Nutrition Tuesday: Proteins!

Protein. My favorite topic. You want to see me get my panties all in a bunch. Let’s talk about protein. If you went out right now and asked 10 people why you eat protein and what it does for your body, I bet all but perhaps a couple people will get the question wrong…. 
Protein is one of the three macronutrients; however, protein is not a sufficient source of energy used by the human body. However, under certain circumstances, dietary protein and/or certain amino acids can have very important roles in muscle metabolism and exercise performance(1). Proteins are similar in molecular structure to fats and carbohydrates, expect for one defining characteristic – proteins contain nitrogen atoms. The word amino literally means “nitrogen containing(1).” Structurally, proteins consist of various lengths and combinations of amino acids that are linked together by peptide bonds. 
Proteins have many functional roles in the human body: 
Specific Role in Human Body
After a protein is degraded (broken down), some amino acids can be changed structurally to form glucose
Growth and maintenance
Proteins are found in numerous body structures, including hair, skin, tendons, muscles, organs, etc.
Some hormones are classified as proteins, such as insulin, glucagon, prolactin and growth hormones
Enzymes are proteins that speed up chemical reactions
Antibodies are proteins produced by specific immune cells to help fight infections
Acid-base balance
Hemoglobin (a protein) not only carries oxygen, but serves as a blood buffer to help regulate pH
Fluid balance
Albumin and globulin (blood proteins) help draw fluid into capillary beds 
Some proteins carry specific substances (i.e. hemoglobin carries oxygen)
The human body begins to digest protein in the stomach. The enzyme pepsin cleaves the peptide bonds that hold amino acids together creating smaller peptides (short chains of amino acids) and some free amino acids. Once the contents of your stomach reach your intestines, enzymes from the pancreas and intestines will finish cleaving the peptide chains to absorbable amino acids(1). Amino acids are then absorbed by the small intestine into the bloodstream. Studies have suggested that about 95% of ingested animal proteins and about 85% of ingested plant proteins are absorbed by the body from one meal, but no one is really certain for sure(1). Before amino acids can be used for energy by the body, it undergoes a reaction to remove its nitrogen-containing compounds. 
There are 20 unique amino acids that make up various proteins. Nine are called essential amino acids, meaning that the human body does not produce these amino acids and we must obtain them through our diets. The remaining 11 are considered nonessential because the human body can synthesize them. 

Much of the debate surrounding protein involves how much should you consume and what types. Traditionally, only animal proteins, such as milk, eggs, meat, and fish, have been considered “complete” protein sources (containing all the essential amino acids). Plants are considered “incomplete” because they lack specific essential amino acids. However, soy is considered a “complete” protein(1). Any vegetarian or vegan can obtain an adequate amount of protein (and all the essential amino acids) through their diet by consuming various food choices throughout the day. Interesting enough, greens have the highest percentage of amino acids per ounce of any food, but since they don’t weigh much, they need to be eaten in greater amounts(2). 
The amount of daily protein intake is much debated. It really varies depending on your weight and what your daily activities are. Currently, the RDA for protein in healthy adults is 0.8 g/kg body weight per day(3). The International Society of Sport Nutrition suggests the exercising individuals ingest protein ranging fro 1.4 to 2.0 g/kg/day(3). They suggest that endurance athletes consume 1.0 to 1.6 g/kg/day depending on the intensity and duration of the endurance exercise. Recommendations for strength/power athletes typically range from 1.6 to 2.0 g/kg/day(3). 
To figure out your protein requirements is quite easy. It’s just a simple math equation. I will use myself for an example. I currently weigh 125 pounds or roughly 57 kg (1 lb = 0.45 kg). I am an endurance athlete with a fairly intense and long training schedule, although it varies day to day. I am going to use 1.3 g/kg/day as my goal protein consumption.
57 kg X 1.3 g/kg = 74 g of protein per day
One relatively new development in sports nutrition is the knowledge that nutrient timing influences the physiological responses to exercise(1). Studies have shown that after exercise a 4:1 or 5:1 carb to protein ratio food or recovery drink is optimal for resynthesis of muscle protein and maintenance of other physiological structures that rely on amino acids, such as the nervous system(4). (This is a topic I plan on discussing in more detail in the future
I’m not a huge fan of the Paleo Diet, but I did read The Paleo Diet for Athletes. One interesting section I found in the book was about why our ancestors chose to eat 6- to 8-ton elephants when they could have easily eaten prey like rabbits, partridges, and fish. Well, it’s because if you eat just protein and way too much of it, it can kill you. Laboratory studies have found that the maximum amount of protein humans can consume on a daily basis is about 40% of our daily calories(4). Anything above that, you become sick. Our earliest settlers learned that the hard way in what they referred to as “rabbit starvation.” Apparently, after eating enormous quantities of very lean meat, they would become nauseated and irritable, lose weight, develop diarrhea, and eventually die(4). What a way to go, huh? Have you ever wondered why you eat lobster with lots of melted butter? It’s because lobster is extremely lean (84% of its energy is protein) and could easily cause poisoning if that’s all you ate! So break out that tub of butter! 
Don’t worry, I will be talking about protein and amino acids in much more detail in the future so stay tuned for some good posts coming up!
  1. Antonio J et al. Essentials of Sports Nutrition and Supplements. Totowa, NJ: Humana Press; 2008.
  2. Brazier B. Thrive: The Vegan Nutrition Guide to Optimal Performance in Sports and Life. Philadelphia, PA: Da Capo Press; 2007.
  3. Campbell B et al. International Society of Sports Nutrition position stand: protein and exercise. Journal of the International Society of Sports Nutrition. 2007; 4:8.
  4. Cordain L, Friel J. The Paleo Diet for Athletes. Rodale; 2005. 
(Disclaimer: Like always, this is for your information only. If you are concerned about your health and diet please seek out professional help from your medical provider and/or registered dietitian.)

