16% fat, and less than 1% carbohydrate. Most of his remaining weight (about 5%) would be attributed to minerals. We will spend a lot more time talking about the finer details of body composition in later chapters.

Table 1.1. Elements of Our Bodies

CELLS ARE THE COMMON DENOMINATOR OF LIFE

How similar are we to other life-forms?

It is obvious that we are not the only life-form or organism residing on this planet. In fact, we are only one of several million different species of organisms. Organisms include everything from mammals, birds, reptiles, and insects to plants, bacteria, fungi, and yeast. But keep in mind that even though a tomato plant and an octopus may seem completely different, they have numerous similarities that strongly suggest a common ancestry for all life-forms inhabiting Earth, which includes humans. On the other hand, we humans have numerous features that are shared with only a few other species, namely apes, and further still, we enjoy other features that no other species enjoys.

What are cells?

Among the millions of species on this planet, the cell is the common denominator. Cells are the most basic living unit. In many species, such as bacteria and amoebae, the entire organism consists of a single isolated cell. But for plants and animals, including us, the organism exists as a compilation of many cells working together. In fact, every adult human is a compilation of some sixty to one hundred trillion cells.

When a human being is conceived, an egg (ovum) from the mother is penetrated by the father’s sperm. This results in the formation of the first cell of a new life. Therefore, everyone you know was only a single cell at first. That cell had to then develop and divide into two cells, which themselves divided to create four cells, and so on. The term cell implies the concept of separation. Each cell can function on its own. In living things comprised of numerous cells, such as humans, individual cells are sensitive and responsive to what is going on in the entire life-form. Therefore, these cells survive as independent living units and cooperatively participate in the vitality of the organism to which they belong.

As a rule of nature, life begets other life, and thus all cells must come from existing cells. This is to say that to create a new cell, an existing cell must divide into two cells. It also suggests that all life-forms on Earth may be derived from the same cell or type of cell. The process of cell division is tightly regulated, and when this regulation is lost and cells divide out of control, cancer can arise.

Figure 1.1. Simple Cell Structure.

CELL STRUCTURE AND ORGANELLES

What do cells look like?

Human cells differ in size and function. Some are bigger; some are longer; some will make hormones, while others will help our bodies move or move things. In fact, there are roughly two hundred different types of cells in the human body. Although these cells may seem unrelated, most of the general features will be the same from one cell to the next. Therefore, we can discuss cells by describing the features of a single cell. General features of a cell are shown in Figure 1.1.

The unique characteristics of different types of cells, including red blood cells, muscle cells, and fat cells, will be described elsewhere. Let’s begin by examining the outer wall or, more scientifically, the plasma membrane of cells. The plasma membrane separates the inside of the cell from the outside of the cell. The watery environment inside the cell is called the intracellular fluid. Meanwhile, the watery medium outside of cells is called the extracellular fluid. It was noted previously that our bodies are about 60 percent water. Of this 60 percent, roughly two-thirds of the water is intracellular fluid, while the remaining one-third is extracellular fluid, which would include the plasma of our blood.

Figure 1.2. Plasma membrane which encloses cells.

What types of substances are found in the intracellular and extracellular fluids?

In our body fluids, we would find small dissolved substances, such as ions, amino acids, and the carbohydrate glucose, as well as larger proteins. The major ions, namely sodium (Na+), potassium (K+), chloride (Cl–), calcium (Ca²+), magnesium (Mg²+), phosphate (PO4³–), and bicarbonate (HCO3–), are also known called electrolytes because they create electrical properties in the fluids in which they are found. Most of us have at least heard of some of the electrolytes because of the composition of our sweat, as well as recipes of common sport drinks.

As shown in Figure 1.2, all of these and other substances will be found in the fluid both inside and outside of cells. However, the concentration of substances dissolved in either fluid varies, and the plasma membrane is bestowed with the awesome responsibility of functioning as a barrier between the two mediums. The concentration of sodium and chloride is more abundant in the fluid outside, while potassium is more concentrated in the fluid on the inside. This sets up the opportunity for these electrolytes to move down their concentration gradients through channels, while oppositely to be pumped against their concentration gradient by energy-requiring pumps.

