Energy Systems

We may not be able to see energy, but we can certainly feel heat, its byproduct whenever we do physical activity. Adenosine triphosphate (ATP) is the molecule within the body that holds energy. For this reason ATP is often referred to as the energy currency of the body. We get energy from the macronutrients we consume like carbohydrates, protein and fat. There’s also energy stored in any alcohol we consume, but we do not consider alcohol a macronutrient because alcohol has no nutrients and is not metabolized in the digestive system in same way that the other macronutrients are. When we eat or drink food, it’s broken down into various molecular components by our digestive system. These molecules are then absorbed into the bloodstream so that the body can put them to use.

Carbohydrates get broken down into glucose molecules. Glucose is a single unit of simple sugar. Dietary fats get broken down into fatty acids. And protein gets broken down into amino acids. Each of these substances is essential to various functions within the body. Ultimately, all will be used to produce energy if necessary. Carbohydrates are the body’s preferred source of energy and as such glucose is stored as glycogen, in ways that make it readily available for quick energy production. If carbohydrates are over consumed the body will convert the glucose molecules into fatty acids, just like those that result from dietary fat metabolism. Fatty acid is stored in adipose tissues (fat cells), so that it may be used for long-term energy needs once the glucose stores have been depleted. In order to do this, and only in the absence of available glucose, your body will convert fatty acids into glucose molecules through a process called gluconeogenesis. Protein may also be used to produce energy, however amino acids play so many other important roles within the body that it will only use it for energy in emergency situations when the other macronutrient storages both glucose and fatty acids, are low. This needs to be considered before attempting lose weight by eating at a calorie deficit without considering what macros that calorie deficit is coming from. If you are losing weight while eating at a protein calorie deficit, you are not just losing fat weight – you’re losing muscle, hormones, and other tissue weight.

There are three main energy systems within the body: the ATP-PC system (also known as the Phosphagen, Phosphocreatine, or Creatine Phosphate system), which is anaerobic, meaning it doesn’t require oxygen; the Lactic Acid System (also known as the Glycolytic) system, which is also anaerobic; and then the Aerobic (or Oxidative) system, which requires oxygen. The ultimate goal of each system is to produce ATP molecules. No single system is used exclusively. While all systems are in use simultaneously, one will take dominance over the others depending on the activities being performed and the energy requirements of that activity.

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 short 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 along with molecules of phosphocreatine, 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 work while another energy system starts working 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 and up to about a minute and a half (90 seconds), the Lactic Acid system will kick in, in order to provide you with the energy necessary. The lactic acid system uses 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 or power. The process of glycolysis breaks glucose down into molecules of 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, those 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 which allows glycogen stores to refill, and then starting a new set of work.

The Aerobic (Oxidative) System

When exercise or physical work lasts longer than a few minutes, the aerobic system kicks in and is the primary source of energy. While this system produces much more ATP than the other two, it doesn’t do it as quickly. As the name suggests, the aerobic system requires oxygen. This system uses aerobic glycolysis (the 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. The aerobic system must kick in quickly, requiring oxygen sooner in order to create the energy needed for the physical activity, than would otherwise occur in people who have fitter, more efficiently functioning systems. Within the mitochondria aerobic glycolysis takes place, creating more than ten times the ATP that anaerobic glycolysis creates from a from a single glucose molecule in the Lactic Acid System.

 

Activity Duration	Dominant Energy System	Major Source of Fuel
< 6 or 7 seconds Examples: initiation of any increase in activity, sprinting across the street, catching a ball, jumping onto a box (once)	ATP-PC System	ATP & Phosphocreatine
7 seconds ~ 90 seconds Examples: Lifting multiple sets of weights, running 200m	Lactic Acid System	Glucose (stored as glycogen within the muscles)
90 seconds ~ 2 hours Examples: walking, long distance running, cycling	Aerobic System	Glucose (stored as glycogen within the muscles) as activity initiates Intramuscular fat & adipose tissue with prolonged activity

The amount of glucose stored as glycogen in the muscles is what limits most exercise performance. Having low stores of glycogen before exercising will lead to early fatigue, reduced intensity and as such a reduced ability to train. This consequently leads to a reduction in exercise gains. For this reason, there is a lot of focus on the role of carbohydrate intake surrounding exercise in the fitness and strength training industries. Fat however, is the major fuel for endurance exercises when the aerobic system is dominant. Endurance athletes can become more fat-adaptive, meaning their body can more readily use fat for fuel and it can do so quicker, by maintaining a higher fat diet. While a Ketogenic style diet may not be the best choice for a strength-training athlete who is relying mainly on the lactic acid and ATP-PC systems, it’s not a terrible choice for the distance runner who relies on the Oxidative system. Depending on the client, their food preferences and activity types we can successfully prescribe either a higher fat lower carbohydrate diet, or a lower fat, higher carbohydrate diet that still supports their quality of life and body composition. Either way, the body will use carbohydrates and fats to create energy, before protein.