    Nutrition Tuesday: Fats Aren’t Evil!


    Fats seem to be a dirty word to many people. Before the low-carb focus in diet trends, dietary fat was the primary target of public and academic condemnation. It is important, especially for athletes, to understand the basic chemistry of fat (not all fats are created equally!) and the metabolism of fat. First, let’s look at some definitions to clear up the air about evil fats…

    Lipids – A class of molecules that are composed of triacylglycerols, sterols, and phospholipids.
    Triacylglycerol (also called triglyceride) – A glycerol “backbone” molecule with three fatty acids. One gram of triacylglycerol (TAG) provides 9 kcal of energy to the body when utilized.
    Glycerol – A three-carbon molecule that is “backbone” of TAGs. By itself, glycerol is a “sugar” and when released from storage, it can be recycled by the liver to create new blood glucose.
    Fatty acids – Chains of carbon atoms of various lengths that attach to a glycerol molecule to form TAGs. A fatty acid with no double bonds is called a saturated fatty acid. A fatty acid with one double bond is called a monounsaturated fatty acid and a fatty acid with two or more double bonds is called a polyunsaturated fatty acid.

    An example of an unsaturated TAG

    Fats are interesting in how they are stored in the body. Once fats are digested, they are packaged into chylomicrons, which are lipoprotein particles that transport dietary fats from the intestines to other parts of the body1. Once the digested fats have reached their targeted cells, they are either stored or used depending on the person’s physiological state. The infamous “fight or flight” hormones (i.e. adrenaline) and muscular contractions (only up to a certain point though) induce fat breakdown and burning in the processes called lipolysis and oxidation. If the body is in the “restive-digestive” state, the hormones secreted during this stage and the relative lack of muscular activity induce fat building/storage called lipogenesis1
    Dietary fats are physiologically different than other major micronutrients because fatty acids can be incorporated into the phospholipid bilayer of the cell membrane, which ultimately can lead to membrane fluidity changes and different physiological effects of the body, such as inflammation or blood clotting. What is the phospholipid bilayer of the cell membrane you ask? Well, think back to your high school biology days… okay, depending on your age, your biology book might not have covered the phospholipid bilayer. In a nut shell, a phospholipid bilayer is a think polar membrane made up of two layers of lipid molecules. Cell membranes of almost all living organisms and many viruses are made of a lipid bilayer. It serves as a barrier to keep ions, proteins, and other molecules where they need to be and prevent them from “running away.” Phospholipids have a “water-loving” head and a “water-hating” tail. Thus the “water-loving” heads face out and the “water-hating” tails always face inward. And that’s all you really need to know about that, but I could go on for days about the topic.