What would we expect to find inside of our cells?

Immersed in and bathed by the fluid within cells are small compartments called organelles. The word organelle means little organ. Two of the more recognizable organelles are the nucleus and mitochondria. Other organelles include the endoplasmic reticulum, Golgi apparatus, lysosomes, and peroxisomes (see cell Figure 1.1). The various organelles are little operation centers within cells. Each type of organelle performs a different and specialized job (see Table 1.2). Each organelle has its own membrane with many similarities to the plasma membrane. Therefore, as we discuss the nature of the plasma membrane below, you can keep in mind that some of these features also pertain to organelle membranes as well.

Also, within the fluid inside certain cells, we would expect to find some energy reserves in the form of fat droplets and glycogen (carbohydrate). The amount of glycogen and fat will vary depending on the type of cell. Another important component of cells is ribosomes, which are the actual site within cells where proteins are constructed.

Table 1.2 Overview of Organelle Function

Do individual cells, and our bodies, attempt to maintain

an optimal working environment?

Just as you clean your house and determine what kind of stuff is found within your living area, so too will our cells clean and regulate the contents of their intracellular fluid. This allows each cell to maintain an optimal operating environment. Scientists often use the term homeostasis to describe the efforts associated with the maintenance of this optimal environment.

Just as it is the responsibility of each cell to maintain its own ideal internal environment, at the same time, many of our organs work in concert to regulate the environment within our bodies. These organs include the kidneys, lungs, skin, and liver. Many of our most basic functions, such as breathing, sweating, urinating, digesting, and the pumping of our hearts, are functions dedicated to homeostasis. Therefore, homeostasis is the housekeeping efforts of all our cells working individually as well as together to provide an environment conducive to optimal function. Some key homeostasis operations include the following:

• Regulation of the ion (electrolyte) concentrations inside and outside of cells

• Blood pressure regulation

• Regulation of optimal levels of blood gases (oxygen and carbon dioxide)

• Maintaining optimal body temperature

• Regulating blood glucose and calcium levels

• Maintaining an optimal pH level

What is the composition of the cell membranes?

Each cell is enveloped by a very thin membrane measuring only about 10 nanometers (nm) thick. A nanometer is one-billionth of a meter—thin indeed. The makeup of the plasma membrane is a very clever combination of lipids and proteins, with just a touch of carbohydrate and other molecules. The plasma membrane uses lipids to provide most of its barrier properties. Molecules that are somewhat like triglycerides (fat), called phospholipids, are arranged strategically to keep water-soluble substances such as electrolytes (e.g., sodium, potassium, and chloride), carbohydrates, proteins, and amino acids from freely moving from the outside to the inside of cells and vice versa. Meanwhile, gases seem to traverse the plasma membrane with greater ease. The plasma membrane will also contain cholesterol, which is also a lipid. Cholesterol appears to increase the stability of the plasma membranes.

If we were to weigh all the components of the plasma membrane, we would find that about half the weight of the membrane is protein. However, this is a bit misleading, as the much smaller lipid molecules of the plasma membrane tend to outnumber protein molecules by about fifty to one. This means that the proteins tend to be larger and complex, which implies that they have important functions, while phospholipids and cholesterol provide more structural support.

Do proteins in the cell membrane have special roles?

Since the plasma membrane functions as a barrier between the outside and inside of the cell, there must be a means (or doorways) whereby many water-soluble substances can either enter or exit a cell. One of the roles of proteins in the plasma membrane is to function as doorways, thereby allowing substances such as sodium, potassium, chloride, glucose, and amino acids to enter or exit a cell.

Let us go into a little more detail about just how some of the proteins function as doorways in our plasma membranes. Some of these proteins function as channels or pores that allow the passage of a specific substance or substances across the membrane. This is like opening the stadium doors for sport fans before a game. The concentration of fans outside the stadium would be greater (higher concentration), and the natural flow is for the general movement of people into the stadium, where there are fewer people (lower concentration). Scientists refer to the movement of higher to lower concentration as diffusion, unless it’s water; then they refer to it as osmosis.

