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Metabolism

Body Science

 

We’ve all heard someone blame their excessive weight on having a slow metabolism, or their ability to eat large amounts of food and not gain any weight on their fast metabolism. But what exactly is metabolism? Technically speaking the word metabolism is defined in biology as the chemical processes that occur within a living organism in order to maintain life. When most people are discussing metabolism though, what they’re actually referring to are just a few of those chemical processes, specifically the ones that convert the macronutrients in the food and drinks we consume into energy or Calories, and then taking into consideration how long it takes for our bodies to use that energy. In this way, our food is our fuel. The molecule known as ATP (Adenosine Triphosphate) contains this energy within its structural bonds. For this reason, ATP is known as the energy currency of the body.

The energy created through metabolic systems is either used immediately or stored in the body for later use, as glycogen, or triglycerides. Your metabolic rate is a measure of the speed at which your body uses, or burns off those calories. This rate is different from person to person and can be quicker or slower based on a number of things like the amount of muscle a person has, how often a person eats, the amount and quality of sleep a person gets, and of course the amount of exercise a person does. We discuss each of these things in detail a bit later.

Creating Energy: Aerobic & Anaerobic Respiration
Our bodies use three different biochemical processes referred to as systems, to metabolize food into energy. One of these systems is aerobic, which means it uses oxygen, and the other two are anaerobic, which means they don’t use oxygen.

The three processes, or systems by which our body makes energy have relatively complicated names. Collectively, breaking down glucose and creating energy is referred to as cellular respiration. The two anaerobic systems are called the ATP-PC System (also known as the phosphocreatine, or creatine phosphate system), and the Lactic Acid System (also known as the glycolytic system). The third system, which uses oxygen, is simply called the Aerobic System.

At any given time your body uses each of our three systems simultaneously. Depending on the amount of work you’re doing and the amount of energy required in doing that work, one system will take more responsibility in creating energy needed, over the other two systems.

It’s not necessary to remember the little details of these systems, the names or even the chemical reactions. I found even in writing the following, I grew tired of the technicalities – so I imagine it’ll be such while reading. As you read through about the following systems, please don’t get stuck on the details but instead try to take in the big picture and overall messages within the paragraphs.

ATP-PC System
Recall that ATP is the energy currency of our bodies. In order to dash across the street, or to quickly catch an unexpected ball being tossed your way, your body requires readily available, easy access energy. A small amount of ATP is stored within our muscles, so that we are able to perform such shorts bursts of work. In fact, during the first few seconds of any exercise, regardless of intensity, this ATP-PC system is relied on almost exclusively. The ATP-PC system uses the ATP molecules that are stored within our muscles, in order to create the energy we need for first 20 or so seconds of all physical work. Because there is only a small amount of ATP stored within our muscles, this system is only able to generate enough energy for short, sharp movements such as a sprint, or to initiate physical work while another energy system starts to initiate (at a slower pace). The ATP-PC system does not require oxygen to produce energy – which explains why for most healthy people, a quick sprint across the street doesn’t have us stopping to catch our breath. This also explains how being out of breath after walking up a flight of stairs may indicate an unhealthy, inactive metabolic system.

The Lactic Acid System
If you do physical work that lasts longer than a few seconds, the Lactic Acid system will kick in, in order to provide you with the energy necessary. The lactic acid system uses carbohydrates in their simplest form as glucose, to create more ATP or energy. This system is where a process called glycolysis occurs. The word glycolysis simply means to break down glucose. This breakdown happens within our mitochondria, which are tiny organelles found inside your muscles. Mitochondria are lovingly referred to as our body’s powerhouses because mitochondria are to the body creating energy, what an energy plant is to a community, creating electricity. The process of glycolysis breaks glucose down into ATP. It can do this with or without using oxygen. The Lactic Acid system uses the process that does not use oxygen, known as anaerobic glycolysis. This way of producing energy can only release about 5% of the ATP that’s contained within a glucose molecule, and it creates a lot of byproducts or waste products in the process. One of these byproducts is Lactic Acid, thus the name of this system. While the Lactic Acid system works for short duration exercises lasting less than a few minutes, it’s not efficient enough to create the energy required for longer duration physical work like long distance running or biking. This system is the primary system used while doing things like weight lifting, as we’re doing moderate amounts of work usually in sets that last less than a few minutes long, before taking a break and starting a new set of work.

The Aerobic
When exercise or physical work lasts longer than a few minutes, the aerobic system kicks in and is the primary source of energy. As the name suggests, the aerobic system requires oxygen. This system uses aerobic glycolysis (the glycolysis process that uses oxygen to break down glucose) in order to create large amounts of ATP. This process requiring oxygen, further explains why people who are less fit, or who have weaker ATP-PC and Lactic Acid energy systems, end up out of breath with less physical exertion, than people who are really fit. In people who are unfit, the initial two systems are capable of producing less energy and working for shorter periods of time, and thus the aerobic system must kick in more quickly, requiring oxygen sooner in order to create the energy needed for the physical activity, than would otherwise occur in people who are fit.  Aerobic glycolysis takes place within the mitochondria, creating more than ten times the ATP that anaerobic glycolysis creates from a glucose molecule in the Lactic Acid System.

Your Body Is Smart
Your body is smart. It uses it’s resources efficiently based on how you use it and the forces you place on it.  This means, if you fail to use these energy systems adequately through regular work, your body will not bother to build them up stronger. If you do use each of these systems regularly your body will build them up, make them stronger and more robust.  If exercise, especially cardiovascular exercise, where The Aerobic System is the main source of energy, is difficult for you today – it won’t always be, so long as you keep using it.  Start small, by using the stairs, or parking further away from your destination and walking.  Every little but counts.  Build your systems up today, for a better, stronger tomorrow.  You’ll be thankful you did.

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