    Here’s a nerdy video of the plasma membrane 🙂
    Certain fatty acids are known to be essential, sort of like certain amino acids, because the human body is unable to create them in the body; thus, these fatty acids must be obtained through diet. These essential fatty acids are called linolenic (Omega 3) and linoleic acid (Omega 6). 
    Omega-3 and omega-6 fatty acids are important in the normal functioning of all tissues of the body. Adequate intake of these essential fatty acids have shown in studies to help prevent atherosclerosis, reduce the incidence of heart disease and stroke, and reduce joint pain among other things2

    Stored cellular fat in the form of TAG is the primary fuel source during fasting periods lacking intense physical exertion1. Research has shown that about 60% of a human’s “fuel mix” at rest – in a fasted state- is from fat1. So, now you think you can become a couch potato and burn fat! Sounds great doesn’t it? But, that’s not how it works. Just sitting there on the couch and watching television requires very little caloric expenditure and thus “60% of nothing is still nothing!”1 So get off your butt and start moving!
    The use of fatty acids during physical activity is an interesting relationship. A couple weeks ago I discussed metabolism. If you need a review, click HERE. There is a direct relationship between fat use and exercise duration. The longer you exercise at a low-moderate pace (i.e. your endurance pace and/or Zone 2 heart rate), the greater your fat breakdown and “burning.” However, there is an inverse relationship between fat use and intensity. The more intense you exercise, the less stored fat can contribute (i.e. your body switches to mainly carbs). 
    It is important to include healthy fats (i.e. monounsaturated and polyunsaturated fats) and in your diet because fats serve the following functions in your body:
    • Fat is used by your body for fuel because it is the most energy-dense macronutrient and they provide most of the body’s tissues and organs (including your heart!) with their energy1
    • Cell membranes are composed of phospholipids
    • Fats are critical for the transmission of nerve signals that generate muscle contractions1
    • Serve as a transporter for vitamins A, D, E, and K1
    • Provide cushioning for the protection of vital organs and insulation from cold environments1
    1. Antonio J, et al. Essentials of Sports Nutrition and Supplements. Totowa, NJ: Humana Press, 2008.
    2. Physicians Committee for Responsible Medicine. Essential Fatty Acids. Available at: Accessed June 4, 2012.   
    (Disclaimer: Once again this is for your information only. If you feel you need help with your diet and health then I urge you to seek professional medical and nutrition advice from an expert.)

    Nutrition Tuesday (Wednesday): A Look at Carbohydrates

    Yesterday I gave an overview of each of the macronutrients needed by the human body for survival. Today I will provide a more in-depth discussion of carbohydrates. I could spend days talking about carbohydrates because there are so many things to talk about, but today I just plan on discussing what exactly a carbohydrate is and what it does in the human body. In the future I plan to discuss other topics such as the glycemic index and how much carbohydrates you should include in your diet (although you should really consult with a registered dietitian to figure out what is best for your body and health). 
    Carbohydrates are groups of molecules that are comprised of carbon, hydrogen, and oxygen atoms. Carbohydrates are subdivided into three main categories: monosaccharides, oligosaccharides, and polysaccharides(1). 
    Monosaccharides (“mono” = one, “saccharide” = sugar) are the simplest carbohydrate molecules. These simple carbohydrates often have a sweet taste and are classified by the number of carbons that make up their physical structure(1). The three most important nutritional monosaccharides are glucose, fructose, and galactose. Glucose is the most common mechanism for transport of carbohydrates in the body and some people refer to glucose to as “the blood sugar”(1). Glucose is a naturally occurring sugar in many foods and also is the end product of the breakdown of more complex carbohydrates. Glucose can be easily broken down by the body and can be 1) used as a fuel substrate to supply energy to the body, 2) stored as muscle or liver glycogen, or 3) converted to triglycerides and stored for later use by the body(1). 