Plasma membrane channels allow the passage or diffusion of electrolytes, such as sodium, potassium, chloride, and calcium, down their concentration gradient. Moreover, since the movement is typically in relatively large amounts, it results in a sudden and significant change in the charged or electrical environment. So, if you weren’t aware of it, our bodies are electrical. For instance, the basic functioning of our nervous systems and different types of muscles (e.g., skeletal, cardiac, and smooth muscle) is electricity. For our nervous systems, this allows rapid communication and responses throughout the body as well as cognition, thinking, problem solving, memories, and imagination. Meanwhile, for muscle, it stimulates contraction, allowing us to move our bodies and move or hold other things, as well as the movement of contents internally (e.g., digestive tract, blood, airways). One example of a channel you might have heard of is the calcium channel, as certain prescription drugs used to manage blood pressure function as calcium-channel blockers.

Channels or pores are not the only type of protein found in our plasma membranes. Other proteins can function as carriers that can transport substances across the membrane. Here again, substances would be moving down their concentration gradient. These carrier proteins tend to transport larger substances, such as carbohydrates and amino acids. Perhaps the most famous example of a carrier protein is the glucose transport protein (GLUT), which is the primary concern in type 2 diabetes mellitus.

Do some membrane proteins function as pumps?

Not all substances move across the plasma membrane down their concentration gradient. Since this type of movement seems to go against the natural flow, to make this happen, certain membrane proteins must function as pumps. Quite simply, pumps will move substances across membranes against their concentration gradient, or from an area of lower concentration to higher concentration. This is hard work and requires energy to happen. In fact, the energy required for pumps associated with our trillions of cells makes a significant contribution to our daily metabolism. We will break down metabolism elsewhere, but as a primer, the three main metabolic (calorie burning) buckets of our bodies are muscle contraction, pumps, and building molecules.

Are some cell membrane proteins receptors?

Some proteins in the plasma membrane function as receptors for special communicating substances in our bodies, such as hormones and neurotransmitters. Typically, receptors will interact with only one specific molecule and ignore all other substances. In a way, then, these proteins can also be viewed as being involved in transport processes; however, what’s being transported isn’t ions or molecules but information. Receptors generally transfer the intended desired information from the outside of the cell to an internal action inside the cell that is the desired outcome. As an example, a chemical signal from the nervous system can interact with receptors on skeletal muscle cells, which sets in play actions that lead to the contraction of muscle. Or, insulin can interact with a receptor on muscle and fat cells, leading to the movement of a specific glucose transporter (GLUT4) to the cell membrane, allowing for an accelerated removal of glucose from the blood, as touched on previously.

What is DNA?

DNA (deoxyribonucleic acid) is found in almost all the cells of our bodies, with a major exception being red blood cells. Within cells, DNA is mostly housed in the nucleus, while a much smaller amount of DNA can be found in mitochondria. DNA contains the instructions (blueprints) for putting specific amino acids together to make proteins. You see, the human body contains thousands of different proteins, and it is up to cells to build them, using amino acids as the building blocks. Without the DNA’s instructions, our cells would not know how to perform such a task.

DNA is long and strand-like and is organized into large structures called chromosomes. Normally, we have twenty-three pairs of chromosomes in a given nucleus. If we were to take a chromosome and find the end points of the DNA, we could theoretically straighten it out like thread from a spool. If we did so, we would find thousands of small stretches called genes on the DNA. Scientists estimate that there are roughly nineteen thousand to twenty thousand genes, which contain the actual instructions for building thousands of specific proteins, serving largely as the structural and functional basis of our bodies.

