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
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2 thoughts on “Nutrition Tuesday: What You Need to Know About Human Metabolism

  1. Man, remembering all those cycles was a pain in the ass. Not sure if you're as dorky as me but every time I feel like I'm going to bonk or cramp I picture those cycles animated in my head.

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