    Fructose is another simple carbohydrate. It is found in fruits, table sugar, honey, and high fructose corn syrup. Table sugar, corn syrup, and honey all contain glucose and fructose, but in differing amounts. During digestion, table sugar is broken down into 50% glucose and 50% fructose(2). High-fructose corn syrup (HFCS) digests into about 55% fructose and 45% glucose. Fructose has come under scrutiny in recent years because of the obesity epidemic in our country. HFCS is made using chemical processes that first convert cornstarch to corn syrup and then convert 42-55% of the glucose to fructose to make it taste sweeter(2). Some research has indicated that fructose is digested, absorbed, and metabolized differently than glucose in ways that favor fat production. The jury is still out on fructose; however, it’s best to avoid soda and other crap that contains HFCS. 
    When monosaccharides bond together, they can form more complex carbohydrates referred to as oligosaccharides. A majority of oligosaccharides are disaccharides, or double sugars. All disaccharides contain glucose and generally are represented by sucrose, lactose, and maltose(1). Sucrose is commonly called table sugar, beet sugar, or cane sugar. Sucrose contains one glucose and one fructose bonded together. Another common disaccharide is lactose, which is formed from the bonding of glucose and galactose. Lactose is commonly referred to as the “milk sugar” because it is only found in milk from lactating animals(1). Lactose can be a difficult disaccharide for some people to digest (like me!) and can result in excessive fluid and gas build up in the bowels, bloating and cramping(1). The third most common disaccharide is maltose, which is created by the bonding of two glucose molecules. Maltose is commonly called malt sugar and is produced when seeds sprout(1). The sprouting process of plants can be altered by heat in the process called malting and is the first step in the production of alcoholic beverages. 

    Polysaccharides are considered to be complex carbohydrate molecules. Generally, polysaccharides are linear or complex branching chains that are composed of more than 10 monosaccharides bonded together. The two most common plant polysaccharides are starch and fiber. Starch is the storage form of carbohydrates in plants and can be digested by humans. Fiber is considered a non-starch structural polysaccharide and may be the most abundant organic molecule on the plant, but it cannot be digested by humans. The primary animal polysaccharide is glycogen. Glycogen is either stored in the skeletal muscle or liver and is composed of subunits of glucose(1). One unique characteristic of glycogen is its branched structure. Generally, a glucose branch occurs on every 8-12th glucose subunit. This branching of glycogen is very important because it allows for very rapid breakdown of the glycogen molecule into its glucose subunits to be used for energy by the body. 


    As previously discussed last week, carbohydrates, mainly in the form of glucose, are used by the body as fuel. Glycolysis is the metabolic pathway that results in the breakdown of blood glucose to produce ATP. Glycogenolysis (which I did not discuss last week) is the process which glycogen stores are broken down to produce ATP. Glycogenolysis is essentially considered the same pathway basic pathway as glycolysis, but each differ in the entry point in which they enter into the metabolic pathway. Generally, because of each enters the pathway, glycogen yields three molecules of ATP compared to glucose metabolism of two ATP molecules(1).


    1. Antonio J et al. Essentials of Sports Nutrition and Supplements. Totowa NJ: Human Press, 2008.
    2. Clark N. Nancy Clark’s Sports Nutrition Guidebook, 4th Ed. Champaign, IL: Human Kinetics, 2008.

    Nutrition Tuesday: Overview of Macronutrients and Micronutrients

    Last week we discussed the basics of metabolism. Now it’s important to discuss what nutrients fuel the body to not only get us through exercise, but our day-to-day activities for survival. Our bodies require two different types of nutrients: macronutrients and micronutrients. 
    Macronutrients are carbohydrates, fats, and proteins, which ultimately provide the energy necessary to maintain body functions at rest and during physical activities and maintain the body’s structural and functional integrity1
    Micronutrients are vitamins and minerals. As their names imply, macronutrients comprise most of a person’s dietary intake, while micronutrients are essential in much lower quantities. With the deficiency of micronutrients, athletic performance in addition to normal physiologic function will suffer. However, with a well-balanced diet, a person should not have to worry about any imbalances. 
    Today’s post will give an overview of each type of nutrient required by the human body. The next few days this week I will post a more in-depth look at carbohydrates, fats, and proteins. Next week I will discuss vitamins and minerals.
    The Macronutrients
    Carbohydrates often get a bad name, but without question, wholesome forms of carbohydrates are the best choices for fueling your muscles and promoting good health. Carbohydrates, as their name suggests, are carbon-, hydrogen-, and oxygen-based molecules that are abundant in most plant foods, especially fruits and grains1. Not all forms and sources of carbohydrates are alike. The carbohydrate family includes both simple and complex carbohydrates2. Simple carbohydrates are monosaccharides (structurally the simplest form of carbohydrates) and disaccharides (two monosaccharides). Glucose, fructose, and galactose are monosaccharides or sometimes referred as the simple sugars2. The three most common disaccharides are sucrose (table sugar), lactose (milk sugar), and maltose (malt sugar)2
    Complex carbohydrates are formed when sugars link together to form long complex chains, similar to a string of pearls. Plants store extra sugar in the form of starch, which is a complex carbohydrate. Humans store extra glucose mostly in the form of muscle glycogen and liver glycogen. This glycogen will become available for energy during exercise. 