To oversimplify one of the most amazing events in nature, when a cell wants to make a specific protein, it makes a copy of its DNA gene in the form of RNA (ribonucleic acid). You see, DNA and RNA are virtually the same thing. However, one of the most important differences is that the RNA can leave the nucleus and travel to where proteins are made in cells, the ribosomes. At this point, both the blueprint instructions (RNA) and the amino acids are available, and it’s the job of the ribosomes to link amino acids together in the correct sequence. Thus, amino acids serve as the building blocks of protein, and the link between them is called a peptide bond.

What does tissue mean, and do our different body tissues work as a team?

Humans are truly a complex array of organs and other tissues designed to support the basic functions and vitality of our bodies. We can process inhaled air and ingested food and regulate body content. We selectively take what we need from the external environment and eliminate what we do not need. We think, move about, and reproduce. Many of these operations occur without us even being aware.

One other term we should be familiar with is tissue. Quite simply, tissue is comprised of similar or cooperating cells performing similar or cooperative tasks. These cells may be grouped together to form fascinating tissues, such as bone, skin, muscle, nerves, and blood. Moreover, tissue working together can form organ systems. (See Figure 1.3.)

Figure 1.3. Main Organ Systems of the Human Body

CELLS NEED ENERGY TO OPERATE

What do cells use for energy to operate?

Cells require energy to operate (metabolize). Energy is provided in foods in the form of carbohydrate, fat, protein, and alcohol. However, these nutrients represent to potential for energy and thus must be processed to a form of energy that cells can use directly. By and large, ATP (adenosine triphosphate) is that energy source and is found within cells in limited quantity. That means that ATP must be made constantly for the cell to operate at its best. Furthermore, those cells with higher energy demands, such as muscle, will need to make more ATP in a given time frame.

Most of the ATP made in our bodies is made in mitochondria (singular: mitochondrion). For this reason, mitochondria are often referred to as the powerhouses of cells. A relatively small portion of the ATP generated in our cells each day will be made in the intracellular fluid outside the mitochondria. As you might expect, cells with higher energy demands will have more mitochondria. This is certainly true for heart and skeletal muscle cells and cells within the liver.

What does the term metabolism mean?

Each second of every day, our cells are engaged in the operations that help keep them alive and well. At the same time, the efforts of each cell also contribute to the proper functioning of the total body. To do so, each cell must perform an incredible number of metabolic operations and reactions every second; the term metabolism refers to them and is reflective of the release of energy as a result. In general, metabolic processes can be grouped into the following buckets: (1) pumping electrolytes and other substances across membranes and against their concentration gradient, (2) making molecules (e.g., protein, lipids, DNA), and (3) movement. Movement includes motion not only of our entire bodies via skeletal muscle efforts, but also of the internal contents, such as the pumping of the heart and the moving of materials along the length of the digestive tract.

The term metabolism is somewhat general and has some utility. For instance, total body metabolism refers to all the energy released from all the chemical reactions and associated processes in our bodies. To say it differently, total body metabolism is the sum of all energy-releasing reactions and operations taking place in all cells collectively. However, if we wanted to describe just those chemical reactions and operations within a specific tissue, such as muscle or bone, we would use the term muscle metabolism or bone metabolism. We can be even more focused and use the term metabolism to describe only those related activities associated with a single nutrient or nutrient class. For example, if we were discussing the reactions that involve only proteins or carbohydrates, we would be discussing protein or carbohydrate metabolism, respectively.

In general, chemical reactions and operations release energy, nearly all of which is converted heat. Since body temperature remains constant, the heat produced in metabolism is removed from the body. Therefore, our total body metabolism can be estimated by measuring how much heat is lost from the body. Researchers can do this in specialized laboratory facilities. Meanwhile, metabolism can also be assessed by measuring how much oxygen is inhaled and used by the body, as described elsewhere.

BONE AND SKELETON PROVIDE THE FRAMEWORK OF THE BODY

What is the skeleton?

The exquisite appearance of the human body is founded upon the skeleton (see Figure 1.4). The human skeleton is a combination of 270 bones at birth, and after some fuse to make bigger bones, the final count is 206 separate bones. In addition to bone, the skeleton includes ligaments, which connect bones, and tendons, which connect bones to skeletal muscle.