    The main functions of carbohydrates are:
    • The primary function is to provide energy to the cells of the body, particularly the brain
    • Facilitate the body’s metabolism of fat
    • Spare muscle protein
    Lipid is the collective name given to a vast variety of water-insoluble chemicals, including fats and oils. Fat or lipids are made up of carbon, hydrogen, and oxygen atoms. The ratio of oxygen to carbon and hydrogen is much lower in lipids than in carbohydrates, and thus lipids are a more concentrated source of energy1. There are three major types of fatty acids that can be distinguished by their molecular bonds and number of hydrogens. Fats can be saturated (the maximum number of hydrogens), monounsaturated (having one carbon-carbon double bond), or polyunsaturated (having two or more carbon-carbon double bonds)1

    The main functions of fats:
    • Fats provide many of the body’s tissues and organs (including the heart) with most of their energy. Fat is the ideal fuel because it contains almost twice the energy as glucose, weighs less, and is easily transported and stored1.
    • Essential for the transmission of nerve signals that generate muscle contraction.
    • Serve as a transporter for vitamins A, D, E, and K.
    • Provide cushioning for the prevention of vital organs and insulation from thermal stress of cold environments.
    Proteins are essential nutritionally because they are comprised of amino acids, which the body needs to synthesize its own proteins and nitrogen-containing molecules that make life possible1. Amino acids are the building blocks of proteins. There are 20 amino acids. Of these 20 amino acids, 9 are considered to be essential because the human body cannot synthesize these amino acids. The remaining 11 amino acids are considered nonessential because the human body can synthesize them. 

    The main functions of proteins:
    • Produce antibodies for the immune system
    • Produce enzymes that are required for various chemical reactions in the body
    • Component of structural hormones:
      • Contractile proteins for muscle tissue (i.e. actin and myosin)
      • Fibrous proteins in connective tissues (i.e. collagen, elastin, and keratin)
    • Component of transport proteins (i.e. hemoglobin)
    • Component of peptide hormones (i.e. insulin, thyroid hormone, etc.)
    • Source of fuel when muscle glycogen levels are low due to prolonged intense exercise 
    The Micronutrients
    Vitamins are metabolic catalysts that regulate biochemical reactions within the body2. They are found in plants that we eat and are created by the plants themselves. Vitamins are categorized into either water-soluble or fat-soluble vitamins. Water-soluble vitamins are found in the fluid portion of our bodies and do not accumulate to a large degree in the body1. Fat-soluble vitamins are stored in the lipid (fat) portion of our bodies and can accumulate in the cells1. Some vitamins include: Vitamin B6, Vitamin C, Vitamin D, and Vitamin A.
    Minerals are natural substances that plants must absorb from the soil2. The human body uses minerals for many different jobs, including building bones, making hormones, and regulating the heartbeat. There are two kinds of minerals: macrominerals and trace minerals3. Macrominerals include calcium, phosphorus, magnesium, sodium, potassium, chloride and sulfur3. Trace minerals include iron, manganese, copper, iodine, zinc, cobalt, fluoride and selenium3

    1. Antonio J et al. Essentials of Sports Nutrition and Supplements. Totowa NJ: Human Press, 2008.
    2. Clark N. Nancy Clark’s Sports Nutrition Guidebook, 4th Ed. Champaign, IL: Human Kinetics, 2008.
    3. MedlinePlus. Minerals. Available at: Accessed May 28, 2012. 
    (Disclaimer: This is for your information. If you need help with your diet and developing healthy lifestyle choices then I suggest seeking out professional help from your medical professional or registered dietitian. If you see any errors, please let me know!)

      Nutrition Tuesday: What You Need to Know About Human Metabolism

      NOTE: I have decided to start a series of blog posts on Tuesdays called “Nutrition Tuesdays.” My goal is to educate more people on the importance of proper nutrition and also dispel common myths that plague the health and fitness industry. I would like to point out that I am not a registered dietitian or nutritionist (although I hope to be in a couple of years!) and thus if you have any serious concerns or questions about your health please seek professional advice. My goal here is to share information as I learn it through my studies.

      Metabolism is essential to sustain life. A majority of people do not understand human metabolism and thus give out bad advice or misinformation. Human metabolism is a very complex topic and one that best needs a degree in biochemistry to fully understand. As someone with a biochemistry degree I free comfortable and qualified to give you the basic understandings of human metabolism to help you better understand the importance of proper nutrition in sports and life. 
      Metabolism is a series of chemical reactions that occur at the cellular level in your body that convert fuel from food into the energy your body needs to do everything from moving to thinking to growing and even sleeping. Metabolism is a constant process that begins when we’re conceived and ends when we die. At any given time, there are thousands of metabolic reactions happening throughout your body that keeps your cells healthy and working!
      Metabolism actually begins with plants! Through photosynthesis, plants take energy from the sun and create sugars from water and carbon dioxide. 
      6CO2 + 6H2O + light energy -> C6H12O6 + 6O2
      When people and animals eat plants they take in energy in the form of sugar along with other important vitamins and minerals. Eating lots of fruits and vegetables are important in your diet! Once food is digested it must be broken down in order for the body to absorb it and use it to fuel its cells. 
      Molecules in our digestive system called enzymes break foods down into their building blocks, or simplest forms. Proteins are broken down into amino acids, fats are broken down into fatty acids, and carbohydrates are broken down into simple sugars. These “building blocks” can then be absorbed into our bloodstream and distributed to cells throughout our bodies. 
      There are actually two forms of metabolism and the body has to perform a balancing act between the two. Anabolism (constructive metabolism) involves changing small molecules into larger, more complex molecules of carbohydrate, protein, and fat. Catabolism (destructive metabolism) is the process that produces the energy that all cells require to function. Cells break down large molecules to release energy. This energy provides fuel for anabolism, heats the body, and enables the muscles to contract and in return move your body! 
      Now for the really fun stuff! As a biochemistry major in college I had to memorize the various cycles of metabolism, but I’m just going to give you a general overview because you really don’t need to know the specific enzymes and substrates that are involved in the processes. Carbohydrates, lipids (fats), and proteins constitute the majority of foods we eat. As mentioned above, carbohydrates are broken down into monosaccharides, or simple sugars. Glucose is the main source of fuel of the body; however, it is not the sole fuel for metabolism! Lipids or fats are broken down into monoacylglycerol and long-chain fatty acids. Proteins are broken down into small peptides and amino acids.   
      Adenosine-5’-triphosphate (ATP) is often called the “molecular unit of currency” of intracellular energy transfer because it transfer chemical energy within cells for metabolism. There are two forms of ATP synthesis: oxidative phosphorylation and substrate-level phosphorylation. 
      Oxidative Phosphorylation
      Oxidative phosphorylation is the main mechanism of ATP synthesis in most human cells. If you think back to your high school chemistry class you remember that all atoms, and thus all molecules are made up of protons, neutrons, and electrons. The exchange or sharing of electrons between two or more atoms is the main cause of chemical bonding. You may also recall reduction-oxidation, or redox reactions. Oxidation is the loss of electrons and reduction is the gain of electrons. You can remember this as LEO (lose electrons oxidation) says GER (gain electrons reduction). During oxidative phosphorylation, electrons are transferred from electron donors to electron acceptors. These redox reactions release energy that is used to form ATP. In humans, these reactions take place in the mitochondria of cells, or commonly referred to as the “cellular power plants.” These redox reactions are carried out by a series of protein complexes in the mitochondrial walls that is ultimately called the electron transport chain (ETS). The energy released by electrons flowing through the ETS is used to transport protons (the positive charge) across the mitochondrial walls to generate an electrical potential across the wall. The protons are allowed to cross the wall through a large enzyme called ATP synthase. This enzyme uses the energy from the protons to generate ATP from adenosine diphosphate (ADP) by adding a phosphate group through a phosphorylation reaction. 
      This ETS is actually in a plant cell because of the Photosystem, but it is pretty much the same pathway as in an animal cell. Note: This is a simplified verison of the real pathway.
      Substance-level Phosphorylation
      In aerobic (with oxygen) respiration all product’s produced by the degradation of carbohydrates, proteins, and fats converge to a central metabolic pathways called the Tricarboxylic Acid Cycle (TCA Cycle). It is also called the Kerb’s Cycle or the Citric Acid Cycle. Through the catabolism of sugars, fats, and proteins, a two carbon product called acetyl-CoA is produced. In the TCA Cycle, acetyl-CoA is oxidized to CO2. The cycle consists of 8 reactions that ultimately produces ATP and reducing agents that will donate their electrons in the ETS. For each molecule of glucose, two ATPs are produced indirectly from guanine triphosphate (GDP). However, the number of ATPs and amount of reducing agents will depend if fat or protein enters the TCA cycle. All of the molecules that enter the TCA cycle must be converted to acetyl-CoA before any reactions can occur. 
      Here is a good animation of the TCA Cycle: How the Krebs Cycle Works