The attachment of bones to skeleton allows for body movement. In addition, bones provide protection. For instance, the skull and the vertebrae enclose the brain and spinal cord, respectively, thereby protecting the invaluable central nervous system (CNS). Meanwhile, twelve pairs of ribs extend from our vertebrae and protect the organs of our chest. Bone also serves as a storage site for several minerals, such as calcium and phosphorus, and is the site of formation for many of our blood cells.

Figure 1.4. Human Skeleton.

By about six weeks of gestation, the skeleton is rapidly developing and is visible in a sonogram. Bones continue to grow until early adulthood, complementing the growth of other body tissue. Up until this point, bones grow in both length and diameter. Around this time, the longer bones of our bodies, such as the femur, humerus, tibia, and fibula, begin to lose the ability to grow lengthwise, and our adult height is realized. Some of the bones of the lower jaw and nose continue to grow throughout our lives, although the rate of growth slows dramatically.

As you may expect, the longest, heaviest, and strongest bone in the body is the femur, or thighbone. These bones extend nearly two feet in some of us and provide much of the support we need against the force of gravity. Meanwhile, the three small bones in the inner ear are the smallest bones in our bodies. In addition, the tiny pisiform bone of the wrist is also very small, having the approximate size of a pea.

What is bone?

Our fascination with the fossil remains of dinosaurs and other ancient creatures may lead us to believe that bone is a hard, nonliving part of our bodies and part of the bodies of other animals, including those from long ago. Although bone is indeed solid and strong, allowing form, movement, and organ protection, it is living tissue and is constantly changing.

Bone contains several different types of cells, which are supported by a thick fluid called the matrix. Within the matrix reside proteins, primarily collagen, and to a much lesser degree, other related substances, such as some unique carbohydrates. Also, in the matrix are mineral deposits, largely a calcium- and phosphate-based crystal called hydroxyapatite, as well as calcium phosphate.

Bone is roughly 60%‒70% mineral complexes, and the remaining bone is largely protein, primarily collagen, which contributes about 30%‒35% of bone weight. Hydroxyapatite crystals are like tiny, long, and flat sheets of minerals that lie on top of and along longer collagen fibers. These mineral deposits provide the hardness and compression-resisting properties to bone. For the most part, it is also these mineral complexes, along with some proteins, that exist as fossils long after the death of an animal.

Bone cells called osteoblasts make collagen proteins that form into collagen fibers that are like rope, here in the matrix of bone. Mineral complexes then adhere to the collagen. Collagen makes bone strong, and minerals make it hard! In addition to some cells, proteins, carbohydrates, and minerals, other tissue can be found in bone. For instance, small blood vessels run throughout bone and carry substances to and away from bone. Some nerves can be found in bone as well.

Is bone constantly changing?

Bone is constantly being turned over. Specific bone cells called osteoclasts are constantly breaking down bone components, such as proteins and mineral complexes. Meanwhile, this is counterbalanced by the actions of osteoblasts. Although this may seem counterproductive, its merit lies in the ability of bone to adapt or be remodeled according to the demands placed upon it. For example, one of the benefits of weight lifting is an increased stress placed on bone, which causes the bone to adapt by increasing its density. In this case, the efforts of cells that build bone will exceed the efforts of cells that will break down bone components. Conversely, prolonged exposure to zero gravity (weightlessness) in outer space will decrease the stress placed upon bone, resulting in a loss of bone density. In this situation, the efforts of cells that break down bone will exceed those efforts of cells that build bone components.

NERVOUS TISSUE IS ELECTRICAL AND EXCITABLE

What is nervous tissue?

Nervous tissue is comprised mostly of nerve cells, or neurons, which serve as the basis for an extremely rapid communication system in our bodies. It also provides the basis for thinking. The central nervous system includes the brain and spinal cord and represents the thinking and responsive portion of our nervous tissue. Links of neurons extend from the central nervous system to various organs and tissues in our bodies, thus allowing the central nervous system to regulate their function. Meanwhile, links of neurons extend to our skeletal muscle, thereby allowing the central