      Glycolysis is the breakdown of carbohydrates, either glycogen stored in the muscle or glucose delivered in the blood, to produce ATP. During glycolysis, one glucose molecule is degraded into two pyruvate molecules. During the initial phase of glycolysis, two molecules of ATP molecules are actually consumed to active glucose and another molecule, called fructose-6-phosphate. The end result of glycolysis is 2 ATP molecules and two pyruvates, which are transported to the mitochondrial matrix to be converted to acetyl-CoA. 
      Fat Oxidation
      Fats are broken down to fatty acids where they can circulate and enter muscle fibers. Free fatty acids enter the mitochondria, where they undergo beta oxidation, a series of four reactions that break fatty acids down and form acetyl-CoA. 
      Amino Acid Oxidation
      Proteins are not a significant source of energy, but can be degraded into amino acids by various metabolic processes. Before amino acids can be metabolically useful, the nitrogen must be removed. Also, 20 different amino acids exist and each one has its own unique degradation pathway. The major amino acids that are oxidized in skeletal muscles appear to branched-chain amino acids (leucine, isoleucine, and valine).  
      Here is a summary of Human Metabolism
      Metabolism is a complex, multistep biological system that is essential to life. It is important to understand that energy stored in the chemical bonds of ATP is used to power muscular activity. The replenishment of ATP in human skeletal muscle is accomplished by three basic energy systems: (1) oxidative phosphorylation, (2) glycolysis, and (3) oxidation via the TCA Cycle. All three energy systems are active at any give time, but, the extend to which each is used depends primarily on the intensity of the activity and secondarily on its duration. I will discuss that more in future weeks. 

      (Author Note: If you actually read through this and didn’t think “What the heck! I thought this was suppose to be a triathlon blog!” Then congratulations! I wrote this post to provide an overview of the metabolism system your body uses to make fuel while you are at rest, doing your daily activities, and working out. I tried to make this blog post super simplified for non-science people, but it was hard to leave out certain terms. This information is important to at least understand the basics on because it will help you to understand my future posts and nutrition in general. If you have any questions or notice an errors in any of the information please leave me a comment! Enjoy!)

      1. Berg JM, Tymoczko JL, Stryer L. Biochemistry, 6th Ed. New York: W.H. Freeman and Company, 2007.
      2. Da Poian AT, El-Bacha T, Luz MRMP. Nutrient Utilization in Humans: Metabolism Pathways. Nature Education. 2010; 3(9):11. Available at  Accessed May 18, 2012.
      3. Coburn JW, Malek MH. NSCA’s Essentials of Personal Training, 2nd Ed. Champaign, IL: Human Kinetics, 2012